Friday, February 3, 2012

Sample of the Week: Sclerotinia Stem Rot of Snapdragon

Drooping snapdragons in flower bed. Jan 23, 2012.
Sclerotinia stem rot (a.k.a. Sclerotinia blight) is a disease we seldom see in ornamentals in North Carolina. The case pictured above is only the second time I've seen it from a landscape ornamental in the last three years. The cause is a cool-weather-loving fungus called Sclerotinia sclerotiorum. Although it can infect a wide range of host plants, it's best known for the damage it does on many vegetable crops. In our state the primary victim is collards, on which the disease is known as "head rot". Cabbage, lettuce, and strawberry are affected occasionally. It would be very unusual to see this disease after mid-May in North Carolina. Note that the related fungi Sclerotinia minor (the cause of Sclerotinia blight on peanuts) and Sclerotinia homeocarpa (the cause of dollar spot on turf) are comfortable with somewhat warmer temperatures.

Stem lesions and visible white mold, evidence
of the causal fungus, Sclerotinia sclerotiorum.
Diseases caused by Sclerotinia sclerotiorum are sometimes called "white mold" because of the fluffy white mycelium of the fungus that grows on affected plant parts under humid conditions.The fungus also wads itself into small, dark, hard lumps called sclerotia, which allow it to survive from season to season in the soil. It would be unwise to plant snapdragon next fall in the pictured flower bed, even if an effort is made to clean up and remove the diseased plant material. You'd also want to avoid planting flowering kale - a collard relative. There is no effective chemical control for this disease in the landscape.

Because the fungus produces airborne spores, it can sometimes even blow into greenhouses and cause stem rots on ornamentals and "collar rot" on tobacco. This is one reason it's so important to avoid leaving piles of dead plant material in and around greenhouses. Of course diseased plant material should never be composted, in this case because of the durability of the sclerotia.

Nota bene! Ojo! Vorsicht! This disease is completely different from Southern blight (Southern stem blight) caused by the unrelated fungus Sclerotiuim rolfsii. The scientific names of the fungi are confusingly similar, both cause stem rots, and both produce visible white mycelium on the plant, but there are key differences. Southern blight occurs from late spring through the heat of summer, not in the winter and early spring. Also, the mature sclerotia of Sclerotium rolfsii are tiny, round, and tan-colored (rather like radish seeds), whereas those of Sclerotinia are somewhat larger, irregular in shape, and black. Both kinds are white on the inside when cut open, and both take longer to form than the mycelium itself.

Above: Sclerotia of Sclerotinia sclerotiorum
(Photo: PP Dept. Slide Collection)
Above: Sclerotia of Sclerotium rolfsii