Of the many diseases that damage alfalfa in Illinois, sclerotinia crown and stem rot can be one of the most devastating. Some years it can be severe, resulting in thinning or destruction of stands and reduced yields, and other years it is uncommon. The disease may also be overlooked because it often occurs sporadically and may kill small patches of plants. Last year (2001) the disease was uncommon in Illinois, and we received only a few reports of it causing damage. Watch for the disease in April and into mid-May.|
Sclerotinia crown and stem rot of alfalfa is most common in the southern half of Illinois, although it can occur anywhere in the state. The disease usually causes most damage in fall-seeded stands, but single or groups of plants in stands of any age can be killed. Sclerotinia in alfalfa is favored by cool, wet weather in the late fall and snow cover in the winter. The disease can easily go unnoticed if only scattered plants or small patches in fields are killed, and may be mistaken for winterkill.
Alfalfa seedlings killed by Sclerotinia crown and stem rot.
Sclerotinia crown and stem rot of alfalfa is fairly easy to recognize. If you see dead plants or wilting or dead stems in April or May, look for white moldy growth (especially in wet conditions) and sclerotia. The telltale sign of infection by Sclerotinia is sclerotia on infected tissue. Sclerotia are small, hard, black fungal structures about 1/8 inch in diameter and nearly round or elongated, up to 1/4 inch or more. If you find dead plants killed by Sclerotinia before they completely decompose, you may be able to find white moldy growth and sclerotia on the dead tissues. In other cases, the infection may have infected the crown but not killed the plants. The crown can be soft and covered partially with white moldy growth, the internal crown tissue will have a yellow-brown color, and sclerotia may be scattered over the surface. Wilting and dead stems are another indication of Sclerotinia infections. The lower half of stems are most frequently infected, and they also often contain white moldy growth and sclerotia. Some or all stems of a plant may be infected.
Split alfalfa stem and crown tissue infected and discolored 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 a different species, Sclerotinia sclerotiorum. Both species can infect alfalfa.
Sclerotinia typically infects alfalfa in the late fall. Sclerotia near the soil surface germinate in the fall to produce small mushroom-shaped structures, called apothecia. The apothecia release thousands of small spores (ascospores) that land on 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.
Alfalfa plant with wilting and dying stems caused by Sclerotinia infection.
Management of sclerotinia crown and stem rot of alfalfa is based on site selection, planting date, crop rotation, and tolerant varieties. 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. Spring planting allows the plants to develop resistance prior to the time that most infection occurs in the late fall. Alfalfa should not be rotated with red clover. Fungicides are not available for control of this disease. Alfalfa cultivars have been developed that can have increased survival and productivity under conditions of low to moderate sclerotinia disease pressure. These may be beneficial where sclerotinia crown and stem rot is a problem.
Help is requested to collect Sclerotinia samples from Illinois for a research project. We encourage anyone who sees or hears about sclerotinia crown and stem rot damage to alfalfa to collect infected plants and report the disease to Dean Malvick. Keep infected plants in a paper bag. If 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