Commodities: Fieldcrops: Peanuts

2009 Peanut Update

2009 PEANUT DISEASE UPDATE

Bob Kemerait, Tim Brenneman, and Albert Culbreath

Peanut diseases are a constant, yet unwanted, companion for growers in the southeastern United States. Coupled with peanut root-knot nematodes, which are particularly troublesome for many producers, diseases cost growers in Georgia and in neighboring states millions of dollars each year in terms of lost yield, reduced grades, and management expenses. Unfortunately, the weather in the southeastern US during the growing season provides near-perfect conditions for the spread of disease. In fact, the warm and wet weather that allows growers to produce bumper crops also sparks and fans the flames of peanut disease epidemics such as white mold, leaf spot, and Rhizoctonia limb rot. The sandier soils, especially in southwestern Georgia, that help farmers harvest top peanut yields are also a perfect home for the root-knot nematodes that wither the plants and gnarl the pods in the field.

Although a few growers may have experienced severe outbreaks of tomato spotted wilt in their fields in 2008, this troublesome disease was of minimal impact in peanut fields across the Southeastern US. It is estimated that that the incidence of tomato spotted wilt on peanut last season in the Georgia-Florida-Alabama region was about 1.0%, among the lowest severity since loss estimates were initiated in 1990. Despite low levels of spotted wilt in 2006, 2007 and 2008, growers should not become complacent in management of this viral disease. Without taking proper management precautions, growers could experience heavy losses to spotted wilt in 2009. Peanut Rx, the peanut disease risk index developed through collaborative efforts at the University of Georgia, the University of Florida, and Auburn University, has been updated for 2009 and offers growers strategies to minimize risk to not only spotted wilt, but leaf spot, Rhizoctonia limb rot, and white mold as well. The complete Peanut Rx is presented elsewhere in this Peanut Update.

White mold was the most important disease of peanuts in Georgia in 2008. White mold became apparent as early as June and caused great concern for many growers and considerable losses in some fields. Many of the questions from peanut growers to the Cooperative Extension offices in July and throughout August dealt with management options for this disease. The key to the outbreak of white mold in 2008 was very warm temperatures in June which fueled the disease. Warm soil temperatures are an important factor in the development of white mold. Rainfall and irrigation certainly increase the potential risk and severity of this disease; however white mold can cause much damage even in a drier year when warm soils are common. In drier year, white mold is likely to cause most of its damage to the pods and pegs lying below that ground as it may be too dry in the above-ground canopy.

Although many growers expressed concern about white mold and potential damage to their fields last season, it seemed that most fungicide programs that included an appropriate soilborne component did relatively well. In managing white mold, note the use of the word “managing” and not “controlling” white mold, growers should not expect 100% effectiveness from any program. It is estimated that 70% control is all that can be expected in the best of situations and 50% control may be all that can be achieved when environmental conditions and factors such as poor crop rotation increase the risk to the disease in a field.

It is extremely difficult to protect a peanut crop from isolated “hits” of white mold in any field. Depending upon the crop rotation in the field, the variety of peanut planted, and the environmental conditions (e.g. weather) during the growing season, a field may have many isolated hits of white mold or fewer hits. An effective fungicide program (to include use of an appropriate fungicide applied at the proper timing with an appropriate spray volume) should minimize the spread of white mold in a field. A grower should be concerned if he notes “runs” of white mold across the field that are several feet in length, or longer, despite use of a soilborne fungicide.

Peanut growers will have the opportunity to use some new and/or updated tools in 2009 to further their battle against diseases and nematodes.

1) “Day versus Night spraying”: Research began in 2007 and was continued into 2008 (both in small plots and in large, on-farm studies) to assess the benefits and potential consequences of spraying fungicides at night for control of soilborne diseases. Because the peanut leaves “fold up” when it is dark, thus opening the interior of the canopy, it is thought that fungicides applied at such time would have better chance of reaching the crown of the plant. For management of soilborne diseases like white mold and Rhizoctonia limb rot, the crown of the plant is targeted for optimum control. Also, it is thought that by spraying fungicides directly into the crown of the plant, the fungicide residues are protected to some degree from sunlight, thus reducing photodegradation and extending the period of efficacy. Below is a summary of findings from the University of Georgia with regards to spraying at night.

a. Although results were not as dramatic in 2008 as they were in 2007, results were similar in both seasons. Control of white mold can be significantly improved by spraying the peanuts at night, there is no significant reduction in leaf spot control, and yields can be significantly improved with night sprays.

b. Improvement of white mold control is more evident in non-irrigated plots than in irrigated plots when fungicides are applied in darkness, though there is likely to be benefit in both situations.

c. Spraying in the early morning hours before dawn tends to offer slightly better results than in spraying in early evening. It is believed that the dew in the early morning further aids in the relocation of the fungicide.

d. It is believed that applying fungicides at night will either maintain yields and control of white mold and leaf spot diseases or improve disease control and yields as compared to daytime applications. There is believed to be little risk to the grower by applying fungicides at night, other than loss of a sound sleep!

e. Note: Only fungicides applied for control of soilborne diseases should be considered for application at night. Fungicides applied only for control of leaf spot diseases and rust should continue to be applied during the day.

2) The 2009 “PEANUT Rx” Disease Risk Index is now available and has been thoroughly reviewed and revised as needed by researchers, breeders, and Extension specialists from the University of Georgia, the University of Florida, and Auburn University. The only change deemed necessary was an update of the risk points and varieties that were included in the Index. All other points/categories remained unchanged from 2008.

3) “Prescription Fungicide Programs”, i.e. specific disease management programs with an increase or decrease in fungicide applications based upon the 2009 “PEANUT Rx”, continues to gain support from the agrichemical industry. In 2009, Syngenta Crop Protection (Abound, Bravo WeatherStik, Tilt/Bravo) and Nichino (Artisan, Convoy) will continue to supported prescription programs (4, 5, and 7 applications) for fields determined to be at low, moderate, or high risk according to PEANUT Rx. Prescription programs using fungicides not from Syngenta or Nichino can also be used successfully by growers; however they would not be endorsed or supported by any company.

4) PROLINE 480SC (prothioconazole) from Bayer CropScience finally received a label for use as an in-furrow fungicide on peanut. PROLINE (5.7 fl oz/A) is a promising component of a complete fungicide program to reduce the impact of Cylindrocladium black rot (CBR) in a field. With the availability of PROLINE, a good integrated pest management program for growers who wish to manage CBR is to a) practice good crop rotation, b) consider planting a variety with some resistance to CBR, c) use PROLINE, 5.7 fl oz/A in-furrow, at planting, followed by d) a 4-block program of PROVOST or at least use of a fungicide program that offers suppression of CBR (e.g. Folicur, Abound, or Headline).

5) Although they were released in 2007, “newer” fungicides PROVOST (triazole fungicide mixture) and Evito 480SC (strobilurin fungicide) will continue to become more familiar to peanut growers this season as they join Abound, Folicur, Headline, Artisan, and a number of generic tebuconazole products for management of leaf spot and soilborne diseases. Arysta Life Science, maker of EVITO, has also labeled EVITO-T for the 2009 growing season. EVITO-T is a premix of fluoxastrobin (EVITO) and tebuconazole. Research data and rate information on this product is limited at this time, but should become more available as the season progresses.

6) ELAST 400F (dodine) has long been important to pecan growers but has only recently received a label for use in peanut production. In preliminary field trials, ELAST (15 fl oz/A as a stand-alone product and 12.8 fl oz/A tank-mixed with products such as Folicur 3.6F) appears to be an effective fungicide for the control of leaf spot. Like chlorothalonil, ELAST is a protectant fungicide and must be applied before disease occurs. ELAST is in a chemical class different from other peanut fungicides and thus could also be a useful tool in fungicide resistance management.

7) QUASH (metconazole) is a new fungicide for peanut labeled by Valent. Although QUASH is an effective fungicide for management of leaf spot diseases, it is unlikely that much will be used on peanuts in Georgia.

CROP ROTATION

Key point for 2009: Although soybeans may be a popular crop for growers in 2009, rotating soybeans with peanuts could help to increase severity of Cylindrocladium black rot (CBR), peanut root-knot nematodes (Meloidogyne arenaria) and will be of little-or-no benefit in the management of white mold and Rhizoctonia limb rot.

The practice of good crop rotation has always been at the foundation of optimum disease management in peanut, affecting not only nematodes and soilborne diseases, e.g. white mold, Rhizoctonia limb rot, and Cylindrocladium black rot, but leaf spot diseases as well. For this reason, Extension specialists at the University of Georgia stress the importance of avoiding planting peanuts in the same field more often than once every three years and rotating with a grass crop, e.g. bahiagrass or corn, if at all possible.

The peanut acreage in Georgia is expected to drop in 2009 from the approximately 675,000 acres planted in 2008. A drop in acreage planted to peanut should help growers to increase the length of time between peanut plantings in a field.

Since the recent change in the Peanut Farm Program, peanut farming in Georgia has expanded into “non-traditional” production areas in the southeastern portion of the state. Growers in this area frequently ask “Can I grow peanuts on my land in back-to-back seasons as I have not grown them here before?” The simple answer is, of course, you can plant peanuts on your land whenever you want to. However, even growers who are planting peanuts on “new peanut ground” should be discouraged from back-to-back peanuts if possible. Reasons for this include:

1) Many peanut growers around the state would love to have access to “new peanut ground” as populations of pathogens attacking the crop should be initially low. Therefore, it does not make much sense to lose this competitive edge in pursuit of the short-term goal of growing two or three crops of peanuts in succession.

2) Many new peanut growers are producing peanuts on land that has been cropped to cotton in recent years. Although cotton is not affected by the peanut root-knot nematode, early or late leaf spot, or Cylindrocladium black rot (CBR), and is only slightly affected by white mold, it is susceptible to diseases caused by Rhizoctonia solani. It is likely that despite previous cropping in a field, there will be significant populations of R. solani and perhaps smaller populations of Sclerotium rolfsii (white mold) in the field when peanuts are first planted. (This was observed in a test plot in Lanier County in 2004.) Without effective crop rotation, these populations may increase quickly.

3) In 2005, we observed an outbreak of CBR in a field in southeast Georgia planted for two consecutive years to peanut, but had not been planted to peanut at any other time. Earlier crops of soybean had introduced this disease to the field and back-to-back years of peanut had intensified the problem.

One of the greatest benefits of crop rotation is that it increases the effectiveness of all disease management programs. Effective crop rotation takes some of the “pressure off” of a fungicide program to minimize the impact of disease. Any fungicide program will be more effective where good crop rotation is practiced. In some situations, fields that are well rotated will require fewer, or at least less expensive, fungicide applications by the grower.

Recommendations from the University of Georgia for crop rotation and peanut production include the following:

1) Avoid planting peanut in the same field more than once out of every three years. Longer rotations, for example once every four years, are even better.

2) The best crops to rotate with peanut are grass crops, such as corn, sorghum, and bahiagrass. These crops will help to reduce the severity of diseases caused by Rhizoctonia solani, as well as CBR, white mold, and leaf spot diseases. Although corn and sorghum are alternate hosts for the peanut root-knot nematode, they are less affected than peanut is. Therefore, planting corn and sorghum should help to reduce populations of peanut root-knot nematode, though perhaps not as fast as when a non-host such as cotton is planted. Bahiagrass is susceptible to the lesion nematode, which can reduce the pod brightness important for the green peanut market.

3) Cotton is a very good rotation crop with peanut and should help to reduce the severity of white mold, leaf spot diseases, and CBR on future crops. Cotton is not a host for the peanut root-knot nematode, so this will be a beneficial effect as well. Cotton is a host for Rhizoctonia solani, so diseases caused by this pathogen will remain a concern in peanut-cotton rotations, especially in conservation tillage where crop debris remains on the surface.

4) Soybeans, other leguminous crops, and many vegetable crops are not preferred for rotation with peanut. Although such rotations are likely to reduce the severity of leaf spot diseases, they may not reduce the severity of white mold, Rhizoctonia limb rot, the peanut root-knot nematode, or, in the case of soybean, CBR.

DISEASE MANAGEMENT IN 2009

Tomato Spotted Wilt. Although tomato spotted wilt was not severe in 2006, 2007 or 2008 in most fields, growers must remain vigilant in the management of spotted wilt and should use the PEANUT Rx as a guide for minimizing risk. Growers may also want to plant at least a portion of their acreage to the newer varieties that have resistance to spotted wilt that is greater than that of Georgia Green.

Every year growers are reminded that the goal of PEANUT Rx is to minimize their risk point total for a specific production field. PEANUT Rx does not dictate when a grower must plant peanuts, for example in the middle of May. The purpose of the index is to allow growers to determine how to minimize their point totals given their own needs. For example, if a grower needs to plant in late April, he or she can still achieve a satisfactory point total by making adjustments to other parts of the index, such as selection of a more resistant variety.

Fungal Diseases. Good crop rotation remains the cornerstone of a good disease management program. We recommend that a grower plant peanut in a field only once every three years, and once every four years is even better. Grass crops, such as bahiagrass and corn, are the best rotation crops with peanuts because they do not share the same diseases or pathogens. (Note: Bahiagrass is a host for the lesion nematode, which does affect peanuts, especially green peanut growers.)

Early and Late Leaf Spot Diseases. Both early and late leaf spot are commonly observed across Georgia’s peanut production region.

Management Points for Leaf Spot

1) Practice good crop rotation.

2) Destroy any volunteer peanuts that may grow in a field and bury/remove old peanut hay that can serve as a source of spores for leaf spot diseases.

3) Do not delay the start of a leaf spot fungicide program.

a. When using chlorothalonil (e.g. Bravo Ultrex, Bravo WeatherStik, Echo, Equus, or other generics), Tilt/Bravo, Echo-PropiMax, Stratego, Elast 400F, or Headline (at 6 fl oz/A), and you have adequate crop rotation,your first leaf spot spray will typically be applied somewhere between 30 and 35 days after planting (unless weather has been dry and unfavorable for development of foliar diseases.

b. In fields where risk to leaf spot has been calculated as low-to-moderate, we have maintained good control of leaf spot when using a single application of Tilt/Bravo 40 days after planting

c. Growers who use the AU-pnut forecasting system, automated at www.AWIS.com, can more effectively time their first application based upon environmental conditions.

d. If you are planting peanuts after peanuts, you will likely need to begin your leaf spot program earlier than 30 days after planting because of the increased risk of disease.

e. If you are using Headline (at 9 fl oz/A) for your first leaf spot spray, it is appropriate to combine your first two fungicide applications for leaf spot control (for example at 30 and 44 days after planting) into a single application of 9 oz of Headline at 38-40 days after planting.

4) Traditionally, fungicides are applied on a 14-day calendar schedule beginning after the first application. This 14-day interval may be modified for reasons such as those below:

a) The interval should be shorter than every 14-days if conditions:

i. Rainfall has been abundant and conditions are favorable for leaf spot.

ii. You are using the AU-PNUT leaf spot advisory and it calls for an early application.

iii. Peanuts follow peanuts in a field and leaf spot is expected to be severe.

iv. Rainfall came on quickly after your last leaf spot spray and you are concerned that some of the fungicide may have been washed off the plants in the field too quickly.

v. You are planting a variety that has poor resistance to leaf spot diseases.

vi. Peanut rust appears in your field prior to the end of the season.

b) It may be possible to extend the spray interval beyond 14-days if:

i. Conditions have been dry and unfavorable for leaf spot, especially if you use the AU-PNUT advisory for spray guidance.

ii. You are using a variety with increased resistance to leaf spot, such as York, Georgia-07W, or Georgia-03L. For example, if pressure from soilborne diseases is not severe, the spray interval for such varieties could be every 21 days and it is possible to treat the most resistant varieties only three times during the season. (Additional information can be obtained from your local Extension Agent).

iii. You use Peanut Rx and determine that the predicted risk of fungal disease in a field is low to moderate and rainfall has not been excessive since your last spray (additional information can be obtained from your local Extension Agent).

iv. Since many fungicide applications are used to manage leaf spot diseases and soilborne diseases, one must consider the effect that an extended spray schedule would have on both types of disease (foliar and soilborne) BEFORE shifting from a 14-day schedule.

The “funky leaf spot”, whose cause is still unknown, typically affects peanut plants very early in the season and can look very much like early leaf spot. It may also cause considerable defoliation of early season foliage. Because this disease typically disappears by the middle of the season, it has not been found to be of real concern. Funky leaf spot has been found to be most severe on peanut varieties such as Georgia-02C and Georgia-03L, but is not thought to cause yield loss for either.

Current fungicides DO NOT control funky leaf spot; so do not be unduly alarmed by the appearance of leaf spots on your peanuts early in the season. Stay on a good fungicide program and have confidence that this program will control the more important early and late leaf spot diseases.

Finding some leaf spot in a field at the end of the season is usually not a problem. As long the diseases are controlled throughout the season, limited defoliation (up to about 30-40%) is not likely to affect your yield. The appearance of leaf spot at the end of the season typically does not mean that your program was ineffective or a failure.

8) Some growers in Florida are mixing chlorothalonil with Topsin-M or Topsin 4.5F or copper fungicides such as Kocide for their final leaf spot sprays to increase peg strength prior to harvest. What do we recommend in Georgia?

a. Combinations of chlorothalonil and Topsin-M currently provide excellent control of leaf spot.

b. Combinations of chlorothalonil and copper are also effective in the control of leaf spot.

c. Data collected at Clemson University demonstrates that peg strength is not increased with use of Topsin-M, Topsin 4.5F, or copper (e.g. Kocide).

9) Failures in leaf spot management in a peanut field are often linked to:

a. Unacceptable delays in starting your program.

b. Improper calibration of equipment (not enough material was applied).

c. Unacceptable delays between applications, such as when weather conditions keep the grower out of the field.

d. Rain events immediately after a fungicide application have washed the fungicide away too quickly.

10) Use of Chlorothalonil:

a. Chlorothalonil is the active ingredient in Bravo products, Echo products, and a number of generics. It is quite effective in the management of leaf spot diseases. Key points:

i. All chlorothalonil products for peanut appear to be effective. Differences between one brand and another are related to the “stickers” and other substances that are added to the active ingredient to increase effectiveness.

ii. There is no difference in efficacy between a flowable and dry- flowable formulation of chlorothalonil.

iii. Two likely benefits from chlorothalonil products when compared to other products for leaf spot control are: Price and Use for fungicide resistance management.

iv. The typical rate for a 720-F formulation is 1.5 pt/A; for a 90-DF formulation is 1.4 lb/A.

v. Chlorothalonil products are not systemic and must be applied to the leaf surface prior to infection by the fungus.

vi. Generally, chlorothalonil products have been on the foliage long enough prior to a rain event IF they have had time to dry completely.

vii. If you feel that your chlorothalonil application may not have had enough time to dry before rain, consider timing your next fungicide application a little earlier to compensate for any reduction in efficacy.

viii. When conditions have been very favorable for leaf spot (a lot of rain), it is generally true that research plots treated with chlorothalonil will have more leaf spot at the end of the seasonthan plots treated with a systemic fungicide for leaf spot control. This increase in leaf spot rarely results in a reduction in yield.

ix. Tank mixing Topsin M with chlorothalonil provides a good option for growers who are looking for a “rescue treatment” when leaf spot is developing too quickly in their field.

11) Use of Elast 400F:

a. Elast (dodine) is in a fungicide class different than others used in peanut production. Thus when used in a peanut program it can help to reduce the chances of fungicide resistance that occur with overuse of certain “at risk” fungicides.

b. Elast is a “protectant” fungicide like chlorothalonil and must be applied before infection by leaf spot pathogens has occurred. If infection has already occurred, application of Elast will be of minimal benefit for disease control.

c. Elast is used at either 15.0 fl oz/A alone or at 12.8 fl oz/A when tank-mixed with a product like tebuconazole (7.2 fl oz/A) for additional leaf spot control.

d. Use of Elast is most appropriate where chlorothalonil would be used.

12) Tilt/Bravo, Echo-PropiMax, and Stratego:

a. Propiconazole + chlorothalonil is marketed as two products, Tilt/Bravo and Echo-PropiMax.

i. The rate of this combination is 2.0 fl oz of propiconazole and 1.0 pt of chlorothalonil/A.

ii. Tilt/Bravo is now marketed as a pre-mix which when applied at 1.5 pt/A, offers the same level of product as described above.

iii. Tilt and PropiMax are systemic, which means that they can be absorbed into the leaf tissue offering some limited curative activity for recent infections.

iv. Fungicide resistance management: improper use of Tilt/Bravo or EchoPropiMax with Folicur or Stratego may increase the risk of resistance to the sterol-inhibitor class of fungicides.

b. Propiconazole + trifloxystrobin is marketed as Stratego.

i. Stratego is also a systemic fungicide with limited curative activity.

ii. For leaf spot control, Stratego is applied at a rate of 7.0 fl oz/A.

iii. Fungicide resistance management: improper use of Stratego with Folicur, Tilt/Bravo, Echo-PropiMax, Abound or Headline will increase the risk of resistance to the sterol-inhibitor and strobilurin classes of fungicides.

c. Where do we see the best fit for these products?

i. Even though these fungicides have a systemic component, they should be applied BEFORE infection occurs in order to obtain maximum benefit.

ii. When conditions for leaf spot are favorable, use of Tilt/Bravo, Echo- PropiMax, or Stratego often provides for better leaf spot control than with chlorothalonil alone.

iii. If growers plan to use one of these fungicides, they are often used early in the season to help insure a good start to leaf spot management.

iv. If conditions have been favorable for leaf spot (abundant rainfall), a grower has been delayed in spraying for leaf spot, or leaf spot is beginning to appear in the field, use of Tilt/Bravo, Echo-PropiMax, or Stratego may provide benefits beyond chlorothalonil.

13) Topsin-M (thiophanate methyl) is a fungicide in the benzimidazole class.

a. Topsin-M can be a very effective part of a leaf spot management program.

b. Growers who use a 4-block tebuconazole program can increase the control of leaf spot by tank-mixing 5.0 fl oz/A Topsin-M with 7.2 fl oz of tebuconazole in alternating applications (either 1 & 3 or 2 & 4).

c. Growers who use a 4-block Artisan program (13-16 fl oz/A on each of four applications, may also want to consider using Topsin as described above.

d. Growers who are looking for an effective fungicide treatment, should leaf spot become a problem in a field, can make an application of Topsin-M (5.0-10.0 fl oz/A) tank-mixed with 1.5 pt/A chlorothalonil. This can be followed up with a second application of the same tank-mix or with an application of Tilt/Bravo.

e. Growers should make no more than two tank-mix applications of Topsin-M per season in order to avoid fungicide resistance problems.

14) Pyraclostrobin is sold as Headline.

a. Headline has been the most effective fungicide labeled on peanut for management of leaf spot.

b. NOTE: Because Headline is our current standard for control of leaf spot diseases, some growers forget that Headline at rates of 12-15 fl oz/A is also an effective white mold/Rhizoctonia limb rot material as well. Growers who incorporate a higher rate of Headline into their fungicide program can expect excellent leaf spot control and effective soilborne disease control as well.

c. Headline has the best curative activity of any fungicide for control of leaf spot.

d. Fungicide resistance management: improper use of Headline with Abound, Evito, or Stratego will increase the risk of resistance to the strobilurin class of fungicides. In most cases, Headline should not be used in a fungicide program that contains Abound, Evito, or Stratego.

e. For leaf spot control, Headline is typically used as follows:

i. Two applications at 6.0 fl oz/A at approximately 30 and 44 days after planting. We generally do not spend much time with this pattern, as the one below is a much better option for the grower.

ii. Combine two traditional leaf spot fungicide applications into asingle application at 9.0 fl oz/A approximately 38-40 days after planting.

iii. Note: Because of its power to control leaf spot, some growers have used Headline as a “salvage” treatment late in the season when leaf spot appears out-of-control in a field. Remember:

1. It would have been better to use the Headline earlier to try and avoid the problem entirely.

2. Headline may slow the epidemic of disease, but it will not cure the problem. You will still have leaf spot; perhaps not as much as you would have had if you had not treatedwith Headline.

3. Using a selective fungicide, such as Headline, when disease is present and severe will increase the risk for the development of fungicide resistance.

15) Abound, Evito, Provost, and tebuconazole products are typically considered to be for control of soilborne diseases; however they must also control leaf spot diseases as well. Provost, Abound, and Evito provide effective leaf spot protection alone. Problems associated with tebuconazole and leaf spot are usually related to fungicide resistance issues or are traced back to rain or irrigation soon after application. To maximize leaf spot and white mold/limb rot control with Folicur/tebuconazole, it is best that the crop dry for 24 hours before irrigation. Where rainfall is abundant and/or resistance is likely, most growers will add a half-rate of chlorothalonil or Topsin to 7.2 fl oz/A of tebuconazole for added leaf spot protection.

SOILBORNE DISEASES

White Mold and Rhizoctonia Limb Rot Diseases: White mold and Rhizoctonia limb rot are likely to occur in nearly every peanut field in Georgia. Losses caused by these diseases can be severe and they are much more difficult to control than leaf spot diseases. Prior to 1994 when Folicur was first labeled, growers did not have any truly effective fungicides to control theses diseases. Since 1994, growers now have six different fungicides from three different classes that can effectively control both white mold and Rhizoctonia limb rot. Still, white mold and limb rot remain troublesome to growers. Two of the reasons for difficulty in control are 1) it can be tough to tell when you need to begin spraying, and 2) it is not easy to get the fungicide to its target where it can affect the pathogen.

Management points for white mold and Rhizoctonia limb rot.

1) Practice good crop rotation.

a. Corn, grass crops, and bahiagrass are good rotation partners reducing effect of white mold and Rhizoctonia limb rot.

b. Cotton will reduce the risk of white mold but will have less benefit on Rhizoctonia limb rot.

2) Choose resistant varieties when available.

a. Some new varieties, such as C99-R, AP3, Georgia-02C, Georgia-03L and York, have increased resistance to white mold over Georgia Green.

b. Georgia Green appears to have better resistance to Rhizoctonia limb rot than many other varieties.

3) Use fungicides.

a. NOTE: No fungicide program will give the grower complete control of soilborne diseases in a field. We estimate that, at best, a good soilborne fungicide program will give 60-70% control under ideal conditions.

b. Initiating fungicide applications is often imprecise and is based upon experience.

c. The timing of fungicides for controlling white mold and limb rot must be early enough to protect the crop when the disease first appears. However, growers should avoid applying soilborne fungicides too early so that they will be available when needed later in the season.

d. Initial appearance of soilborne diseases is related to the soil temperature, the growth of the crop, and rainfall/irrigation.

e. In Georgia, we generally start spraying for soilborne diseases approximately 60 days after planting. At this time in the season, the growth of the crop and the environmental conditions are suitable for disease to occur. Because white mold and Rhizoctonia limb rot can occur earlier than this, the grower should watch his fields carefully to determine when the diseases appear.

f. Example: In 2003, rainfall was abundant and we predicted that severe white mold would occur early in the season. However, white mold did not appear until later in the season and was much of a late-season problem. The most probable reason for this was temperature. Although the moisture was suitable for white mold (and limb rot), the cooler-than- normal summer temperatures delayed the onset of white mold. In 2006, white mold was severe across much of the production region of Georgia despite dry conditions. Again, the warm soil temperatures resulted in outbreaks of white mold, though the drought reduced the severity of Rhizoctonia limb rot.

g. Fungicides are applied to the foliage, but must reach the crown and limbs of the plant in order to be effective against soilborne diseases.

i. The fungicides can be moved by rainfall and irrigation. If rainfall or irrigation occurs too quickly after application, the fungicide may not provide enough protection for leaf spot.

ii. If the rainfall or irrigation is delayed, absorption of the fungicide into the foliage may reduce the amount available to fight soilborne disease.

iii. In a dryland situation, lack of rainfall, and thus movement down the plant, will reduce the effectiveness of a soilborne fungicide. Still, the fungicide was probably not wasted; some of the product likely reached the desired target with the spray mix.

iv. If fungicides are applied during the night after the leaves have folded, more fungicide will reach the crown of the plant where it is needed to control soilborne disease.

h. Management with Tebuconazole:

i. Tebuconazole is marketed as Folicur, Tebuzol, Orius, Tri$um, Integral, Muscle, Tebustar, etc.

ii. Tebuconazole is effective against white mold and Rhizoctonia limb rot.

iii. Tebuconazole remains effective against early and late leaf spot; however the fungicide is not as effective as it once was due to development of resistance by the fungal pathogens.

iv. It is recommended that tebuconazole remain on the leaf surface for 24 hours after application to insure enough is absorbed for leaf spot control.

v. If tebuconazole is washed from the leaves too quickly, leaf spot control may suffer, though the grower may get maximum control of white mold and limb rot.

vi. In extremely wet weather, or when the threat from leaf spot diseases is elevated or where resistance has developed, growers should choose to mix 0.75-1.0 pt of chlorothalonil or 5 fl oz Topsin with 7.2 fl oz of tebuconazole to insure leaf spot control. 7) At one time the addition of chlorothalonil was thought to impede the movement of Folicur from the foliage; however this has not found to be a problem. Note: Topsin is added to two alternating applications of tebuconazole in a 4-block program.

vii. Tank-mixing tebuconazole with the product Prevam has, in some trials, helped to reduce the severity of leaf spot over Folicur applied alone.

viii. Tebuconazole is applied at a rate of 7.2 fl oz/A, beginning approximately 60 days after planting.

ix. In the most traditional program, tebuconazole is applied in a four block program, on a 14-day interval.

x. Fewer than four applications of tebuconazole may be sufficient in some low disease situations; however this will be an off-label program.

xi. Improper use of tebuconazole with Stratego, Tilt/Bravo, or Echo- PropiMax could increase the risk of fungal resistance to the sterol inhibitor fungicides.

i. Management with Provost (tebuconazole + prothioconazole):

i. Provost is available to peanut growers in 2009 from Bayer CropScience.

ii. Based upon results from the University of Georgia, Provost appears to have better systemic activity than other soilborne fungicides. This means that Provost can be more easily translocated within the plant from where it was applied to other regions for greater protection.

iii. Bayer CropScience recommends that Provost be used in a 4-block program like Folicur.

iv. The standard rate for Provost is 8.0 fl oz/A; however the rate can be effectively increased to as much as 10.7 fl oz/A when pressure from white mold or limb rot is severe.

v. Because Provost is a combination of two fungicides within the same chemical class (triazoles/DMI fungicides), it is EXTREMELY important that growers practice good fungicide resistance management principals with this product in order to maintain its efficacy over an extended period of time.

vi. From University data, Provost has provided excellent control of leaf spot diseases and control of white mold, Rhizoctonia limb rot, and CBR that is at least as good as that of Folicur.

vii. To avoid causing injury to the foliage, growers should carefully read the Provost label before tank-mixing this product with other fungicides.

j. Management with azoxystrobin:

i. Azoxystrobin is marketed as Abound and is typically applied at 60 and 90 days after planting at 18.5 fl oz/A.

ii. A lower rate (12.0 fl oz/A) is allowed by label in dryland situations or in reduced-risk “Prescription Programs”; however it must be used with caution, as it will not have the “power” of the full rate. We typically do not recommend this rate unless each Abound application is alternated with applications of tebuconazole at 7.2 fl oz/A OR a grower is carefully using a prescription program in a reduced risk field.

iii. Abound is effective against leaf spot diseases, white mold, and is excellent for management of Rhizoctonia limb rot.

iv. For maximum efficacy against white mold and limb rot, the field should receive irrigation or rainfall within 72 hours after application.

v. Fungicide resistance management: To avoid problems with fungicide resistance, Abound should not be used in the same program with Evito, Absolute, Stratego or Headline.

k. Management with fluoxastrobin:

i. Fluoxastrobin is marketed as Evito 480SC.

ii. Evito is in the same chemical class (strobilurins) as are Headline, Abound, Stratego, and Absolute and should not be used in the same fungicide programs as these products.

iii. Recommended use for Evito is two applications of product (5.7 fl oz/A) timed approximately 60 and 90 days after planting.

iv. We have limited research data on this product; however from fungicide trials Evito seems to be an effective component of a peanut disease management program.

v. Evito is NOT “generic Abound”.

l. Management with flutolanil:

i. Flutolanil is an excellent fungicide for the management of white mold and is also effective against Rhizoctonia limb rot. It is not effective against leaf spot diseases.

ii. Flutolanil is marketed as Moncut, Artisan and Convoy.

1. Moncut 70 DF must be mixed with another fungicide for the control of leaf spot. Moncut 70 DF is typically applied at 1.07 lb/A, in the middle of the rate range.

2. Convoy, like Moncut, only contains flutolanil and must be mixed with the full-rate of another fungicide for control of leaf spot. Convoy is typically applied at 26 fl oz/A twice (60 and 90 days) or at 13 fl oz/A in a four-block program.

3. Artisan is a combination of flutolanil and propiconazole. Therefore, it will control leaf spot, white mold, and limb rot. Artisan can be applied at a rate or 26 or 32 fl oz/A.

4. Moncut and Artisan are typically applied at 60 and 90 days after planting, though Artisan and Moncut can also be applied in a 4-block program.

5. When using Artisan in a 4-block program, it is applied at rates between 13 and 16 fl oz/A and tank-mixed with an additional leaf spot material, e.g. 1.0 pt chlorothalonil/A or perhaps an alternation of chlorothalonil with Topsin at 5 fl oz/A.

6. When using Moncut 70DF fungicide in a 4-block program, it is typically applied as 0.5 lb/A tank mixed with a FULL rate of some leaf spot material.

7. As a final note, the flutolanil products Artisan and Moncut performed exceptionally well in 2003, 2006, and 2007 in field trials where white mold was severe. It is expected that Convoy would offer similar levels of control of white mold as well.

m. Management with pyraclostrobin:

i. Pyraclostrobin is sold as Headline (as discussed in the leaf spot section).

ii. Headline is effective in a soilborne disease management program against white mold and limb rot when applied at the 12-15 fl oz/A rate.

iii. Headline is not used as a “stand-alone” soilborne fungicide, but rather is used in combination with tebuconazole, or perhaps Artisan or Moncut.

iv. Headline is not used with Evito, Absolute, Stratego or Abound for fungicide resistance management concerns.

v. Use of Headline at 12.0 fl oz will provide adequate control of white mold and limb rot when used as a part of a soilborne program and will provide exceptional leaf spot control.

vi. An ideal use of Headline would be 9 fl oz/A at 40 days after planting, 7.2 fl oz/A Folicur at 60 days after planting, and 12.0 fl oz/A Headline at 74 days after planting.

n. Management with mixed programs. Some peanut growers in Georgia are experimenting with fungicide programs that mix different fungicides for the control of soilborne diseases and the results can be outstanding. The goal in mixing fungicides is to capture the best control available through the use of multiple chemistries. While some of these programs, like the alternate use of Folicur and Abound, for a total of four soilborne fungicide applications, appear to be quite effective, the grower must accept all responsibility if his program is off-label.

o. Managing White Mold with Lorsban 15G. Prior to Folicur, the insecticide Lorsban 15G was one of the only chemicals that growers had to manage white mold. As Folicur and then Abound were labeled, growers turned away from Lorsban for control of white mold. However, results from field trials in 2003 demonstrate that application of Lorsban 15 G (13.6 lb/A) in conjunction with fungicides may provide control of white mold beyond that of the fungicides alone. It appears that Lorsban 15G may still have a place in white mold control.

Cylindrocladium Black Rot (CBR): CBR is a very challenging disease to control and of increasing importance to growers across the state. Crop rotation away from peanut and soybean is an important management tool. Also, it is important that growers not introduce infested soil from fields where CBR occurs to fields where it is not yet present. This can be done best by cleaning equipment and vehicles before traveling between fields. In recent years, it has been proven that CBR can be transmitted via seed, though at a very low rate. Growers should try to obtain seed produced in fields free of CBR. They should also recognize that much of the seed for Virginia varieties is produced in the Virginia-Carolina region where CBR is of even greater importance than it is in Georgia.

Management Points for CBR

1) Crop rotation away from peanut and soybean. Unfortunately, once CBR is established in a field, it is very difficult to eliminate. Not only can the fungal pathogen survive for long periods of time in the soil, but it can also infect common weeds such as beggarweed and coffee weed.

2) Proline 480SC (prothioconazole) is a fungicide that is labeled to be applied in-furrow at planting time for management of CBR. The in-furrow rate is 5.7 fl oz/A. The in-furrow application of Proline promises to be a critical component for the management of CBR when followed by foliar application of the effective fungicides noted below. From numerous studies, it is demonstrated that liquid inoculants can be mixed with Proline without loss of efficacy of the fungicide or the inoculant.

3) Provost, Folicur, Abound, and Headline are labeled for the “suppression” of CBR. This means that these fungicides may reduce the symptoms of disease and possibly increase yields above other fungicides. Growers who are battling CBR may choose to use Provost, Folicur, Abound, or Headline for CBR suppression, though results are variable and sometimes disappointing.

4) Varieties with some level of resistance were not available to growers until recently. In the past several years, varieties GA-02C, Tifguard and Carver, have been released and appear to have at lest some level of resistance to CBR. Growers who have fields where CBR is found may want to consider planting these varieties.

5) It has been found that CBR is more severe in fields where the peanut root-knot nematode also occurs. Therefore, growers who manage nematodes with either Telone II or Temik 15G may find some suppression of CBR as well.

6) Fumigation with metam sodium (e.g. Vapam) at 10 gal/A directly beneath the row 10 days prior to planting is currently our best management strategy for the control of CBR. Results can be quite dramatic and can allow growers to plant peanuts in fields where it would otherwise be nearly impossible to grow a crop.

Prescription Fungicide Programs

“Prescription fungicide programs” are defined as strategies designed to maximize yields and maintain disease control in a field using the appropriate number and type of fungicide applications based upon the risk to disease in the field. The goal of prescription fungicide programs is too use the right amount of fungicide for the level of disease expected in a field and to modify the fungicide use as the risk of disease increases or decreases as the season progresses.

Fields where the risk to disease is high, for example where fields have shorted crop rotation, are planted to less resistant varieties, and weather favors disease development should receive at least seven fungicide applications during the season, and perhaps more.

Fields where the risk to disease is reduced to a low or moderate level, for example where fields have longer rotations and are planted to more resistant varieties, typically do not need the same fungicide program as a higher risk field in order to maximize yields. Research data from many on-farm and small plot studies conducted at the University of Georgia have demonstrated that growers who manage their crop so as to reduce the risk to leaf spot, white mold, and Rhizoctonia limb rot can also reduce the number of fungicide applications and increase the value of their crop by cutting production costs. In low risk fields, it is quite possible to reduce the number of fungicide applications from seven to four, so long as the grower is willing to watch the field to insure that disease does not begin to develop unnoticed.

Growers interested in developing prescription programs should first assess the risk in their field(s) using the PEANUT Rx Disease Risk Index and then contact their local county agent for guidance on a suitable fungicide program. Syngenta has developed their on prescription programs with input from University researchers. Growers who use a Syngenta prescription program in reduced risk field can have the confidence that Syngenta will “stand behind” these programs as long as risk level has been appropriately assessed and the appropriate fungicide program has been used. Nichino has also developed similar prescription programs with input from university researchers that can be used with confidence.

Managing Seedling Diseases: Seedling diseases were typically not a concern for peanut growers in Georgia prior to the arrival of the tomato spotted wilt virus. Even if some plants were lost in a stand, the neighboring peanut plants were often able to compensate for the loss by growing into the vacated space. However, it is clear that spotted wilt can be devastating when fields have poor stands. For this reason, getting a good stand has become critical for growers. Below are some management techniques to reduce seedling diseases (primarily caused by Rhizoctonia solani and Aspergillus niger).

1) Rotate peanuts with grass crops to reduce the populations of Rhizoctonia solani.

2) Plant the peanut crop when soil temperatures are warm enough to produce rapid, vigorous germination and growth. This can help protect the plants from disease.

Excessive moisture at planting will also increase the risk of seedling diseases.

3) Use quality seed that has a good germination rating and will grow vigorously.

4) Choose varieties that are known to germinate and emerge uniformly and with vigor.

5) Use only seed treated with a commercial fungicide seed treatment. The seed treatments that are put on commercial seed prior to purchase are outstanding and provide protection for the seed and seedling. Seed treatments include:

a. Vitavax PC

b. Dynasty PD (azoxystrobin + mefenoxam + fludioxonil)

c. Trilex Optimum (trifloxystrobin + metalaxyl + carboxin)

d. Trilex Star (trifloxystrobin + metalaxyl + carboxin + thiophanate methyl)

6) Use an in-furrow fungicide where the risk of seedling disease is great or where the grower wants increased insurance of a good stand.

a. Abound at 6.0 fl oz/A in the furrow at planting can provide increased control of seedling diseases, including Aspergillus crown rot.

b. Terraclor (64 fl oz/A) also provides additional control of seedling diseases when applied in-furrow.

c. Growers who are most likely to yield benefits from these in-furrow fungicides are those that have poor crop rotation and a history of seedling disease in the field.

Managing root-knot nematodes: Peanut root-knot nematodes are a severe problem in some fields in Georgia, especially in the sandy soils in the southwest corner of the state. Growers initially become aware of the problem when they note stunted plants across patches in their field. At harvest, many of the pods and pegs from these fields are galled and of poor quality. Based upon conversations with growers, it is likely that many fields across the state have problems with root-knot nematodes, but growers may fail to attribute the cause to nematodes. Below are some management options.

1) Use crop rotation to avoid building large populations of nematodes in a field. Cotton is an excellent rotation crop with peanut to reduce levels of nematodes.

2) Plant the root-knot nematode resistant variety ‘Tifguard’.

3) Treat the field with Temik 15G. From our trials, Temik at 10-lb/A in-furrow followed by 10 lb/A at pegging provides good control. It appears that the 10-lb/A application at pegging-time is critical. Note: growers must not apply Temik to the crop any later than 90 days before harvest.

4) Telone II at a broadcast rate of 6 gal/A or an in-furrow rate of 4 gal/A can provide control of the root-knot nematodes as well on peanuts. The Telone must be applied 7-14 days before planting to avoid damaging the crop. Applications of Temik 15g at 10 lb/A at pegging may still be advisable, even when Telone II was used at planting.