Specialty Bean Tissues No Treat for Nematodes

Jack beans, sun hemp and coffee senna aren't your typical, garden-variety beans. But these little-known legumes may soon find favor among southern farmers and gardeners. Greenhouse studies by Agricultural Research Service (ARS) and University of Georgia scientists in Griffin, Ga., show that mixing dried bean plant material into the soil reduces root-knot nematode numbers there. In the South, this roundworm species attacks peanuts, soybeans, corn, cotton, tobacco and other crops, causing yield losses and control costs of $53 million annually.

Farmers strike back with chemicals, crop rotation and resistant cultivars. But new weapons are always needed. Discing bean material from a cover crop of the legume prior to planting a high-value crop like cotton may offer farmers greater flexibility in how they control the pest, notes ARS agronomist Brad Morris.

In tests, mixing dried bean material into potting soils resulted in a 67- to 89-percent reduction in the number of nematode galls on the roots of test tomato plants versus control plants. Scientists attribute this nematicidal activity to natural substances produced in the legumes’ leaves, stems and seeds.

Of 18 legume species tested, Jack bean (Canavalia ensiformis) earned the highest marks, according to Morris, with ARS’ Plant Genetic Resources Conservation Unit in Griffin. There, he regenerates and distributes seed from a "special-purposes" legume collection. It includes 64 genera, or genetic groups, representing hundreds of semitropical legume species collected from around the world. Most have multiple uses, from controlling weeds and erosion to providing drug companies with pharmaceutical compounds.

Jack bean, a shrublike plant adapted to southern climates, flowers in July through August. Though normally grown as a cover crop, the bean plant's seeds can be eaten if properly boiled. In addition to concanvalin-A, a lectin protein with many biochemical properties, Jack bean also is a source of the enzyme urease. Pharmaceutical companies extract both concanvalin-A and urease from the plant for use in diagnostic tests.

ARS is the U.S. Department of Agriculture's chief scientific research agency.