Anthracnose fruit rot disease, caused by fungal Colletotrichum species, is one of the most significant disease problems of commercial strawberry production in Georgia and the Southeast as a whole. Dark, sunken lesions on fruit are the main disease symptoms. 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 crop protection applications from flower bud emergence to harvest. The most commonly used single-site crop protections are quinone outside inhibitors (QoIs); the QoI active ingredients (a.i.) 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 crop protection, and resistance has been confirmed. In 2019, county agents submitted numerous samples to the Plant Molecular Diagnostic Lab (PMDL) in Tifton, GA. Dr. Md Emran Ali, the lab director, collected 108 strawberry fruits with visible rot symptoms to test for crop protection resistance; these samples were from seven different strawberry farms scattered throughout Georgia. These farms had received multiple applications of QoI crop protection during the 2019 growing season, as well as in previous seasons. Dr. Ali identified all isolates as Colletotrichum acutatum. For further confirmation of QoI resistance, he tested all 108 isolates for the presence of the G143A substitution using the PCR-Restriction Fragment Length Polymorphism (PCR-RFLP) assay. His results showed the presence of the G143A mutation in all QoI-resistant C. acutatum isolates, 87% 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.
For effective control of this disease, growers need to focus on using multi-site crop protection, 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 Dr. Rebecca Melanson (Mississippi State University), provides excellent information relative crop protection 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, and 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 crop protection 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.
Source: University of Georgia
