In early May, the reports on alfalfa progress were optimistic, but development slowed because of the cool temperatures as the month progressed. During the past week, the Plant Disease Clinic has received several samples, and we have had other reports of struggling alfalfa stands. Samples have come from across central Iowa and range from stands seeded last year to those in their third year of production. Some show similar symptoms, but it is likely there are different factors involved.
In general, we think the cool weather has greatly slowed the growth of the plants and brought out symptoms of some other underlying problems. Whether scattered plants or patches of plants, they all have stunted shoots with very short internodes near the top. Leaves on these shoots have withered and sometimes whole shoots have died back or are wilted and drooping. In several samples healthy new shoots were appearing at the base of the dead or dying shoots.
Leaf disease symptoms are severe on many of these plants, primarily spring black stem and leaf spot (Phoma medicaginis), but also Leptosphaerulina leaf spot (Leptosphaerulina trifoliorum). See the May 13, 2002, Integrated Crop Management newsletter  for descriptions of these two diseases. Lesions of spring black stem can girdle the shoots and cause them to die back. In some of the samples, this disease seems to be the main culprit. In other samples, the pattern in the field suggests other causes. A few plants are showing deformed growth that seems virus-like in appearance. Crown rot does not seem to be a factor, but some plants are showing discoloration of the taproot, which could mean a wilt disease. There are three wilt diseases: bacterial wilt, Fusarium wilt, and Verticillium wilt. We are attempting to isolate the wilt pathogens from these plants.
|Severe discoloration of alfalfa taproot.|
|Alfalfa plant with stunted, withered shoots.|
|Spring black stem lesion on alfalfa petiole.|
Factors other than diseases may also be involved. In one of the fields, pea aphids were found in high numbers and they are capable of causing the stunting and wilting that was observed. Although the other fields showed similar symptoms, aphids were not evident. Late spring frost can cause leaf injury and irregular growth, but plants usually grow out of the condition.
The healthy new shoots suggest that many of these plants may recover, but it is impossible to say at this point how well they will recover. Regardless of the cause of the slowed growth or degree of plant disease, consider that these alfalfa plants are recovering under stress. For these plants to regain physiological 'vigor' it is important to minimize further stresses during the remainder of the growing season. Maintain a good fertility program. Carefully monitor and manage insect populations, particularly of potato leafhoppers. And, if possible, allow one of the summer regrowth cycles to reach early- to mid-bloom stage.
This article originally appeared on page 89 of the IC-488(11) -- June 3, 2002 issue.