Issue No. 3, Article 6/April 9, 2004
Early-Season Diseases of Alfalfa, the "Queen of Forages"
Alfalfa in Illinois in April can be looked at in at least two ways. One is the established alfalfa fields, which are growing vigorously in most areas of Illinois and are well on the way to producing the first crop for 2004. Another is the fields planted this spring. Both are susceptible to multiple diseases. With the value of alfalfa and the costs for reseeding in mind, it can pay to closely monitor and scout fields to determine whether and which diseases are causing problems so that management plans for specific diseases can be planned to maximize yield, quality, and profit for the alfalfa crop. Information and photographs for the most important alfalfa diseases in Illinois can be found at the University of Illinois Field Crop Diseases Web site (www.cropdisease.cropsci.uiuc. edu).
Early-season growth of alfalfa in a 1-year old field in north-central Illinois. (Photo by J. Morrison).
Early-Season Diseases in Established Stands
Several diseases can cause significant damage to alfalfa stands early in the season. Some of these are the same ones that cause later problems, including bacterial and Fusarium wilts, Verticillium wilt, anthracnose, crown rot, and Phytophthora rot. These diseases, with the exception of crown rot, can generally be managed with good alfalfa varieties that have high levels of disease resistance. Two other diseases can be of particular concern at this time of the year: spring black stem and Sclerotina crown and stem rot.
Spring black stem and leaf spot (caused by the fungus Phoma medicaginis) is a common disease in the cool times of the year that can reduce yield and forage quality. Dark brown to black spots develop on the leaves, which can expand and cause yellowing. Leaf drop may result, especially in the lower canopy of dense stands, which often stays moist. Dark lesions may develop on stems and and kill them. This disease is favored by wet and cool conditions and can develop quickly when conditions are favorable. The alfalfa crop should be harvested as soon as possible if this disease becomes severe in order to minimize losses of yield and quality. High levels of resistance to spring black stem are not available in alfalfa cultivars. However, good new varieties adapted to your area may suffer less damage from this disease than older varieties. Losses from foliar diseases can generally be minimized with good management practices and fertilization, especially using potassium.
Another disease that can cause considerable damage to young alfalfa stands, especially stands in the southern half of Illinois that were seeded last fall, is Sclerotinia crown and stem rot. At this time of the year, damage from this disease may be seen as dead patches of plants or plants with wilting or dying stems. On closer inspection, infected plants often are soft and rotting, covered in part with white fungal growth, and they contain small (1/8 inch to 1/4 inch), rounded black fungal structures. These plants typically have infected internal crown tissue that is brown to yellow in color. Sclerotinia crown and stem rot of alfalfa in Illinois is thought to be caused primarily by the soilborne fungus Sclerotinia trifoliorum, which is a different species than what causes white mold of soybean. This disease is managed by planting in spring, rotating away from alfalfa and clover, and using the most resistant cultivars available.
Diseases in Newly Planted Fields
Soilborne diseases that cause pre- and postemergence damping-off of alfalfa seedlings can be a major concern for successful stand establishment. Three diseases of particular concern are Pythium rot and damping-off, Phytophthora rot and damping-off, and Aphanomyces root rot. All of these diseases are favored by wet soil conditions. Pythium and Phytophthora often kill seedlings rapidly, before plants become severely discolored, whereas Aphanomyces tends to kill plants more slowly, while causing stunting and yellow-purple discoloration of cotyledons and leaves. Each of these diseases can be managed to some degree by avoiding wet fields. Treatment of alfalfa seed with the fungicides metalaxyl (Allegiance) or mefenoxam (Apron XL) provides effective protection for seed and seedlings against Pythium and Phytopthora. Aphanomyces cannot be managed with fungicides. Cultivars are not available that are resistant to Pythium. Many alfalfa cultivars have excellent resistance to Phytopthora and race 1 of Aphanomyces.
Aphanomyces root rot, caused by Aphanomyces euteiches, may be of particular concern, because we have found this pathogen to be very widespread in Illinois alfalfa fields. This disease is typically most damaging to seedlings and can dramatically thin stands and reduce vigor and yield of infected plants. Races 1 and 2 of Aphanomyces euteiches are very common in Illinois alfalfa fields. Resistant alfalfa varieties should be used to manage Aphanomyces root rot, especially where fields are prone to slow drainage and where seedling establishment problems have been noted in the past. Most certified varieties are resistant (R) or highly resistant (HR) to Aphanomyces root rot race 1. Resistance to race 1 does not protect against race 2. Thus, based on our new knowledge of race 2 in Illinois fields, resistance to races 1 and 2 is needed in many fields to protect plants against Aphanomyces root rot. Several alfalfa varieties are available that are resistant to both races of Aphanomyces.
Field of alfalfa seedlings severely thinned by soilborne diseases, primarily Aphanomyces root rot.
For diagnosis of these and other crop disease problems, send samples to the University of Illinois Plant Clinic in Urbana (217-333-0519; www.cropsci.uiuc.edu/research/clinic/clinic.html).--Dean Malvick