We’ve had several reports this week of Plectosporium blight appearing in pumpkins in Ohio. Sometimes also called white speck, this is a disease of pumpkins and squash caused by the fungus Plectosporium tabacinum. The symptoms on vines, leaf petioles and veins on the back of the leaves are small, white and diamond-shaped lesions; on fruits the lesions are also small and white, generally
round. Under favorable environmental conditions (rainy, moderate temperatures) the lesions can coalesce, and the affected tissues appear white and become brittle. Plectosporium produces spores in the lesions that are dispersed long distances by the wind. The fungus survives in soil associated with plant debris for several years.
Plectosporium blight management requires an integrated approach that includes crop rotation, cultural practices and fungicide applications. Although pumpkin and squash varieties vary somewhat in susceptibility to this disease, none are resistant. In a study we conducted in 2018 at the OSU South Centers in Piketon, OH, the varieties ‘Hulk’, ‘Cronus’, ‘Warty Gnome’ and ‘Bayhorse Gold’ had less Plectosporium blight than other varieties in the trial.
Plectosporium blight management:
- Rotate out of cucurbits for 2-3 years.
- Choose a site with good air circulation to allow plants to dry quickly.
- Use drip irrigation instead of overhead irrigation.
- Scout fields, looking for diamond-shaped lesions on leaf veins, vines and petioles within the canopy; if present, apply fungicides on a 7-day schedule.
- Chlorothalanil (e.g. Bravo), the strobilurin fungicides such as Quadris, Quadris Top and Pristine, and Topsin M are the most effective fungicides against Plectosporium blight, but do not fully control the disease. It is important to get good coverage of the fungicides well into the canopy. Use high water volumes – at least 40 gal/acre. Strobilurin fungicides have translaminar activity and move through the leaves, improving coverage.
- After harvest plow down the crop residue to encourage rapid decomposition.