Submitted to: Phytoparasitica
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/8/2016
Publication Date: 7/21/2016
Citation: Windham, G.L., Williams, W.P. 2016. Effect of inoculum concentrations of Aspergillus flavus and A. parasiticus on aflatoxin accumulation and kernel infection in resistant and susceptible maize hybrids. Phytoparasitica. 44:333-339.

Interpretive Summary: Aflatoxin is a cancer causing compound produced by the fungi Aspergillus flavus and A. parasiticus, and is a major problem for corn producers in the southern United States. The development of corn lines with resistance to aflatoxin accumulation in grain has been a goal of the USDA Agricultural Research Service. To identify resistant corn lines, developing ears in the field are inoculated with millions of fungal spores. This is necessary to produce the disease consistently in the field tests, but it is considered a harsher treatment compared to disease which starts naturally in a grower's field. A three year study was conducted to determine if we could produce similar levels of fungal infection and aflatoxin production using lower concentrations of A. flavus or A. parasiticus spores during the ear inoculation process. Six spore concentrations ranging from 4,000 to 400,000,000 per ear were used. Ears were injected with fungal spores using a tree marking gun fitted with a stainless- steel needle. Aflatoxin contamination and grain infection by the fungi were determined approximately two months following inoculation. The inoculation treatment with the lowest concentration of spores produced high enough levels of aflatoxin to distinguish between a resistant corn hybrid and a susceptible corn hybrid. More variability was observed in the amount of infection of the grain, but infection was adequate to produce aflatoxin. Using lower levels of spore concentrations to inoculate ears may allow researchers to identify plants, with moderate levels of resistance to aflatoxin accumulation, that might be missed using higher spore concentrations.

Technical Abstract: Over a three year period, we compared aflatoxin accumulation and kernel infection in maize hybrids inoculated with six inoculum concentrations of Aspergillus flavus isolate NRRL 3357 or A. parasiticus isolate NRRL 6111 which is a norsolorinic acid producer. Aflatoxin resistant and susceptible maize hybrids were inoculated with six conidial concentrations (ranging from 1 x 103 to 1 x 108/ml) of either A. flavus or A. parasiticus using the side-needle technique 7 days after midsilk (50% of the plants had silks emerged). Aspergillus kernel infection and aflatoxin contamination was determined at ca. 63 days after midsilk. For each hybrid, aflatoxin levels were very similar regardless of the inoculum concentration. With the exception of the A. parasiticus treatments in 2007, the lowest inoculum conidial concentration (1 x 103/ml) for both fungal species was sufficient to produce statistical differences between the resistant and the susceptible maize hybrids for aflatoxin contamination. There was more variability in kernel infection between inoculum concentrations that that observed in aflatoxin accumulation. Since our aflatoxin evaluations are conducted in a warm, humid environment, putting less disease pressure on plants via lower inoculum concentrations may make it possible to identify genotypes that have moderate levels of resistance to aflatoxin accumulation.