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Review of Scab Resistant Cultivars

Introduction

After the wet summer we had in 2003, and the difficulty many growers had in controlling scab, it is not surprising that there is renewed interest in planting more resistant cultivars. In this report I hope to give an introduction to some of the more commonly available cultivars with various levels of scab resistance. Unfortunately, there is no perfect pecan cultivar combining scab immunity with good production and quality. What we have instead is a diverse group of cultivars, each with its own limitations and advantages. The importance of each of these factors depends largely upon the grower and his or her particular situation. What I will try to do in this article is introduce those cultivars that are most commonly available, review their positive and negative aspects, and finally introduce some questions a grower should ask in determining which one is best for their situation.

Before we talk about cultivars, I would like to mention one aspect of scab resistance. Remember that historical evidence and recent research has shown that the scab fungus is composed of multiple races, each suited to grow best on a small number of cultivars. What this means is that the resistance level of a cultivar will be influenced by what races are present in the orchard in which it is grown. An orchard composed largely of 'Desirable' trees will probably have a large proportion of scab fungus which is suited to grow on 'Desirable'. If you plant one or a few trees of 'Sioux' in this orchard, 'Sioux' may not at first be much affected by scab. This is because the majority of scab conidia which land on 'Sioux' trees are from 'Desirable' trees and are not suited to growing on 'Sioux' leaves. However, the few conidia that successfully grow on 'Sioux' will reproduce and over time more and more scab may appear. The greater the number of 'Sioux' trees that are planted the more quickly this may occur. If additional 'Sioux' trees are planted in this orchard, they will likely be infected with scab much more quickly now that the 'Sioux' scab race is present. This leads to the cycle we commonly see where new cultivars start out appearing highly resistant, but over time appear more and more susceptible. 'Cape Fear', 'Stuart', and 'Desirable' are examples of cultivars which once had good levels of resistance but which now appear fairly susceptible. The cultivar itself is not changing, rather the pathogen is adapting to the cultivar. A few cultivars like 'Elliot' have had excellent levels of resistance for many years. Currently its not easy to predict how a cultivars resistance will last over time. You should therefore keep in mind that resistance levels may decrease. The variability in scab races in different locations may also lead to differences in scab incidence on a particular cultivar in different locations. If all this leads you to believe that scab resistance is a confusing issue, you are exactly correct. Right now it appears that the best we can offer is generalizations about current resistance levels and encourage growers to graft in different varieties and see how they perform in their orchard.

Let's talk a little about what "scab resistance" means. Everyone has their own idea about what constitutes "resistance." In the western regions where 'Wichita' and 'Western Schley' are king, 'Desirable' would be considered to have good levels of scab resistance. In the southeast, scab can be very difficult to control on 'Desirable' in many locations and resistance is considered to be poor. To simplify this topic, I will limit our discussion to four classes of resistance as seen in the southeast: Excellent, Good, Mediocre, and Low. Excellent resistance means that the cultivar will in most locations exhibit little or no scab symptoms, and a reduced spray program may be used. Good resistance is a defined as resistance in most locations, trees should be sprayed, but missing a couple of sprays will not result in an epidemic. Mediocre resistance is exhibited by trees which in most locations will have less scab incidence than 'Desirable', however, a full spray program is needed to control scab on these cultivars. Those cultivars with low resistance need a full spray program and scab may still be difficult to control in some locations. 'Desirable' would be considered to have low resistance. Cultivar ratings are not set in stone, but are assembled from a variety of sources including what I have seen in our orchards, published reports, and from talking to a variety of other researchers and growers.

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Cultivars with excellent levels of resistance

First let's talk about cultivars with an excellent level of resistance. Probably the best known of this group is 'Elliot'. 'Elliot' has long been the standard by which we measure resistance to scab. 'Elliot' produces a small round nut with very good kernel quality. Size ranges around 77 nuts / lb. with 51% kernel. 'Elliot' is well-known by most buyers, and its reputation for a high-quality kernel partially makes up for its small size. The limitations to 'Elliot' are its strong tendency to alternate as a mature tree, and it leafs out early in the spring, making it susceptible to freeze damage. The alternate bearing tendency of 'Elliot' is less severe in its impact because even with a heavy crop the nuts tend to be well-filled. Also, 'Elliot's' strong resistance means that fewer sprays can be used making it less expensive to care for the tree in the "OFF" year.

Those growers in more northern areas may wish to try 'Kanza' instead of 'Elliot'. This variety was released by the USDA in part because of its cold tolerance. Nut size and shape is very similar to 'Elliot' and would probably pass as 'Elliot' in most locations. Scab resistance in this cultivar so far has been outstanding, but it has not been grown much in the Southeast, so we don't know if it will hold up like 'Elliot'. Like 'Elliot', 'Kanza' tends to alternate as a mature tree, but percent kernel remained good at around 51%. Harvest date is early, averaging around Oct. 8 in Tifton, Ga.

Three cultivars with excellent resistance that we no longer recommend are 'Curtis', 'Gloria Grande', and 'Barton'. 'Curtis' produces a small nut (89 nuts / lb.) with a 54% kernel. The kernels sometimes have a flecking on the testa which may be objectionable. The late harvest date, small size, and mediocre production potential of this variety lead us to no longer recommend it for planting. 'Gloria Grande' has a very high level of resistance. The nut size is large at 45 nuts / lb. and the trees have shown a consistent good yield. However, it is difficult to produce good quality kernels with 'Gloria Grande'. The kernels often are dry and have packing fuzz attached, and the color is dark. The very thick shell of this cultivar and tendency to be poorly filled lead to a low 45% kernel. This cultivar is also favored by black aphids and must be closely monitored for this pest. At one time this cultivar was believed to have much promise as a new cultivar. While 'Gloria Grande' is a large nut with excellent resistance levels, the negatives outweigh the positives and we do not recommend it for planting. 'Barton' is an older USDA cultivar with outstanding diseases resistance. Nut size is medium (68 nuts / lb) but kernel percentage is low (50% kernel). This variety overloads on the "ON" year with very poor quality. The tree is vigorous, but seems much more adept at producing wood than nuts.

Recently some newer cultivars have appeared that have been specifically selected for high levels of scab resistance. Five of them are from the Auburn University program and were selected for low-input situations. The cultivars; Gafford, Syrup Mill, Jenkins, Carter, and McMillan, were all selected for very high resistance to scab as well as other disease and insect pests. 'Gafford' and 'Syrup Mill' have so far shown even higher levels of resistance than 'Elliot' in scab tests, although they have not yet been as widely planted. We have these in our variety test, but they have not yet fruited so I can't comment much on them. I will refer the reader to Dr. Bill Goff and Dr. Monte Nesbitt for further information on these selections.  

McMillan has been in our test since 2002. So far, it has been an excellent producer of medium sized nuts. Kernel quality has been fair to good. Its not as pretty as some other cultivars, but we’ve seen no sign of scab at all in this sprayed planting.  Another interesting selection in this group is the 'Excel' cultivar. This cultivar was selected from a seedling tree by Mr. Andy Clough in Blackshear, Ga. 'Excel' produces a large nut, 45 nuts / lb., with a very bright attractive kernel. It has now been widely planted around Georgia, and has not yet shown any signs of scab susceptibility.

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Cultivars with good levels of resistance

The next group of cultivars are those with good resistance levels. These cultivars will need to be sprayed to control scab, but in most locations scab incidence will be reduced, and missing a few sprays should not lead to large amount of disease. 'Sumner' is the only cultivar we recommend which falls into this group. Most growers are relatively familiar with 'Sumner' as it is becoming widely planted, primarily due to its increased disease resistance in comparison to 'Desirable'. 'Sumner' produces a large nut (55 nuts / lb.) with relatively good quality (52% kernel). Resistance to scab has been ranked higher than either 'Desirable' or 'Stuart'. The largest drawbacks to this cultivar is an increased susceptibility to black aphids, and a very late harvest date of around Oct. 29 in Tifton.

Another less well-known cultivar with this level of resistance is 'Candy'. 'Candy' produces a very vigorous precocious and productive tree. Unfortunately the nut size of 'Candy' is so small (78 nuts / lb.) that it would likely bring seedling prices. For this reason alone we don't recommend this variety.

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Cultivars with mediocre levels of resistance

The next lower level of resistance is mediocre resistance. These are cultivars which definitely need a spray program to control scab. However, at this time scab incidence is usually less on these cultivars than on 'Desirable'. Scab incidence also appears to vary quite a bit by location, making it difficult to predict how they will perform in any particular orchard.

The first cultivar of this group that we recommend is 'Oconee'. 'Oconee' produces a large nut (48 nuts / lb.) with a 53% kernel. Kernels are attractive and the nut shells well. Harvest date is relatively early, averaging around Oct. 12. One concern with this cultivar is that controlling black aphids can be challenging. We also seemed to note that this cultivar did not appear to do well when the trees began to crowd each other. Otherwise a nice cultivar that deserves more attention.

The second cultivar we recommend in this group is 'Caddo'. 'Caddo' set the standard in our tests for high and consistent production of good quality kernels. 'Caddo' is a precocious cultivar which is able to continue to produce good quality kernels as a mature tree. Harvest date is relatively early, averaging around Oct. 11. The biggest limitation to 'Caddo' is a medium sized nut (67 nuts / lb.).

'Forkert' is an older variety that has always performed well in our tests. Nuts are large at 53 nuts / lb. and percentage kernel averaged 58%. The high percent kernel is due in part to a very thin shell, but kernels are nearly always well developed. Some growers have complained about low production, but it has performed well in most published studies. It may require the correct pollinator to be productive. Attention will need to paid to disease control measures for each of these cultivars, but they can all be profitably grown.

There are several cultivars in this category that we no longer recommend for planting. 'Cape Fear' was once considered a good choice for scab resistance. However, the resistance of this cultivar appears to be declining. I have seen several orchards where 'Cape Fear' was scabbing as bad or worse than 'Desirable'. 'Cape Fear' also appears to be prone to a bacterial leaf scorch disease and it is difficult to produce a good quality kernel in older trees due to over production. 'Moreland' is an older cultivar that has been recommended in several states partially due to its scab resistance. In our experience, 'Moreland' begins to alternate in production as a mature tree, leading to a significant decline in quality. In addition, the harvest date of this variety is fairly late averaging around Oct. 24. Everyone is familiar with 'Stuart', and older 'Stuart' trees continue to be profitable in many orchards. We no longer recommend that 'Stuart' be planted because kernel percentage is marginal and there is a tendency towards dry fuzzy kernels. Young 'Stuart' trees also take longer than most varieties to come into production. 'Kiowa' is a cultivar in this category that has been borderline for recommendation. While it scabs, in many orchards it seems to be an easy keeper. Nut size is similar to 'Desirable', but the kernel is darker in color. 'Kiowa' is more precocious than 'Desirable' with a larger cluster size. As 'Kiowa' matures it tends to overproduce in the "ON" year with a decline in quality. 'Kiowa' nuts seem to be strongly attached to the stem, making it difficult to remove the excess crop via shaking. In the short term 'Kiowa' seems to be profitable, but it may be difficult to keep up quality in older trees.

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Cultivars with low levels of resistance

The final group of cultivars is those with low resistance. These are cultivars that will scab in nearly all locations. A full spray program is needed to control scab on these cultivars, and in some locations, scab will be difficult to control even with a full program. 'Desirable' falls into this category. 'Desirable' is well known of course. It produces a large nut with a high percentage kernel. Perhaps its strongest feature is the ability to produce consistent high-quality crops year to year. These strengths have lead to a large number of 'Desirable' trees being planted over the years. However, 'Desirable' is not an easy cultivar to manage, and requires excellent management to be at its best.

'Pawnee' produces a large nut (56 nuts / lb.) with a high percent kernel (54%). 'Pawnee' has an exceptionally early harvest date and is often ready in the first week of October or the end of September. Alternate bearing may be a problem, especially if orchards become crowded. The early harvest date means that animal predation may become a serious problem if only isolated trees are planted or if harvest is not prompt. Growers should plan to manage this cultivar differently than other cultivars in order for it to be profitable.

'Sioux' is the other cultivar I placed into this category. It was difficult to define 'Sioux' because it has not really been planted much in the Southeast and little literature is available. However, in our orchards we have seen moderate levels of scab, comparable to 'Desirable'. 'Sioux' is a relatively small nut (71 nuts / lb.) with a beautiful light colored kernel. 'Sioux' tends to alternate as a mature tree, but kernel percentage has remained high (54%). We recommend 'Sioux' in situations where a very high quality kernel is desired.

Now that we have our cultivars defined a bit, lets put them all together into a table (Table 1). As you can see there is not a cultivar that combines excellent disease resistance with a proven track record of high quality production. This means when deciding on a cultivar you are going to have to make choices and weigh options. The current tradeoff seems to be that the higher you go in resistance levels the less the quality of nuts that are produced. For example 'Elliot' and 'Kanza' each produce relatively small nuts, and 'Sumner' has a very late harvest date. Some of the nuts recommended for trial like 'Carter' and 'Excel' have good nut size, but only very limited production data exists for these new cultivars.

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Table 1. Cultivar Recommendations for Commercial Georgia Orchards

Resistance Level Recommended Recommended
for Trial
Not Recommended
for Most Situations
Excellent Elliot Gafford Gloria Grande
  Kanza (in north) Syrup Mill Curtis
    Jenkins Barton
    Carter  
    Excel  
Good Sumner McMillan Candy
Mediocre Oconee   Stuart
  Caddo   Moreland
  Kiowa   Cape Fear
  Forkert    
Poor Sioux Pawnee  
  Desirable    

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Questions to think about when deciding on a cultivar

  1. How am I doing controlling scab now?
    If you are growing 'Desirable' and feel scab control is acceptable, you may be able to focus on the high quality cultivars at the bottom of the table. On the other hand if scab is getting out of control, you will want to move up in the table and choose a cultivar with higher levels of resistance than what you are growing now.
  2. Do I have the time to adequately spray the trees I am going to plant? 
    The ability to apply sprays in a timely manner and get good coverage of the tree is the prime factor in the success of a disease control program. If you have a difficult time spraying your current acreage, but still wish to have additional trees, you may wish to plant a cultivar with a higher resistance level. This way you can focus your spray time on the high value but more susceptible cultivars. Conversely, if you have a limited amount of trees and feel you could adequately spray additional trees, you may wish to focus more on the higher value susceptible cultivars.
  3. What cultivars do I have now?
    Take stock of what you have now in your orchard and how a new cultivar might enhance your program. Do you have several older cultivars with a later harvest? Perhaps an early maturing cultivar would allow you to take advantage of the early market. Do you have a lot of acreage with an early harvest? If so, then perhaps the lateness of 'Sumner' wouldn't be an issue since you couldn't get to it early anyway. Don't forget pollination dates, information on pollen shed and pistil receptivity can be found here.
  4. How many trees am I planting?
    The amount of trees you are planting is important. If you are adding just a few trees, then some of the trial trees may provide you with valuable information on their potential in your situation. The downside to planting just a few trees of a new cultivar is that the nuts will have to be sold in a small volume and you might not be able to bring the price you would get for a standard cultivar like 'Desirable' or 'Sumner'.
  5. How will I be selling the crop?
    Many times the value of a nut will be influenced by how it is sold. If you are selling to a local accumulator in relatively small quantities, then it often pays to have a cultivar with which they are familiar. If you are selling in larger quantities, you may have increased flexibility to take advantage of a newer cultivar by marketing them separately in large lots.
  6. How much risk am I willing to take? 
    All of the trial cultivars entail a certain amount of risk. It is extremely difficult to judge a pecan cultivar as a young tree. In the past, many cultivars have looked good early on, only to fizzle out as mature trees. However, there are several promising selections available which appear to combine good quality together with higher levels of disease resistance. The amount of risk you are willing to take will also depend upon how your current cultivars are performing. It may be just as risky staying with a cultivar that isn't working in the hope that it will get better.

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