When people mention the Big Three auto manufacturers in Detroit, Michigan, most people think Ford, GM and Chrysler. When someone mentions the Big Three soil borne diseases in pumpkins, you should think of Phytophthora, Fusarium and Plectosporium. All of these fungal diseases can cause plant death and significant fruit loss. The spores can live several years in the soil, requiring proper moisture and weather conditions to initiate the infection cycle on a susceptible host, such as pumpkin.
This past week at the Western Ag Research Station in South Charleston, there was a spectacular infestation of Plectosporium blight on pumpkin leaves, veins, petioles, vines and fruit handles. This disease has previously been called white speck or Microdochium blight. After weeks of periodic heavy rains splashing infected soil onto the plants, a small percent of lesions were noted on a few plants July 30. Five days later, about 90% of the plants in the trial had a full blown infestation, severely damaging foliage and killing some plants in the trial. Because this disease attacks all parts of the plant, including the fruit handle and rind, it can lead directly to marketable fruit losses. Once infected, the leaves and petioles become very brittle and can snap off easily when handled or moved.
Plectosporium signs on pumpkin leaves. Note tiny white specks / spots on early infestation.
Note spindle or diamond shaped lesions on the lower leaf surface veins and petioles. These lesions will expand and coalesce into a large bleached area as the signs progress.
This infected fruit handle has become bleached over time. Notice all plant parts can be infected (leaves, veins, petioles, vines, fruit and handle) on right hand image.
Lighter colored foliage in this field wide image is infected and will eventually turn necrotic and fall off of the plant. Based on severity of infection, the entire plant may die.
Protectant fungicides used in powdery mildew applications such as Manzate should help prevent infection but after disease is identified, other chemistries such as the strobilurons (FRAC 11) are quite effective at control. Cultural management includes rotation away from cucurbits for at least three years to reduce soil borne inoculum. Biofumigation with mustard cover crops was researched for two years (2019-20) as a potential mitigating practice but was not found effective in reducing plant and fruit losses. A video outlining the biofumigation process can be seen on the OSU IPM Video Library on YouTube (https://youtu.be/Taz-PhDphhA?si=U11CrjsdAJXR01TD).
Cover crops such as rolled or crimped winter rye left on the soil surface forming a mat between the soil and the plants/fruit should help reduce soil splash and therefore limit disease progression on plants. Winter rye should help reduce Fusarium infections by preventing soil contact but will not be very effective against Phytophthora. Most pumpkin hybrids have very little resistance to Plectosporium blight but squash hybrids tend to be more resistant to infection, so looking for resistant hybrids in seed catalogs won’t yield any results.