March 1984
Crown Rot of Alfalfa
Crown rot is a very common disease problem in Utah alfalfa fields. It consists of a slow rot in the tap root and crown area. Unlike most plant diseases, crown rot cannot be attributed to a single pathogen. Several fungi (Fusarium spp., Phoma, Pythium, Rhizoctonia) as well as bacteria (Pseudomonas, Serratia) have been implicated in the disease. Often, a complex consisting of several of the pathogens attacks the plant. The disease progresses relatively slowly and, consequently, may go unnoticed for several years, particularly in a young field. When the stand is young, there are enough plants to compensate for some losses, but eventually weeds fill in the vacant spaces resulting in weedy fields. Yield losses may not be apparent in the first five years of a stand.
SYMPTOMS
The fungal and bacterial pathogens are already present in most soils and may invade the plant at any time. Any condition that weakens or injures the plant in any way makes it more susceptible to crown rot. Foliar diseases, insects, frost damage, low soil fertility, poor drainage, and summer drought are common stresses in alfalfa fields that predispose plants to crown rot. Physical injury due to soil heaving, tractors, or grazing animals is especially harmful because it opens the crown to infection.
A dry rot begins to develop in the crown and root cortex. Brown necrotic lesions are visible in these areas. Over the course of several years, the lesions grow larger and kill crown bud meristems (where the stems arise). Plant vigor declines and stems are stunted and wilted. Eventually the whole crown will be rotted and killed. Secondary organisms often invade and contribute to the decline.
Not much work has been done on determining yield loss due to crown rot. In young stands (less than 3-4 years) yield reduction appears to be insignificant; plants adjacent to a space left vacant by the death of a plant compensate for the loss by producing more stems. Several years later, however, the field may be thinned out considerably, and surrounding plants cannot compensate for the losses. Yields in these fields may be significantly less than in a healthy field and will be a lot more weedy.
CONTROL
There is no specific control for crown rot; however, the following cultural practices will probably reduce disease incidence.
- Select well-adapted, hardy varieties resistant to the common diseases in your area. Consult your county extension agent for this information. There are no varieties resistant to crown rot.
- Control foliar diseases and insect problems. They weaken the plant, and insects can leave wounds open to other pathogens.
- Fertilize and irrigate properly to produce a vigorous field.
- Avoid planting in heavy or poorly drained soil; avoid low areas where spring runoff and irrigation collect.
- Follow a suitable cutting schedule. Leave 3 - 4 inches of growth in the fall to overwinter.
- A certain amount of damage is unavoidable when mowing or grazing animals, so some disease will probably always be present. Avoid cutting hay when soils are wet. Avoid heavy grazing.
- Avoid vigorous cultivation for weeds in the spring which damages the crowns. Use herbicides for weed control.
- Do not replant alfalfa in the same field for two to three years. Crop rotation with small grains or corn will reduce crown rot and other alfalfa diseases.
Sherman V. Thomson/Extension Plant Pathologist
Scott C. Ockey/Plant Disease Diagnostician
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