Production of Macrophomina phaseolina Conidia by Multiple Soybean Isolates in Culture

Macrophomina phaseolina (Tassi) Goid., a pycnidia-producing fungus, is one of the most important soilborne pathogens, infecting over 500 plant species in more than 100 plant families around the world (1,18). It causes a major disease of soybean (Glycine max (L.) Merr.) called charcoal rot (18). Symptoms on soybean include blight of emerging seedlings, wilting, early maturation, and incomplete pod filling (18). The disease is most pronounced when plants are stressed by adverse environmental factors such as drought and heat stress during the reproductive growth stages. Charcoal rot of soybean occurs in all soybean production areas throughout the world (18). Diagnostic signs of charcoal rot are the microsclerotia that are often imbedded in infected host debris. The pycnidial stage is commonly found in nature on garden bean and jute bean (18), occasionally on soybean and other host plants (1), and has even been reported on cats (5). Pycnidia have been reported to develop on agar media such as potato dextrose agar (PDA) (3,20) and soybean meal agar (10), although infrequently. In vitro production of large numbers of pycnidia has been accomplished using sterilized plant parts, filter paper treated with peanut meal-ether extract placed on the surface of a basal agar medium, and incubation under UV and long-wave near-UV irradiation, with variable results depending on M. phaseolina isolate and substrate (1,2,4,7–11,13). Various kinds of inocula have been used to inoculate plants with M. phaseolina, including microsclerotia suspensions, a mixture of sclerotia and soil, or infested millet (12,18,19); however, there are no reports of the use of a conidia suspension as inoculum, which has the advantage that it can be easily quantified with precision and then standardized based on the number of conidia per unit of inoculum. Standardized inoculum is essential for sound comparative studies (2), especially when differences between treatments, such as between partially resistant host plant genotypes, are relatively small. Standardizing inoculum and inoculation procedures also maximizes reliability and repeatability of inoculations. A method to produce abundant M. phaseolina conidia from different soybean isolates to use as inoculum would facilitate screening for partial resistance among soybean genotypes and, possibly, also could be used with other crop hosts. (1,2,4,7–11). The objective of this study was to select a method that would induce multiple soybean isolates to produce a sufficient quantity of viable and infective M. phaseolina conidia that could be used to inoculate soybean plants in a large resistance-screening program.