Diseases of alfalfa (
Phymatotrichum root rot (Cotton or Texas root rot)
Phymatotrichum root rot is also known as Cotton root rot or Texas root rot. It is caused by the soil borne fungus Phymatotrichum omnivorum (Phymatotrichopsis omnivora). Disease occurs in alfalfa at low to moderate elevations throughout Arizona, but does not occur in the high elevation production areas in Apache, Coconino and Yavapai counties. Alfalfa planted in the fall may become infected and die during the first summer. Plants often wilt and die quickly, but it is not unusual to have some plants that decline slowly. Kill patterns vary from small isolated areas to areas that become large circles. The first symptoms of infection are lesions on tap roots. Later the entire root is rotted. The characteristic strands of the fungus usually can be observed on infected roots viewed under10-20X magnification (photo 2); the cruciform hyhae in strands is diagnostic and is observed by looking at strands under a compound microscope at 100X or higher (photo 3).
There is no control for Phymatotrichum root rot and there is no tolerance in any alfalfa cultivars. Besides loss of alfalfa yield, the intrusion of weeds in disease areas may be problematic for hay quality (photo 1). These areas may have to be treated with site application of herbicide to prevent weed contamination.
The fungus also attacks cotton, grapes,
and stone fruit and many ornamental plants. Vegetable crops or grains are good rotations to
use in infested fields. For more information concerning Phymatotrichum
root rot on other hosts see Cotton
Root Rot bulletin AZ1150.
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August 28 , 2008