Guido Schnabel, Professor and Extension Specialist, Clemson University
In plasticulture strawberry production, plant disease management is critical to ensure yield and fruit quality. Typically, we worry about gray mold, anthracnose, root rot, crown rot, and more recently Neopestalotiopsis disease. When, early in the 2022-2023 season, growers at multiple farms in South Carolina reported major plant decline, Andy Rollins (the local Clemson Cooperative Extension County Agent) and I were worried about yet another outbreak of a well-known disease. It turned out that the farmers had received the affected plants from a single nursery source. They were also from the same shipment and, while several cultivars were shipped, only one (Ruby June) was severely affected. Did something go wrong at the nursery level or during transport, was Ruby June especially susceptible to disease, or were we dealing yet again with an emerging disease that we would need to worry about regularly? Those were all open questions, and some are still unanswered.
The plants went down gradually, and multiple symptoms were observed. They included failure to take off after planting, a slow start leading to small and stunted plants, and plants that made it to decent size but lost a lot of older leaves. During fall and early spring, multiple samples of declining plants were sent to various plant problem clinics, and results came back with a plethora of suggested problems. These included the aggressive strain of Neopestalotiopsis, the “non-aggressive” strain of Neopestalotiopsis, Phytophthora root and crown rot, Fusarium, and gray mold, and sometimes no pathogen was detected. Gnomonia leaf blotch (GLB) was also mentioned but disregarded as it has historically been a minor disease.
After walking the fields, we noticed that GLB was widespread and responsible for major leaf decline, contributing to at least some of the plant stunting. But Phytophthora crown rot was also detected frequently by A. Rollins when using the Agdia Immunostrip test for Phytophthora. Ultimately, I think, these two diseases were mostly to blame for the disaster. The plants did not look right at the nursery level. Pictures revealed weak plug plants and leaf decline in the trays. The lesson learned here is to be picky and reject plants that do not meet the highest standards. Perhaps even drive up to the nursery and check on the plants before shipment. Clean planting stock is the best way to hit the ground running for the upcoming season.
Figure 1. 2022-2023 disease outbreak in South Carolina plasticulture fields. Some decline was attributed to Phytophthora crown rot (left) but Gnomonia leaf blotch (middle) commonly took out mature leaflets and contributed to weak plants. The disease is now featured in MyIPM with images (right) and updated management information.
Symptoms of GLB include brownish to purplish lesions that begin small but expand to large areas. Lesions can have various shades, and V-shaped blotches often occur on the end of a leaflet (Fig. 1 middle). As the disease progresses, the outer trifoliate leaves of affected strawberry plants often die, which is what we observed on the plants in the 2022-2023 outbreak. Frequently, the lesions have small, raised, black bumps (pycnidia) visible to the naked eye. (If you are like me, you will need your reading glasses to see them.) These release spores that can be spread by wind and water to other plant organs such as flowers and fruit.
My lab collected a bunch of symptomatic leaflets and recovered the pathogen. After analyzing isolates from South Carolina and North Carolina farms, we determined the causal agent was, without exception, Gnomoniopsis fructicola. This was the first report of this pathogen in our neck of the woods. One other GLB outbreak in the southeastern US was reported by Dr. Frank Louws (NCSU) about 20 years ago and attributed to G. comari. To investigate cultivar differences, we obtained Ruby June, Albion, and Camarosa plants (thank you, Barclay Poling!!!) and conducted inoculation trials in the greenhouse. We were unable to establish any plant organ sensitivity difference between cultivars. In other words, based on our results, Ruby June was/is not more or less susceptible to GLB than the other cultivars we tested. My lab teamed up, pushed ongoing research aside and determined in lab studies that FRAC 3 (DMIs, e.g., difenoconazole), 11 (QoIs, e.g., pyraclostrobin), and 12 (phenylpyrroles, e.g., fludioxonil) products are most effective against this pathogen. FRAC 7 (SDHIs, e.g., boscalid) and 17 (SBI class III, e.g., fenhexamid) products are ineffective, and FRAC 1 (MBCs, e.g., thiophanate methyl) and 2 (dicarboxamides, e.g., iprodione) products only have suppressive action. This information should help limit future outbreaks. Please consult the spray guide at smallfruits.org or the MyIPM app for more information on fungicides and their FRAC codes.
While Phytophthora crown and root rot is rather common these days and often can be halted with mefenoxam and oxathioapiprolin injections, and/or foliar phosphonate applications, the GLB outbreak was rather odd. Discussions with nursery representatives also led to no clues as to what may have happened. The disease has not recurred since, at least not in any significant shape or form, and it may not come up anytime soon either. But we learned from this outbreak and hopefully will be better prepared with the data we collected.