Weather ripe for disease development on pumpkins.

By Andy Wyenandt and Dr. Mac Riedel, Dept. of Plant Pathology, Ohio State University

With the wettest August weather in Ohio history and wet weather to start September pumpkin growers need to be well aware of potential disease problems. This summer pumpkin growers have been faced with more disease pressure than in the past because of all the heavy, frequent rain. Growers in Ohio needed to start their fungicide maintenance programs a few weeks early this year because of Anthracnose which began to show up on foliage in early July. Left untreated, the fungus can cause damage to fruit. Anthracnose produces distinct symptoms and growers should look for orangish/pink spore masses which develop on the veins on the underside of leaves. Similarly, small concentric lesions can develop on fruit if spores are splashed around during rainfall. Another problem which has shown up in the past month on pumpkin foliage is Downy Mildew. Symptoms of Downy Mildew are very similar to Powdery Mildew. Downy mildew will cause the upper sides of leaves to yellow and brown out and produce grayish/white fungal masses only on the underside of leaves. Powdery Mildew will produce white fungal masses on the upper and lower leaf surface and the stem ruining quality if left uncontrolled. Also if downy or powdery mildew cause high defoliation the fruit exposed to direct sunlight on hot summer days may begin to show symptoms of sunscald. Powdery Mildew will be more prevalent during drying weather; Downy Mildew will begin to show up more often during cool, wet weather. Microdochium blight, or ‘white speck’, seems to become more and more prevalent each year in Ohio pumpkin fields. The fungus will produce small white lesions which can develop very quickly on all parts of pumpkin foliage and then spread to the topside of fruit causing aesthetic damage to the fruit and stem. With all this extremely wet weather over the Labor Day weekend Phytophthora blight can become a serious problem if fields remain under water. Phytophthora blight can become serious problems in low lying areas and can cause green vines to look wilted while nearby vines look healthy. Infected fruit will begin to develop white cottony growth and eventually rot off the vine. Fusarium fruit rot is one disease that growers may not realize is present until they begin to harvest. The fungus will invade the belly side of fruit that is in direct contact with the soil. Symptoms include small to large red/purple expanding circular lesion that often has white, cottony centers. Another disease common in pumpkin fields this year has been Bacterial Spot. The bacterium causes small, circular lesions that can be confused with insect feeding damage, although the pathogen appears to be contained on the surface of the fruit it can penetrate the seed cavity and cause the fruit to rot prematurely. Another bacterial disease which will cause wilting and death of vines is Bacterial Wilt. Petioles of infected plants will begin to appear stand upright and the leaf will begin to curl down with the area between major leaf veins turning yellow then brown. A good insecticide program for the control of striped cucumber beetle when the first populations appear is necessary keep bacterial wilt infection to a minimum. The striped cucumber beetle is often confused with the corn rootworm. A very simple way to tell them apart is that the striped cucumber beetle has a black belly and the corn rootworm has a yellow belly and does not transmit the bacterium. Symptoms of virus infection are beginning to show up on infected plants and fruit. Infected plants can be stunted, often have irregular leaf shapes, and reduced fruit set if vines are infected early in the growing season. The most common virus in Ohio, Watermelon Mosaic Virus, is transmitted by aphids. Virus infection can cause fruit to become bumpy, show ring spots and cause uneven (mosaic) or premature ripening. Unfortunately, it is difficult to control virus infection because it is unfeasible to spray for aphid control. Growers may avoid virus problems by planting their pumpkins earlier in the growing season. For more information on the control of these diseases and insects growers should consult their Ohio Vegetable Production Guides or contact their local extension agent or crop advisors. Growers can also contact Dr. Mac Riedel @ riedel.1@osu.edu or Andy Wyenandt, @ wyenandt.1@osu.edu or by phone at The Ohio State University, Dept. of Plant Pathology at (614)292-9355.