Submitted to: Mycopathologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/7/2007
Publication Date: 11/1/2007
Citation: Windham, G.L., Williams, W.P. 2007. Systemic infection of stalks and ears of corn hybrids by Aspergillus parasiticus. Mycopathologia. 164:249-254. Interpretive Summary: Aflatoxin is a cancer causing toxin produced by the fungus Aspergillus flavus in corn grain while still in the field. This fungus can infect corn plants by growing down the silks into the ear and then entering the kernels. Insects may also play a part in infection by carrying fungal spores into the ear and providing a site of entry at their feeding sites. Other possible routes of infection exist that have not been studied. No information is available on infection of the ear from the fungus growing up through the stalk. We studied the movement of an Aspergillus fungal isolate up the corn stalk and into the ear. Toothpicks infested with the fungus were inserted into corn stalks in the early stages of flowering. After two weeks, stalk and ear tissues were brought to the lab, sectioned, and placed in agar Petri plates. The fungal isolate we used in this study was easy to detect because it produces chemical in culture that is a bright orange color. The fungus had moved from the infection site all the way up the stalk in to the ear tissue by the first harvest date. When aflatoxin resistant and susceptible hybrids were compared, the fungus was found to move readily through the stalks of all of these hybrids. The fungus was also found in kernels of ears collected from infected plants. The levels of kernel infection were very low. Our study demonstrated that an Aspergillus sp. can move from infections sites on the stalk through the corn stalk, and infect cob tissue and kernels.
Technical Abstract: Aspergillus parasiticus is one of two species that commonly causes aflatoxin contamination of corn kernels. Modes of entrance into plant tissues and kernels are not well known. In this study, an A. parasiticus mutant which produces a norsolorinic acid (a visible orange intermediate of the aflatoxin biosynthetic pathway) was used in field studies to monitor systemic infection of corn stalk and ear tissues. Corn hybrids resistant and susceptible to aflatoxin contamination were grown in the field and inoculated prior to tasselling by inserting A. parasiticus infested toothpicks into stalks between the 5th and 6th node below the lowest ear shoot. Beginning two weeks after inoculation, fungal growth and movement was determined weekly by collecting ear shank tissue and stalk tissue from the nodes between the infection sites and the developing ears. Ears were collected at the end of the growing season to determine the level of kernel infection by the NOR mutant. In two studies, the A. parasiticus NOR mutant was isolated from stalk tissues at all node positions and from ear shank tissue of susceptible corn hybrids at the first harvest date two weeks after inoculation. The NOR mutant was also isolated from stalk and ear tissue of a resistant hybrid. The NOR mutant was isolated from kernels of the susceptible hybrids in 2003 and 2004. Infection rates of kernels were very low (< 1.0 %). In 2005, the fungus was found in only one kernel from an ear of the resistant hybrid. The NOR mutant was not isolated from stalks, ears, or kernels from control (uninoculated) plants grown in the plots with inoculated plants. Although infection levels of corn kernels were low, systemic movement of the A. parasiticus up the stalk appears to be a possible route to infection of developing corn ears.