Plectosporium Becoming Common in Pennsylvania

Plectosporium blight was first reported in Tennessee in 1998. Since then, the disease has been reported in New England states and Southern states. Sometimes called white speck, the fungal pathogen was previously known as Microdochium blight.

Plectosporium is not hard to diagnose once you are familiar with the symptoms. Characteristic symptoms include diamond-shaped lesions on the vines and handles of pumpkins. Small, white specks on the developing fruit are often present and small white to light tan spots on the leaves. As the disease progresses, the diamond-shaped lesions grow together and can create a bleached appearance in the field, and the stems and handles can become brittle. In severe infestations, large portions of the field will wilt and desiccate, exposing the fruit to sunlight. Though the fruit blemishes are only on the surface, the bad handles and fruit "pimples" often cause the fruit to be unmarketable and susceptible to other rots.

Host plants include pumpkin, squash, especially zucchini (see Image 1), and gourds. Fungal spores can be spread by wind or rain-splash. In general, plectosporium requires moist or wet conditions to infect plants. It is often found later in the growing season when dewy nights and moderate temperatures allow for the disease to develop. Once observed in a field, the grower is more likely to have a plectosporium infection the following year unless the weather is not conducive for the disease to advance. The disease can overwinter in the soil on crop debris.

Growers who experience plectosporium blight should rotate away from the cucurbit family in that field for three years. Scouting is crucial for detecting plectosporium in its early stages. Chlorothalonil can be used as a protectant fungicide, and more targeted fungicides can be used if the disease continues to develop and rain is in the forecast. Group 11 fungicides such as Quadris Top (3+11), Cabrio (11), and Flint Extra (11) are labeled for plectosporium management and can be tank-mixed with chlorothalonil. Other control options that can be rotated with Group 11 include Inspire Super (3+9) and Aprovia Top (3+7).

Refer to the Pumpkin chapter of the  Mid-Atlantic Commercial Vegetable Production Guide for more recommendations. Always follow the pesticide label directions for the product you have on hand.