(Click on image to view large version)
Overview: Pustule has been reported in most soybean-growing areas of the world where warm weather and frequent showers prevail during the growing season. The disease may cause premature defoliation, which may decrease yield by reducing seed size and number.
Research: Commercial varities have been screened for disease resistance.
Symptoms: Early symptoms are minute, pale green spots with elevated centers on either or both leaf surfaces. Later, a small, raised, light-colored pustule forms in the center, usually in lesions on the underleaf surface. The spots vary from specks to large, irregular, mottled brown areas, which develop when lesions coalesce. Leaves become ragged when dead areas are torn away by wind. Severe disease often results in some defoliation. Leaf spots sometimes form without developing pustules.
Causal Organisms: Xanthomonas axonopodis pv. glycines. Colonies on beef infusion agar are pale yellow, become deep yellow with age and are small, circular, and smooth, with an entire margin. It is slow growing in culture.
Disease Cycle: The bacterium overseasons in seeds, in surface crop residue, and in the rhizosphere of wheat roots. Strains infect common bean and cowpea. The bacterium spreads via splashing water or windblown rain and during cultivation when foliage is wet.
Management: Use resistant cultivars. The Rxp gene, originally from cultivar CNS and in the cultivar Lee, confers resistance by requiring an increase in the number of bacterial cells necessary for disease development. The AvrRxv gene inhibits disease development by inducing a hypersensitive reaction. However, some genes required for hypersensitive elicitation by other phytopathogenic bacteria are not required in X. axonopodes pv. glycines. The Rxp locus is identified with Linkage group 20.
Researchers: At the Laboratory for Soybean Disease Research, Curt Hill is currently conducting research related to this disease.