Late Season Alfalfa Diseases

In mid- to late-summer there are a host of leaf diseases that can attack alfalfa and reduce yield. They include but are not limited to:

• Common Leaf Spot
• Summer Black Stem and Leaf Spot
• Leptosphaerulina (Lepto) Leaf Spot

Symptoms of common leaf spot, caused by Pseudopeziza medicaginis,include small, circular brown to black spots on the leaflets of lower leaves before moving upward. Mature lesions usually remain isolated from each other and may have a size of 1/8 inch (Figure 1). A raised, light brown fungal fruiting body (psuedoapotheicum) within the lesion may appear on the upper leaf surface. The infected leaves turn yellow and drop from the plant as the disease progresses. Summer growing conditions that are dry and warm reduce disease development. Cool, moist growing conditions during the fall favor disease development. Management includes the planting of moderately resistant alfalfa varieties, harvesting prior to leaf drop to help maintain quality, and timely application of labeled foliar fungicices.^1,2

Summer black stem and leaf spot is caused by Cercospora medicaginis. If not diagnosed when leaf spots appear, premature leaf defoliation which starts with the lower leaves and moves upward may be the first sign of infection. The circular leaf spots are initially brown before turning ash-gray and are surrounded by a yellow margin. After the leaf spots, the long, reddish brown to chocolate brown stem lesions develop. Severe stem infection can cause defoliation and plant death.^3,4 The diseases reduce yield and quality and are generally more of a concern with the second and third alfalfa cuttings. High humidity, warm temperatures, and a dense plant canopy favor the disease. Avoid planting particularly susceptible alfalfa varieties and as with all leaf diseases, harvest early before symptoms are severe.

Leptosphaerulina (Lepto) leaf spot is caused by Leptosphaerulina briosiana. Symptoms often start as small, black spots (“pepper spots”) on new leaves and eventually enlarge to form an “eyespot” which is oval and brown with dark margins and yellow halos. Lesions form parallel to veins, come together, and cause leaves to drop and possible stem stunting. Young leaves are prone to infection; however, the disease can develop on older leaves and petioles. It is favored by cool, wet weather throughout the growing season in northern climates. It may develop in midwinter in southern states. Management includes varietal tolerance, early harvest, and labeled foliar fungicides.⁵

Channel Agronomist
Mike Binsfeld

Sources

¹Telenko, D. and Johnson, K. 2019. Foliar diseases of alfalfa. Purdue University. https://extension.entm.purdue.edu/newsletters/pestandcrop/article/foliar-diseases-of-alfalfa/

²Common leaf spot of alfalfa. Crop Protection Network. https://cropprotectionnetwork.org/encyclopedia/common-leaf-spot-of-alfalfa#:~:text=Common%20leaf%20spot%20is%20caused,foliar%20lesions%20that%20rarely%20coalesce.

³Wegulo, S.N. 2011. (Watkins, J.E. and Gray, F.A.) Spring and summer black stem and leaf spot diseases of alfalfa. NebGuide. G2079. https://extensionpubs.unl.edu/publication/g2079/html/view

⁴Summer black stem and leaf spot of alfalfa. Crop Protection Network. https://cropprotectionnetwork.org/encyclopedia/summer-black-stem-and-leaf-spot-of-alfalfa

⁵Leptosphaerulina leaf spot of alfalfa. Crop Protection Network. https://cropprotectionnetwork.org/encyclopedia/leptosphaerulina-leaf-spot-of-alfalfa

Web sources verified 5/5/24. 1110_430801