No. 13/June 19, 1998
Foliar Diseases in Alfalfa
Alfalfa leaf diseases have been reported to be increasing, especially in the southern parts of Illinois. These diseases not only cause a decline in yield but also can affect feeding quality of the forage. The fungi that parasitize alfalfa leaves use the proteins and other nutrients for their own growth and often reduce the general feeding value of this crop.
Warm wet weather is ideal for alfalfa leaf spots, particularly common leaf spot, pepper or Lepto leaf spot, and yellow leaf blight. All of these fungal diseases thrive in a damp and warm environment, such as is found in the lower stem areas close to the plant crown. Infection by any of these fungi can cause leaf damage as well as defoliation, especially when harvests are delayed. Studies conducted here at the U of I about 12 years ago indicated that letting stands mature beyond about 1/10-bloom stage could result in significant disease levels and severe defoliation.
Management of leaf spots is somewhat restricted compared to diseases of other field crops. Because alfalfa can be fed directly to livestock, there are limitations on the availability of foliar fungicides. Common materials such as mancozeb and other products that have been shown experimentally to improve yields cannot be applied to alfalfa. Currently, only copper-based fungicides such as Kocide can be used. Applications may be made every 10 to 14 days when disease conditions warrant.
One important factor to remember in managing alfalfa leaf spots is that these fungi cause their greatest damage from the crown outward to the buds. When assessing leaf spot damage levels, it is very important to examine the entire stem area and not just the outer portions. Otherwise, you may find that the stems have been defoliated for most of their length.
Fungicides should be applied with a spreader-sticker. Because the copper compounds are protectant in nature and do not enter the leaves, thorough coverage is necessary for maximum protection. Always apply according to the manufacturer's label directions and be aware of any days-to-harvest restrictions. Some commercial varieties of alfalfa posses some level of resistance to leaf spots, and this is an excellent way of reducing damage.Other agronomic practices that maximize plant health are also recommended. Plants grown under stress always have greater levels of disease when compared to those not so stressed.
H. Walker Kirby (email@example.com), Crop Sciences Plant Pathology,(217)333-8414