This is the time of the year when the effects of Sclerotinia crown and stem rot are often noticed in Illinois alfalfa fields. This disease is most common in the southern half of Illinois. Sclero-tinia crown and stem rot may be noticed as death of plants in large or small patches. The disease is typically most destructive in fall-seeded stands, where large patches of the young plants can be killed. Single plants or groups of plants may be killed in established stands. |
Sclerotinia crown and stem rot is favored by cool, wet weather in the late fall and snow cover over the winter. The disease can easily go unnoticed if only scattered plants or small patches in fields are killed, and when noticed may be incorrectly called "winter kill." This disease can destroy stands of alfalfa or thin out stands to result in poor yields.
Alfalfa plants killed by Sclerotinia crown and stem rot.
Sclerotinia crown and stem rot of alfalfa in the Midwest is thought to be caused primarily by the soilborne fungus Sclerotinia trifoliorum. A very similar disease, white mold of soybean, is caused by Sclerotinia sclerotiorum. Several reports suggest that S. sclerotiorum can also infect alfalfa.
Sclerotinia infection causes soft rot of infected plant tissues. If conditions are wet, infected tissues are often mushy and covered with white, moldy growth. Young plants that are killed by this disease degrade quickly and are seen as brown, dead tissue lying on the ground. Symptoms on established plants are often noticed as wilting and death of individual stems. These plants typically have infected internal crown tissue that is a yellow-brown color, and the infection may spread to kill the crown and all stems. In all cases, the telltale sign of infection by Sclerotinia is sclerotia in or on infected tissue. Sclerotia are small black fungal structures about 1/8 inch in diameter and nearly round or may be elongated up to 1/4 inch or more. These sclerotia look similar to, but usually are smaller than, those associated with white mold of soybean.
Sclerotia on alfalfa stem.
Alfalfa crown and stem tissue discolored by Sclerotinia infection.
Alfalfa is typically infected with Sclerotinia in the late fall. Sclerotia, which survive in the soil, germinate in the fall to produce small mushroom-shaped structures called apothecia. The apothecia can release thousands of small spores (ascospores) that land on alfalfa plants and initiate infection when the weather is cool and wet. The infection may quickly kill plants or may progress slowly over the winter and into spring.
Management of Sclerotinia crown and stem rot of alfalfa is based primarily on planting date, plowing, and crop rotation. There are no fungicides available for control of this disease. Spring planting allows the plants to develop resistance prior to the time infection occurs in the late fall. Deep plowing may bury sclerotia and reduce disease incidence. If possible, new fields of alfalfa should be established where there is no history of severe Sclerotinia crown and stem rot of alfalfa or red clover. Alfalfa cultivars with resistance/tolerance to this disease have been developed that can be more productive than other cultivars under conditions of low to moderate disease pressure. The cultivars have been tested in field plots for resistance to Sclerotinia crown and stem rot in Ohio and Kentucky; however, similar tests have not been conducted under Illinois conditions.
Individuals are encouraged to report Sclerotinia crown and stem rot damage to alfalfa and to collect infected plants. The disease and collection should be reported by e-mail to firstname.lastname@example.org. Keep infected
plants in a paper bag. When you report, please include the date the disease was observed, the age of the stand, the field location (county, township, section number), and the level of damage.--Dean Malvick