Submitted to: Aflatoxin Elimination Workshop Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: August 15, 2006
Publication Date: September 29, 2006
Citation: Timper, P., Holbrook, Jr., C.C., Wilson, D. 2006. Mechanisms of preharvest aflatoxin contamination in peanut infected by root-knot nematodes [abstract]. In: Proceedings of the Aflatoxin Elimination Workshop, October 24-26, 2005, Raleigh, NC. p. 115. Technical Abstract: Infection of peanut by root-knot nematodes (Meloidogyne arenaria) can lead to an increase in aflatoxin contamination of kernels when the plants are subjected to drought stress during pod maturation. It is not clear whether the increased aflatoxin contamination is primarily due to greater invasion of the galled pods by toxigenic Aspergillus spp. or whether root galling is also involved. Nematode damage to the pods and/or roots may also delay pod maturity. Small, immature peanuts are more prone to aflatoxin contamination than are mature, undamaged peanuts. Our objectives were: 1) to determine the contribution of root and pod galling caused by root-knot nematodes to the increase in aflatoxin contamination, and 2) whether nematode infection increases the percentage of immature peanuts. A greenhouse experiment was conducted in which pods and roots were physically separated. Pod set was restricted to soil-filled pans (41 cm dia. x 10 cm depth), while the roots grew underneath the pan into a pot. Root-knot nematodes (RKN) were applied to the root zone of half the plants, the other plants did not receive nematodes in the root zone. Plants received three pod treatments after pod set: one application, two applications, and no application of RKN. The treatments were arranged in a completely randomized design with 12 replicates/treatment. Conidia of Aspergillus flavus and A. parasiticus were added to each pan when the plants started to flower. Plants were subjected to drought stress 40 days before harvest. The results were similar among the two trials of the experiment so the data was combined for analysis. Adding nematodes to the pod zone had no effect on aflatoxin concentrations in the peanut kernel. However, the lack of an effect may have been to due to the low occurrence of galling on the peanut hulls. In pots where nematodes were added to the root zone, 50 to 80% of the root system was galled. Adding nematodes to the root zone increased (P = 0.003) aflatoxin concentrations in the peanut kernels from 37 ppb in the control to 67 ppb. A field microplot study was conducted in 2003 and 2004 to determine whether infection of peanut by RKN increases the percentage of immature kernels. Half of the 12 plots were inoculated with nematodes at two different times (at plant and after pegging) and the other half were not inoculated with nematodes. All plots were inoculated with A. flavus/A. parasiticus. Drought was induced 5 to 6 weeks before digging. In both 2003 and 2004, the presence of nematodes did not increase aflatoxin concentrations in the peanuts. In 2003, plants infected with RKN produced a greater (P = 0.0001) percentage of immature kernels than uninfected plants; however, in 2004 nematodes had no effect on the percentage of immature kernels even though root galling and yield reductions were greater in 2004 than in 2003. In summary, infection of peanut roots by the peanut root-knot nematode increases aflatoxin contamination of the kernels. Nematode damage to the roots results in greater drought stress which may result in greater susceptibility to aflatoxin contamination. The contribution of pod galling and immature kernels on the increase in aflatoxin levels in nematode-infected peanut are still unclear.