A Single Fungicide Application Can Control Exobasidium Leaf and Fruit Spot, an Emerging disease of Blueberries in the Southeastern U.S.

Summary

Exobasidium leaf and fruit spot of blueberry has increased considerably in prevalence and severity over the past 3 years, leading to significant yield losses and even abandonment of plantings in some cases. Recent field trials in Georgia have shown that the disease can be controlled effectively with a single late-dormant application of lime sulfur, a topical fungicide, providing blueberry producers with a simple and inexpensive means of managing the disease.

Situation

Blueberries have become Georgia's number 1 fruit crop, now exceeding $250 million in farm gate value per year. However, as acreage and production intensity have increased, a number of plant diseases threatening yield, fruit quality, and plant longevity have emerged. The most recent of these emerging diseases is Exobasidium leaf and fruit spot, caused by the newly described plant-pathogenic fungus Exobasidium maculosum. During the 2011-2013 harvest seasons, we have had frequent reports of the disease in rabbiteye and highbush blueberries throughout the Southeast. Although scattered, where Exobasidium leaf and fruit spot occurs it can cause significant losses (60-70% in some locations), primarily because affected fruit are unmarketable and it is difficult to remove all berries with this symptom from the packing line. In addition, fruit drop occurs with some varieties. Although the pathogen has been formally described by CAES mycologists this past year, its epidemiology and disease cycle are still unknown.

Response

Since 2012, field trials have been conducted at three locations in commercial rabbiteye blueberry plantings in southeastern Georgia to identify management options and shed light on the epidemiology of Exobasidium leaf and fruit spot. These trials, conducted in close collaboration with local Cooperative Extension agents, evaluated different fungicide active ingredients applied during different timing windows synchronized with the phenology of the blueberry host. Detailed leaf and fruit spot counts were collected and analyzed from each trial prior to fruit harvest.

Impact

Three main conclusions can be drawn from the trials conducted to date: 1) liquid lime sulfur, a topical fungicide, applied during the late-dormant stage in the winter (evaluated in 2013 only) yielded strong suppression of the disease with a single application; 2) other fungicides applied in-season were effective when applied in multiple sprays from bloom through early cover sprays; and 3) pyraclostrobin + boscalid, a commonly used fungicide in blueberries, was ineffective in most trials, indicating that the pathogen is insensitive to this product. The results suggest a single winter application of lime sulfur, possibly combined with spring applications of in-season fungicides targeted against other flower or leaf diseases, as a simple and inexpensive means of managing the disease. From an epidemiological perspective, the results suggest that the pathogen overwinters in the bush, likely epiphytically, and that the infections causing leaf and fruit spots occur early, possibly during bud emergence. The increased use of pyraclostrobin + boscalid in blueberries in the recent past could have selected for insensitive isolates of E. maculosum and thus may have contributed to the emergence of the disease.

State Issue

Agricultural Profitability and Sustainability

Details

• Year: 2013
• Geographic Scope: Multi-State/Regional
• County: Bacon
• Program Areas:
  • Agriculture & Natural Resources