Leaf Spots


Symptoms

A wide variety of different leaf spots may be present. Leaf spots can be tan, brown, or black, circular to oblong, and may be raised above the leaf surface. Smaller spots are scattered over the entire surface of the leaf; larger spots may be oblong and show concentric rings of light to dark colors. Yellow areas may surround some spots. Leaves with several or many spots may turn yellow and fall to the ground. Note that some foliar pathogens also infect stems and may produce similar spots there.

Common Leaf Spot

Common (Pseudopeziza) Leaf Spot

Cause

Common Leaf Spot, Downy Mildew, Rusts, Bacterial Leaf Spot, Yellow Leaf Blotch, Stemphylium Leaf Spot, and Summer and Spring Black Stem and Lepto Leaf Spot all cause symptoms called "leaf spot". Moist weather conditions and dense stands favor all leaf spots. Certain leaf spots are favored by cool temperatures (Lepto Leaf Spot, Spring Black Stem) while others prefer warmer temperatures (Stemphylium Leaf Spot, Summer Black Stem).

Leaf spots may occur after cutting (Lepto Leaf Spot) or after several weeks of growth depending on the leaf spot agent and the environmental conditions.

IPM Techniques

  • Examine plants in the field every one to four weeks from May 1 through September. Examine leaves of plants within a 10 foot radius. Report severity of the disease according to the following rating scale:

0 = no spots observed on leaves or stems

1 = a few lower leaves contain spots, but little or no defoliation

2 = many leaves contain spots, some premature defoliation

3 = leaves and stems peppered with spots, many leaves fallen on ground, all plants affected

  • If the field receives a rating of 2 or more, prompt cutting of the alfalfa will reduce loss in yield and quality. However, do not harvest fields that have not reached the mid-bud stage, in order to allow sufficient time for replenishment of root reserves.

References and Additional Information

  • IPM-1 Kentucky Alfalfa IPM Manual.
  • PPA-10d Kentucky Plant Disease Management Guide for Forage Legumes, P. Vincelli, Extension Plant Pathology, University of Kentucky
  • ID-104 Managing Diseases of Alfalfa, P. Vincelli, Extension Plant Pathology, and G.D. Lacefield and J.C. Henning, Agronomy, University of Kentucky
  • Compendium of Alfalfa Diseases. 2nd ed. Stuteville, D.L. & D.C. Erwin, (eds.). 1990.

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