Seedborne black Aspergillus species as maize seedling pathogens: role of fumonisin production and interaction with soilborne Pythium species
Twenty-six strains of black Aspergillus (Aspergillus section Nigri) were studied as seedling pathogens of maize. There were two major research components. The first component was an evaluation of the pathogenicity of isolates from several black Aspergillus species and a comparison of the pathogenicity of isolates with and without fumonisin production. This was accomplished by testing inoculated seeds in warm germination and cold tests, and by evaluating growth of inoculated seeds in rolled paper towel assays. In the second component, four of these species were selected for evaluation of interactions between Aspergillus and Pythium as a seedling disease complex in maize. Seeds inoculated with Aspergillus were planted in soil infested with Pythium sylvaticum, Pythium torulosum, or a control in a cup assay, and scored on several criteria for seedling growth.
Species of Aspergillus in section Nigri are commonly associated with maize kernels but there is little information about their effects on maize seed germination and seedling health. It has recently been discovered that some strains in this group have the capacity for producing the fumonisin mycotoxins, but it is not known what effect, if any, this has on pathogenicity in seedling disease. We compared 9 strains from Iowa, 4 from Illinois, and 13 from Italy in two seed-inoculation assays assessing their ability to affect germination and seedling growth. Some were fumonisin-producing strains and others were non-producing. Representatives of A. awamori, A. niger, A. phoenicis, A. tubingensis, and a single strain of A. carbonarius were included. Maize seeds of two different hybrids were inoculated with spores of each of these strains. They were evaluated for germination and seedling growth using a warm germination test, a cold test, and a rolled paper towel assay. Strains of each species reduced germination or seedling growth of one or both hybrids, but there was high variability among strains within species. There were no consistent differences between fumonisin-producing and non-producing strains.
While many pathogens of maize seedlings have been studied extensively in isolation, little is known about their interactions with each other. In the second portion of this study we investigated the relationship between Aspergillus section Nigri, seedborne fungi that cause ear rot and seedling disease in maize, and Pythium spp., which are soilborne and cause seedling disease. Maize seeds inoculated with one of four strains of Aspergillus or not inoculated were planted in cups filled with non-infested sterile field soil, or soil infested with P. sylvaticum or P. torulosum. The cups were placed in a growth chamber at 25°C in a randomized complete block design (RCBD) and assessments done at 7 and 14 days after planting (DAP). An interaction was detected between Pythium and Aspergillus on seedling height at 7 DAP and percent emergence. Percentage healthy mesocotyl, height at 14 DAP, and shoot weight were reduced by Pythium only. Root weight was affected by both Pythium and Aspergillus, but with no interaction. For the variables with an interaction, P. torulosum caused more severe symptoms when associated with seed-borne Aspergillus, while P. sylvaticum caused severe symptoms regardless of the presence of Aspergillus spp. The results suggest that seedborne Aspergillus can exacerbate seedling disease caused by Pythium spp. under some conditions. This highlights the need for further study of seedling pathogens with reference to the entire soil ecosystem, rather than simply in isolation.