Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 10/22/2007
Publication Date: 2/1/2008
Citation: Windham, G.L., Williams, W.P. 2008. Effect of stalk inoculation site on kernel infection of corn by Aspergillus parasiticus [abstract]. Proceedings 2007 Annual Multi-Crop Aflatoxin/Fumonisin Elimination and Fungal Genomics Workshop. p. 63.
Technical Abstract: Aspergillus flavus and A. parasiticus may infect corn kernels on developing ears by several routes. Kernel infection is postulated to occur inside ears primarily by the colonization of silk tissues. Airborne conidia land on exposed silks, germinate, and then colonize the silks inside the ear. Once inside the ear, fungi colonize the glumes at the base of the kernels and the kernel surfaces. Another method of infection involves the vectoring of conidia by insects. Ear-feeding insects transport fungi inside the husks and predispose the kernels to infection by wounding kernels as they feed. Systemic movement through the stalk is another possible route of kernel infection by mycotoxigenic fungi. Recently, we demonstrated systemic movement of an A. parasiticus mutant through the corn stalk and into the ear shank tissues. Plants were inoculated by inserting infested toothpicks in the stalks between the 5th and 6th node below the lowest ear shoot. In as little as two weeks, the mutant moved up through the stalk into the ear shank tissues. Kernel infection levels in inoculated plants were found to be low (< 1.5 %). It is possible that inoculating the stalk closer to the site of ear attachment may increase the level of kernel infection. This study was conducted to determine if the site of A. parasiticus inoculation on stalks would have an effect on kernel infection. An A. parasiticus color mutant (NRRL 6111) was used in field studies conducted in 2006 and 2007. This isolate can be distinguished from wild-type Aspergillus spp. by its reddish-brown color and the production of norsolorinic acid (NOR) which is a visible orange intermediate of the aflatoxin biosynthetic pathway. The NOR mutant was cultured on V-8 agar with sterilized toothpicks placed on the surface. Three A. flavus susceptible corn hybrids (TV2100, RX938, Mp305 x Mp339) and a resistant hybrid (Mo18w x Mp313E) were inoculated at the VT stage by inserting A. parasiticus infested toothpicks into stalks 1, 2, 3, 4, or 5 nodes below the lowest ear shoot. At maturity, five ears from each inoculation treatment from each hybrid were harvested, dried, and individually hand shelled. Three-hundred and ninety kernels from each ear were surface sterilized and plated on Czapek solution agar amended with NaCl (7.5%). After 7 days, the Petri dishes were examined for presence of the NOR mutant. Ears were also collected from non-inoculated plants interspersed among the inoculated plants, and processed as described above as a control treatment. In 2006, the susceptible hybrids TV2100 and Mp305 x Mp339 had the highest levels of infection. Systemic infection of kernels in these hybrids was highest in plants inoculated 1, 2, or 3 nodes below the ear. The highest rate (1.5 %) of kernel infection for any of the inoculation treatments was in TV2100 plants inoculated 1 node below the ear. The resistant hybrid Mo18w x Mp313E had the lowest level of infection. The NOR mutant was only detected in two ears from plants inoculated 1 node below the site of ear attachment. The fungus was not detected in ears from plants inoculated at 2 to 5 nodes below the ear. The NOR mutant was not detected in any of the ears from the un-inoculated plants. In 2007, the fungus was not detected in ears of any of the hybrids regardless of the inoculation site. Our studies have demonstrated that A. parasiticus can readily move up the stalk and into the ear/cob tissue. Kernel infection by the NOR mutant was highest in plants with inoculation sites closest to the ears. However, kernel infection rates remained relatively low. These findings are similar to those of other researchers working with F. verticilliodes on corn. Infection of kernels systemically via the stalk is possible, but it appears that the most likely route of infection by mycotoxigenic fungi is by colonization of silk tissues.