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

Plants can also exhibit foliar symptoms that are characteristic of Leptosphaerulina leaf spot. However, if you look past the leaf spots it can be noted that the leaf margins show browning from the edges more characteristic of plants suffering from water stress.

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.

Control
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.  FG

—From University of Illinois Extension Bulletin, No. 12, Article 9

Loretta Ortiz-Ribbing, Extension Specialist – Crop Systems, University of Illinois