PATHOGENICITY OF MACROPHOMINA-PHASEOLINA TO MATURE PLANT-TISSUES OF ALFALFA AND WHITE CLOVER

Macrophomina phaseolina has been observed on alfalfa and white clover in North America, but its pathogenicity to mature plants of these spec ies has not been adequately documented and Koch's postulates have not been fulfilled. Isolates of M. phaseolina from alfalfa and white clove r were evaluated for pathogenicity by inoculating tissues of mature pl ants with infested toothpick pieces. Excised leaf tissues also were in oculated with mycelium. in stolons of white clover and stems of alfalf a, M. phaseolina caused a brown-black, basipetally progressive necrosi s of vascular tissue with subsequent collapse of the surrounding pith and epidermis to produce radially constricted, expanding lesions. In t aproots and crowns of alfalfa, M. phaseolina caused dark discoloration of vascular tissues in bands or streaks above and below inoculation p oints with subsequent invasion and death of cortical tissues, lateral roots, and stems. Sclerotia formed in all tissues of both species. Exc ised leaf tissues were rapidly parasitized, but significant difference s in rates of parasitism between genotypes suggested that differences in host resistance to M. phaseolina may be present in both species. Py cnidia formed on leaves of bean, lima bean, and cotton. All isolates o f M. phaseolina were reisolated from margins of necrosis in all types of inoculated tissues and regrown in pure culture. These results fulfi ll Koch's postulates for M. phaseolina as a pathogen of mature white c lover and alfalfa in North America, and they demonstrate its capacity to parasitize a variety of tissues of both species in the absence of o ther pathogens. Results indicate that M. phaseolina should be consider ed a potential cause for lack of persistence of white clover and alfal fa during summer months in the southeastern United States.