Wilt in Alfalfa - May 10, 2012, UNL CropWatch
Figure 1. Fusarium-infested alfalfa, evident by yellow-green color of plants. (Photos by Michael Rethwisch)
May 10, 2012
Figure 2. Interveinal chlorosis noted with early onset of alfalfa Fusarium wilt.
Figure 3. Alfalfa root infested with Fusarium. Note diagnostic reddish brown streak in the the center of the root. (Photo by Stephen Wegulo)
Symptoms of wilt diseases of alfalfa have been observed in several fields in southeast Nebraska (Figure 1). The symptoms include wilting shoots, bleaching of leaves (Figure 2) and stems, and a reddish-brown discoloration of the center of the taproot (Figure 3). Several pathogens including the fungi Fusarium and Verticillium and the bacterium Clavibacter can cause wilt in alfalfa. Fusarium and Verticillium cause leaf bleaching; bacterial wilt is characterized by yellow-green foliage. Fusarium was isolated from samples submitted to the UNL Plant and Pest Diagnostic Clinic.
Symptoms became evident several weeks ago during the unusually warm, dry conditions that caused plants to undergo moisture stress. Growers noted yellowing of established alfalfa in low field areas and thought nighttime frost/freezing temperatures might have caused the injury.
Closer examination indicated the symptoms were not consistent with frost injury. Damaged plants were shorter than surrounding healthy green alfalfa, plant stand was sometimes reduced, and upper leaflets often had distinct white interveinal areas prior to leaflets becoming yellow (Figure 2).
These plants were near waterways in upper field areas as well as in lower elevation areas, and were consistent with high moisture areas conducive to fungal root disease development. Wilting plants are often the first symptom of these diseases, although interveinal white areas and yellowing plants preceded wilt in 2012. Other symptoms include bleaching of the leaf and stem, and a reddish tint to the leaves, especially with Verticillium wilt. In infected alfalfa, stress helps to express the symptoms. Roots have a dark reddish-brown streak in the stele (center of root) when infected with Fusarium wilt (Figure 3).
Alfalfa wilt caused by Fusarium or Verticillium is often chronic, causing plants and stands to decline over time (Figure 1).
Disease Resistance and Management
Differences in varietal resistance range widely in alfalfa. Several varieties provide only 31-50% resistance, and highly resistant (HR) varieties can range from 51% to 100%. Several Fusarium species can affect seedling alfalfa as well.
Fusarium and Verticillium wilt of alfalfa can be managed by planting resistant cultivars, crop rotation, weed control, pathogen-free seed, and harvesting younger stands before older, diseased stands.
Extension Educator, Butler County
Extension Plant Pathologist