University of Illinois
No. 12/June 13, 1997
Leaf Diseases of Alfalfa
Alfalfa is commonly attacked by a number of leaf and stem diseases that cause a loss of vigor and reduce both the hay quality and yield. Diseases are the worst when the spring season is cold, wet, and late and when there are frequent showers and heavy dews. Seedling stands often become heavily infected, especially under a thick nurse crop (such as oats). A high stubble and weeds' matting around the alfalfa plants also contribute to disease severity.
- Common, or Pseudopeziza, leaf spot. Caused by the fungus Pseudopeziza medicaginis, this leaf spot occurs wherever alfalfa is grown. The disease appears to be most serious on soils that are acidic or low in fertility. Plants may be severely weakened, lack vigor, and become stunted the first year; but little permanent damage occurs. Common leaf spot starts on the lowermost foliage and progresses up the plant. Later cuttings are usually the one attacked most severely.
Small, circular, dark brown to black spots, about 1/16 inch in diameter, develop first on the lower and inner leaves. In the thickened center of fully developed spots, a tiny raised light brown disk-shaped fungus fruiting body forms on the upper leaf surface. This fruiting body is easily visible with a hand lens or reading glass. Large numbers of microscopic spores are shot into the air during cool to warm wet weather and are carried by wind currents and rainsplash to other plants. Thus, the disease may spread quickly throughout a field. When the spots are numerous, the leaflets soon turn yellow and fall off. Premature defoliation reduces the quality and quantity of forage. The causal fungus overwinters in fallen, undecayed leaves and leaf fragments on the soil surface. It is not known to be seedborne. As indicated, disease attacks are most damaging on susceptible varieties during prolonged periods of cool, wet weather in spring, early summer, and autumn. The incidence of disease decreases during dry, warm summers.
- Spring black stem and leaf spot. All above-ground parts of the plant are attacked by the fungus Phoma medicaginis var. Medicaginis. Numerous spots develop on the lower leaves, petioles, and stems in early spring. These spots are small, dark brown to black, and irregular. Young shoots are often girdled and killed. Leaf lesions may enlarge and merge, killing large areas of the leaflets. The leaves turn yellow and often wither before dropping off. Stem and petiole lesions may enlarge, girdle, and blacken large areas near the base of the plant. The fungus may extend into the crown and upper root, causing a crown and root rot. Affected stems are brittle and easily broken. When the disease is severe, entire stems are blackened and killed.
The spring black stem and leaf spot fungus mostly overwinters in old stems and fallen leaves where minute, brown to black, pimplelike fruiting structures (pycnidia) are produced. Large numbers of microscopic spores formed within the pycnidia, ooze out in cool, wet weather and are spread primarily by splashing water, but also by wind and insects. In cool, humid areas, seed pods may become infected and the fungus becomes seedborne as mycelium in the seed coat. Infection of new alfalfa shoots occurs as the shoots grow through the residue or stubble of a previous alfalfa crop. The first harvest is usually damaged the most.
- Grow well-adapted, high-yielding alfalfa varieties. Varieties differ in their resistance to common leaf spot, lepto leaf spot, spring black stem, antracnose, downy mildew, other diseases; and in winter hardiness.
- Plant in warm, well-drained soil in a well-prepared seedbed that is only slightly acidic to neutral (pH of 6.5 to 7.0).
- Practice balanced soil fertility. Maintain adequate amounts of phosphate and potash in the soil, based on soil tests.
- Cut heavily infected stands in the mid to late bud stage before bloom appears for high yields, minimal leaf loss, and high quality. Cutting before leaf drop maintains the quality of the hay and removes the infected leaves that are the source of infection for later growth. Thus, later cuttings have a greater chance to remain healthy. Cut early to avoid rank foliar growth that favors fungus growth (disease buildup).
- Cut only when the foliage is dry. This avoids spreading fungi and bacteria that cause leaf and stem diseases, wilts, and crown and root rots.
- Cut short, leaving a stubble of 1-1/2 to 2 inches. This removes sources of infection for the recovering shoot growth.
- Control weeds. Prevent thick growth of weeds that mat around the alfalfa plants.
- Control insects. Follow the suggestions given by University of Illinois Extension entomologists.
- Where feasible, rotate alfalfa at least 2 years with corn, soybeans, small grains, sorghum, or forage grasses that are free from volunteer forage legumes.
- Applications of a copper-based fungicide are also helpful. Products such as Kocide should be applied as soon as diseases appear on the lower leaves. Multiple applications may be made if disease pressures are heavy. The label states that applications should not be made closer than 10 to 14 days before harvest.
H. Walker Kirby, Department of Crop Sciences, (217)333-8414