Fusarium Dry Rot of Beans
Fact Sheet Page729.20 Date 11979
COOPERATIVE EXTENSION NEW YORK STATE CORNELL UNIVERSITY
by Arden Sherf Dept. of Plant Pathology Cornell University
Dry root rot caused by the fungus Fusarium solani f. phaseoli directly affects only the roots of the plants; however, the parts above ground are stunted and may turn yellow, wilt, and die before the plants mature. If infection is only moderate and rather general, the plants remain alive until harvest; but the whole field will have the appearance of being undernourished as from a lack of nitrogen. The plants may be affected as soon as they emerge from the ground; however, the trouble is usually more common on older plants. When these are pulled, the side roots are found to be rotted away and the tap root to have turned brick red and become hollow and dry. New side shoots may have formed on the stem above the lesion. Dry beans are affected more severely than snap beans.
The causal fungus lives in bean refuse and also in the soil for several years. It grows into the roots and up through the water ducts, thereby causing the plant to wilt. The fungus rarely fruits before the infected stems or roots have started to decompose. The spores and the mycelium are carried into the soil on tools and in bean straw manure. They may also be splashed by rain or carried by floods. The fungus affects no other crop. A Fusarium disease on peas looks very much like the one on beans, but the two are distinct insofar as the hosts are concerned.
When putting into practice root rot control measures, one must remember that the pathogen is not seedborne, but is strictly a soil organism. Because it is carried with the bean straw, this should never be fed to animals, for the manure will carry the organisms. The bean refuse should always be hauled where beans probably will not be grown for 6 or more years. It is not known how long the root rot fungus can live in the soil; but where a 6-year or longer rotation is practiced, the disease is held in check sufficiently to grow a profitable crop. Where the usual 3-year rotation is practiced, root rot increases until finally bean growing in those fields becomes impossible.
In addition to correct disposal of the bean straw and long rotations, a few general recommendations can be made. Any diseased bean refuse left on the field should be turned under deeply by fall plowing. Beans should be planted only on well-drained, well-fertilized soil that is likely to give an excellent growth of vines. Close cultivation should be avoided. If the base of the plant rots off and new side shoots form above the lesion, the plant may live if the newly formed roots are not cut off by the cultivator. The more plant food and moisture available in the soil, the faster these side roots will form, and the more chance there is for recovery and production of an acceptable, although reduced, crop. Plant breeders are working to develop a variety resistant to root rot.
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