Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/10/1998
Publication Date: N/A
Interpretive Summary: Alfalfa, clovers, and other forage legumes grown for pasture or hay in the southeastern USA are seriously limited by a lack of persistence. These species are considered to be perennial, meaning that they should survive and produce forage or hay for several to many years after planting. However, often they disappear or become unthrifty and unproductive by only the second year. Diseases caused by soil fungi such as Macrophomina phaseolina often are considered possible causes for the lack of persistence of forage legumes in the Southeast. However, the importance of M. phaseolina and other disease-causing fungi is open to question because their ability to kill mature plants or plant tissues has never been adequately proven under controlled conditions. In this study, cultures of M. phaseolina were evaluated for disease-causing ability in inoculated stems, leaves, taproots and crowns (root-stem juncture) of alfalfa or white clover in the absence of other diseases. M. phaseolina caused progressive rotting and death of all inoculated plant tissues within 3 to 5 weeks. Initially it spread internally in water-conducting elements, and later it spread outwards to kill all adjacent plant tissues. Disease symptoms were most severe in crowns because from there the fungus often spread upwards to invade and kill individual stems or whole plants. Diseased tissues usually developed a blackened appearance. These results prove that M. phaseolina has an ability to kill all mature plant tissues of alfalfa and white clover. Therefore, this fungus should be considered a major potential cause for premature death of alfalfa and white clover in the southeastern USA.
Technical Abstract: Macrophomina phaseolina has been observed on alfalfa and white clover in North America, but its pathogenicity to mature plants of these species has not been adequately documented. Isolates from alfalfa and white clover were evaluated for pathogenicity by inoculating tissues of mature plants with infested toothpick pieces. Excised leaf tissues also were inoculated with mycelium. In stolons of white clover and stems of alfalfa, M. phaseolina caused a brown-black, basipetally progressive necrosis of vascular tissue with subsequent collapse of the surrounding pith and epidermis to produce constricted, expanding lesions. In taproots and crowns of alfalfa, M. phaseolina caused dark discoloration of vascular tissues in bands or streaks above and below inoculation points with subsequent invasion and death of cortical tissues, lateral roots, and stems. Excised leaf tissues were rapidly parasitized, but significant differences in rates of parasitism between genotypes indicated that resistance is present in both species. Two isolates from alfalfa often were more virulent on tissues of both species than two isolates from white clover. All isolates of M. phaseolina were reisolated from margins of necrosis in all types of inoculated tissues and regrown in pure culture. These results fulfill Koch's postulates for M. phaseolina as a pathogen of mature white clover and alfalfa in North America, and they demonstrate its capacity to parasitize a variety of tissues of both species in the absence of other pathogens. Results indicate that M. phaseolina should be considered a potential major cause for lack of persistence of white clover and alfalfa during summer months in the southeastern USA.