Fungicide Resistance in Georgia Strawberry Fields

By Phil Brannen, Md Emran Ali, Jeff Cook, Sumyya Waliullah and Owen Hudson

Anthracnose fruit rot disease, caused by fungal Colletotrichum species, is one of the most significant disease problems of commercial strawberry production in the Southeast.

Dark, sunken lesions on fruit are the main disease symptoms (Figure 1). Hot, humid weather and significant rainfall make Colletotrichum-induced fruit rot a widespread problem in strawberry production.

For disease control, growers mainly rely on preventive fungicide applications from flower bud emergence to harvest. The most used single-site fungicides are quinone outside inhibitors (QoIs). The QoI active ingredients azoxystrobin (e.g., Abound) and pyraclostrobin (e.g., Pristine) are often utilized to manage anthracnose fruit rot. If appropriate resistance-management strategies are not implemented, QoIs are at increased risk of resistance development and subsequent control failure.

The QoIs have been marketed since 1996, and resistance development is expected with long-term use, but limited surveys and in vitro efficacy tests conducted in 2004 and 2008 did not confirm QoI resistance in Georgia. However, more recently, producers have complained of control failure when using QoI fungicides, and resistance has been confirmed.

RESISTANCE CONFIRMATION

In 2019, county agents submitted numerous samples to the Plant Molecular Diagnostic Lab in Tifton, GA. Md Emran Ali, the lab director, collected 108 strawberry fruits with visible rot symptoms to test for fungicide resistance. These samples were from seven different strawberry farms scattered throughout Georgia. The farms had received multiple applications of QoI fungicides during the 2019 growing season, as well as in previous seasons.

Ali identified all isolates as Colletotrichum acutatum. For further confirmation of QoI resistance, he tested all 108 isolates for the presence of the G143A mutation using the PCR-Restriction Fragment Length Polymorphism assay. His results showed the presence of the G143A mutation in all QoI-resistant C. acutatum isolates, 87 percent of isolates with moderate resistance, but none with reduced sensitivity or sensitive isolates (Table 1). These findings suggest that there is a high risk that resistance has developed in C. acutatum populations wherever QoIs have been utilized over time for control of anthracnose fruit rot in Georgia – and likely elsewhere.

GROWER RECOMMENDATIONS

For effective control of this disease, growers need to focus on using multi-site fungicides, such as Captan products, and alternation with classes other than QoIs. The Southeast Regional Strawberry Integrated Pest Management Guide for Plasticulture Production (www.smallfruits.org), edited by Rebecca Melanson of Mississippi State University, provides excellent information on fungicide selection under various conditions of resistance to anthracnose and/or botrytis fruit rots. 

Moving forward, growers should have their anthracnose populations tested for QoI resistance. Use of QoIs may be limited in future management strategies as a result of widespread resistance development. The Plant Molecular Diagnostic Laboratory, a lab service of the University of Georgia Department of Plant Pathology, is now providing fungicide resistance testing support for several plant pathogens like anthracnose of strawberry. The clinic can accept symptomatic fruit samples (generally 10 per site) to test for resistance.

In 2019, funds were provided by the Southern Region Small Fruit Consortium for resistance testing of both anthracnose and botrytis — free of charge to producers from member states until the funds ran out. Check with your local county agent on the status of resistance testing funds. If funds are not available, you are still encouraged to have both anthracnose and/or botrytis profiled for your location. The tests currently available, their pricing, a submission form and submission information are available at the Plant Molecular Diagnostic Lab web page at https://site.caes.uga.edu/alimdl/fungicide-resistance-testing/. See the form at https://site.caes.uga.edu/alimdl/files/2019/02/resistant-profile-form-003.pdf.

For more information of questions, contact Ali at emran.ali@uga.edu, 229-386-7230 or 229-386-7285.

Growers are highly encouraged to take advantage of this service. It is very important to know the resistance profile for anthracnose at your location — fungicides that should work and those that will not. If you have questions or need help, contact your local county agent for additional information. It is recommended to overnight samples to the Plant Molecular Diagnostic Lab and to communicate with the lab so it can expect the samples on the day of arrival.

Fungicide resistance can be devastating, so use these services to ensure that the fungicides you are utilizing are active. Spraying inactive fungicides is the equivalent of spraying water on your strawberry plants. If a fungicide is not active, you waste money on the fungicide, and you can lose your entire crop to disease as well — adding insult to injury.

This story was from the August edition of VSCNews Magazine. To subscribe, see http://specialtycropgrower.com/subscribe/.