SCLEROTINIA STEM ROT ON POTATO

Sclerotinia stem rot of potato, also called white mold, is caused by the fungus Sclerotinia sclerotiorum. This disease is a problem in the Pacific Northwest during very moist conditions and is especially common under center pivot irrigation. Practices that promote dense vine growth and keep high relative humidity and free moisture in the crop canopy for extended periods of time favor disease development.

Sclerotinia stem rot first appears as watersoaked spots, usually at the point where stems attach to branches, or on branches or stems in contact with the soil. A white cottony growth of fungal mycelium develops on the lesions, and infected tissue becomes soft and watery. The fungus may spread rapidly to nearby stems and leaves if moisture is present for several hours. Lesions then may expand and girdle the stem, causing the foliage to wilt. During dry conditions, lesions become dry and will turn beige, tan, or bleached white in color and papery in appearance. Hard, irregularly shaped resting bodies of the fungus, called sclerotia, form in and on decaying plant tissues. Sclerotia are generally 1/4 to 1/2 inch in diameter, initially white to cream in color but become black with age. They frequently develop in hollowed-out centers of infected stems. Sclerotia eventually fall to the ground where the fungus is able to survive until the next growing season.

Sclerotia are very durable and can survive in the soil without a host plant for at least 3 years. Sclerotia require a conditioning period of cool temperatures before germination. During the growing season, sclerotia within 1 to 2 inches of the soil surface usually germinate when the canopy of the growing crop shades the ground, and when soil moisture remains high for several days. After germination, small (about 1/2 inch in height), pink to beige, flat to slightly cup-shaped, mushroomlike structures called apothecia (singular is apothecium), form. Apothecia frequently occur in wheat that is cropped after potatoes, beans, or another susceptible host. Sclerotia also may germinate directly and form mycelium when conditions are wet and dead plant material serves as an energy source.

Lesion and white mycelial fungus growth on potato stem.

Millions of spores (ascospores) form in each apothecium and are ejected into the air. Air currents carry spores up to several miles away to colonize dead or dying plant tissue. Yellow leaves and blossoms lying on the ground serve as an energy base for the fungus to colonize green plant tissue that touches the growing mycelium.

Sclerotinia sclerotiorum has a wide host range, infecting over 300 broadleaved crop and weed species. Among these are potato, bean, peppermint, tomato, carrot, radish, pea, peanut, and sunflower.

Reduce the severity of Sclerotinia stem rot by combining practices such as limiting potato vine growth through nitrogen fertilizer management, avoiding overirrigation, and foliar applications of fungicides. Initiate practices before stem rot begins developing in fields. High nitrogen fertility promotes lush, dense, crop canopies, which provide a favorable environment of high relative humidity and prolong wet periods for disease development. Restrict irrigation during rainy weather, and on cool, cloudy days, whenever possible. Avoid practices that promote long periods of leaf wetness or high relative humidity within the crop canopy.

Areas that have severe disease pressure may need protectant fungicides. Spores are usually discharged over a period of 2 to 8 weeks after row closure. Applying a single fungicide treatment at row closure may be sufficient in areas where spores are discharged over a short period. Use repeated applications, beginning at row closure, in areas where spores are discharged for extended time periods.