Sunflower is starting to bloom and the beautiful yellow fields are a pleasure to observe. At this time of the year sudden sunflower plant wilt may be observed. The disease, sclerotinia in sunflower, is expressed in three different ways: sclerotinia wilt, middle stalk rot and head rot
(Figure 1).

Soilborne sclerotia infect sunflower roots causing Sclerotinia wilt. Middle stalk rot and head rot are caused by spores transported in the air. Sunflower planted in fields with sclerotia in the soil can have severe disease infection. Oil and protein content of the sunflower seeds can be significantly reduced if plants wilt within 6 weeks after flowering, and yield reductions can be substantial. Sclerotinia is one of the major diseases in sunflower in all major production areas of the world.

The Pathogen
The fungus has a wide host range of over 374 species including alfalfa, buckwheat, canola, dry bean, flax, peas, soybean, and other crops. The fungus will produce hard, black sclerotia which can survive in the soil for up to 5 years. The black sclerotia are able to withstand several environmental stresses and are resistant to invasion by hyperparasites. The black bodies can overwinter and are the primary inoculum in the sunflower crop. The sclerotia can either give a white mycelium (vegetative hyphae) which can infect a plant, or (carpogenically) a tiny mushroom called apothecia. The incidence of wilt is highest where the sclerotia are buried 2 inches deep close to the sunflower seed. When the sclerotia are close to the surface, or deeper than 2 inches, the incidence of wilt is reduced. The disease disseminates from plant to plant via root contact

The apothecia will produce millions of microscopic ascospores which can infect aboveground plant parts that are susceptible. The spores are carried by the wind and may survive a number of weeks on plants during dry conditions. Carpogenic germination of sclerotia requires high moisture. Moisture is very important in the development of above ground symptoms of Sclerotinia. Ascospore germination and infection require a film of water for 42 consecutive hours on the plant tissue. As we have had dry conditions the risk of ascospore production in the region is low.

Sclerotinia wilt will show up as basal stalk rot. The sunflower plant may suddenly wilt and the roots show rotting. The stem base shows a brown lesion which will become wet and soft. The base of the stem develops a canker. The lateral and tap roots of infected plants decay and the plants may lodge. The majority of plants start to show wilting symptoms around flowering of the sunflower. Possible reasons why sunflower is attacked by Sclerotinia wilt after anthesis is that the sunflower plant is more susceptible and there is more root contact between plants when the sunflower is fully developed.

Wilted plants are often found in groups within a row. Lesions in the bottom foot of the stalk will develop and the appearance of the stalk is ragged. Sclerotia, the resting bodies of the fungus are produced in the decaying stalk. During moist conditions mycelium can develop at the base of the stalk giving Sclerotinia its other name "white mold."

Stalk rot normally appears after the flowering of the sunflower, usually in the middle or upper portion of the stalk. A watersoaked spot where the leaf is connected to the stalk marks the beginning of the rot. The decaying tissue weakens the stalk and the plant will fall over. The plant parts above the stalk rot usually die. At the end of the season the area where the rot occurred has a bleached and shredded appearance and sclerotia may have been produced.

Head rot often starts as spots on the receptacle. Within the receptacle mycelium is growing causing decay of the head. The head has a ragged and bleached look. Some of the tissue is soft and seeds drop on the soil. Black hard sclerotia developed in the head are often harvested and are difficult to remove from the sunflower seeds. Presently there is a low risk of stalk rot or head rot developing as we have had dry conditions.

For further information:
Nelson, B., and A. Lamey. 2000. Sclerotinia Diseases of Sunflower. NDSU Extension Circular PP-840.

http://www.ext.nodak.edu/extpubs/plantsci/rowcrops/pp840w.htm

Hans Kandel, Regional Extension Educator
Regional Extension Center, Crookston