No. 12 Article 9/June 15, 2007

Alfalfa Crown Rot: Time to Renovate or Rotate?

Alfalfa fields in Illinois have had a rough time lately. If the droughty weather last year did not stress the crop enough, the winter ice or freezing temperatures in April certainly could have. In retrospect, it may not be surprising to find fields with rotted crowns.

Damage to the crown and roots of alfalfa plants is a chronic problem caused by a complex of soil microorganisms that include fungi, bacteria, and nematodes. Some of the fungi in this complex include those in the genus Fusarium, Phytium, Phoma, as well as Rhizoctonia solani. Development of crown rot is favored by plant stress, age of the plant, damage to the crown, or insect damage. These rot organisms interact with nonpathogenic soil microbes when plants are stressed, damaged by insects, or exposed to unfavorable environmental conditions; the end result is rot of the alfalfa crown and root. These organisms can infect and rot the crown and roots of alfalfa throughout the year.

Symptoms. Crown rot is generally described as brown, dead areas in the crown of the plant that can extend down through the cortex of the root. At later stages of the disease, the center core may become completely rotted and hollow.

Alfalfa crown rot in the crown and root cortex.

As crown rot develops, plant vigor declines, and alfalfa plants appear stunted; they begin to wilt and eventually die.

Severely rotted alfalfa crown.

What caught the attention of Tim Bewley, Crop Production Services, in an alfalfa field near Biggsville was foliar symptoms that are characteristic of Leptosphaerulina leaf spot.

However, look past the leaf spots and note that the leaf margins show browning from the edges more characteristic of plants suffering from water stress. In this case, Leptosphaerulina leaf spot may have infected already stressed plants.

Foliar symptoms on alfalfa plants with crown rot.

Scouting. To scout for crown rot and other root and crown disorders, it is best to monitor your alfalfa stands over a three- to four-year period. Make several stand counts in five locations per field. Record the numbers of wilted plants, plants with stem lesions, and plants with crown and root rots in five separate areas, each approximately 300 square feet in size. Cultural practices to help improve alfalfa vigor and reduce crown rot development include maintaining proper fertility and soil pH, avoiding harvesting or grazing too frequently at the end of the growing season, and controlling insect pests that damage crowns.

It is time to renovate or rotate to a nonlegume crop for at least three years when your alfalfa stand becomes too thin. Refer to the Illinois Agronomy Handbook, chapter 8, for recommendations on pasture management and renovation. The handbook is also available for purchase through PubsPlus.--Loretta Ortiz-Ribbing

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