ABSTRACT Peanut kernels are susceptible to colonization by some species of Aspergillus which, under conditions of drought and high temperatures, can produce aflatoxins prior to harvest. The objective of this research was to determine the mechanism by which the peanut root-knot nematode ( Meloidogyne arenaria ) increases aflatoxin contamination in peanut. Research determined 1) the role of nematode infection of roots vs. pods in increased aflatoxin contamination and 2) whether increased aflatoxin production in nematode-infected peanut is due to a greater percentage of small or immature kernels. An additional objective was to determine whether a peanut cultivar with resistance to M. arenaria would reduce the risk of preharvest aflatoxin contamination. In the greenhouse, researchers physically separated root growth from pod set and inoculated each location with M. arenaria or a water control in a 2 × 2 factorial design with 12–15 replications. Of the six trials conducted, data indicated that pod and root infection by M. arenaria was associated with elevated aflatoxin concentrations in one and three trials, respectively. This suggests that root infection by the nematode can increase aflatoxin concentrations in the peanut kernel. Another 2 × 2 factorial experiment was conducted with two peanut genotypes (Tifguard and TifGP-2) and two nematode treatments (with and without M. arenaria ) with six replications. The cultivar Tifguard is resistant to M. arenaria and TifGP-2 is susceptible. The experiment was carried out in 24 field microplots equipped with a rainout shelter. The experiment was conducted five times from 2006 to 2010. Infection of TifGP-2 by M. arenaria did not lead to greater percentages of small kernels. In only one year (2007), nematodes appeared to increase the percentage of damaged kernels, though aflatoxin concentrations were not affected by nematodes in that year. In the rainout shelter experiment, 2006 was the only year where nematode infection of peanut increased aflatoxin concentrations. In that year, there were lower aflatoxin concentrations in the nematode-resistant cultivar Tifguard than the susceptible germplasm TifGP-2 (12 vs. 136 ng/g).

Aflatoxin contamination costs the U.S. peanut ( Arachis hypogaea L.) industry over $20 million annually. The development of peanut cultivars with resistance to preharvest aflatoxin contamination (PAC) would reduce these costs. Screening techniques have been developed that can measure genetic differences in aflatoxin contamination and they have been used to identify accessions that exhibited relatively low PAC in multiple environments. Significant reductions in PAC have been identified in peanut genotypes with drought tolerance. These sources of resistance to PAC have been crossed with cultivars and breeding lines that have high yield, acceptable grade, and resistance to spotted wilt caused by Tomato spotted wilt tospovirus (TSWV). Due to the large environmental variation in PAC, breeding populations can only be evaluated in late generations when there is less heterozygosity and adequate numbers of seed are available for field testing using multiple replications. Evaluation of numerous breeding populations has identified several families and individual breeding lines with relatively low PAC, relatively high yield, and acceptable levels of resistance to TSWV. To increase breeding efficiency, studies on mechanisms of resistance to PAC are being conducted. The most promising mechanisms identified thus far are resistance to drought and resistance to the peanut root-knot nematode. Late generation breeding lines have been developed with resistance to drought, several of which also exhibited reduced aflatoxin contamination in multiple environments. Tifguard, the first cultivar with high levels of resistance to both TSWV and the peanut root-knot nematode [ Meloidogyne arenaria (Neal) Chitwood race 1] was released from this program. Testing is ongoing to determine if this cultivar can be used to reduce aflatoxin contamination in nematode infested fields.

Peanuts become contaminated with aflatoxins when subjected to prolonged periods of heat and drought stress. The effect of drought tolerance on aflatoxin contamination is not known. The objectives of this research were to evaluate preharvest aflatoxin contamination in peanut genotypes known to have drought tolerance and to determine the correlation of drought tolerance characteristics with aflatoxin contamination. Twenty genotypes with different levels of drought tolerance were grown in Yuma, AZ (a desert environment) and under rain-protected shelters in Tifton, GA. Two drought-tolerant genotypes (PI 145681 and Tifton 8) and an intolerant genotype (PI 196754) were selected for further examination in a second experiment with two planting dates in 1997 at Tifton. Drought and heat stress conditions were imposed for the 40 d preceding harvest. The drought-intolerant genotype had greater preharvest aflatoxin contamination than Florunner (the check cultivar) in the tests conducted in 1997. Both drought-tolerant genotypes had less preharvest aflatoxin contamination than Florunner in these tests. Significant positive correlations were observed between aflatoxin contamination and leaf temperature and between aflatoxin contamination and visual stress ratings. Leaf temperature and visual stress ratings are less variable and less expensive to measure than aflatoxin contamination. Leaf temperature and visual stress ratings maybe useful in indirectly selecting for reduced aflatoxin contamination in breeding populations.

Peanut ( Arachis hypogaea L.) is susceptible to aflatoxin contamination when pods are developing under drought conditions in the field. The development of cultivars which resist preharvest aflatoxin contamination would be advantageous, but has been limited by the lack of genes for resistance. Several genotypes have been suggested as potential sources of resistance. Conflicting results have been reported on how useful this resistance may be, and some of these sources have never been specifically examined for resistance to preharvest aflatoxin contamination. The objective of this study was to evaluate aflatoxin contamination under drought stressed conditions in potentially resistant peanut genotypes. Twelve peanut genotypes were planted in a randomized complete-block design in field plots in Yuma, AZ in 1991 and 1992. Ten of these genotypes were also planted in a randomized complete-block design in field plots in Tifton, GA in 1992. All plots were inoculated with Aspergillus inoculum and were subjected to 40 to 50 d of drought stress immediately prior to harvest. After harvest, aflatoxin contamination (ppb) of seed was measured. None of the genotypes included in this study were more resistant (P≤0.05) to preharvest aflatoxin contamination than Florunner. The results of this study indicate that it would be desirable to identify higher levels of resistance to preharvest aflatoxin contamination in peanut.

Efforts are being made to screen peanut ( Arachis hypogaea L.) genotypes for resistance to invasion of seed by the Aspergillus flavus (Link ex Fries) group fungi and subsequent contamination by aflatoxins. Field inoculation studies are needed to develop reliable screening techniques that insure infection of susceptible peanut seed by the A. flavus group fungi and minimize the number of escapes. The objectives of this study were to evaluate the effects of A. parasiticus (Speare) inoculum formulation and timing of application on shell and seed colonization and aflatoxin contamination. Soil population densities of A. flavus and plant survival were also measured. Plots were inoculated using various rates either at-plant or at mid-bloom of an organic-matrix infested with A. parasiticus NRRL 2999, and/or using an aqueous conidial suspension of the fungus. Mid-bloom inoculation produced an increase of A. flavus group population densities in the soil. There were no statistical differences between treatments in aflatoxin content, or colonization of shells or seed. At-plant inoculation was associated with reduced plant emergence or survival. Inoculation at mid-bloom with the organic-matrix resulted in peanuts that ranked high in aflatoxin content. There were no correlations between aflatoxin content, colonization of seed or shells, and population densities of A. flavus in soil. Inoculation at mid-bloom with the organic-matrix, resulting in greater soil population densities and relatively higher aflatoxin levels, was an effective method to use in screening for resistance to invasion by A. flavus group fungi and contamination by aflatoxins.

Florunner peanuts ( Arachis hypogaea L. ) were grown during the 1984 season near Tifton, Georgia. Two planting dates spaced one month apart were used to obtain four weekly digging dates. Peanuts from each harvest were windrow dried for 4 days, mechanically harvested and placed into a ventilated wagon for 1 to 4 days. Peanuts were subsequently removed from the wagon, shelled at 8 to 22% moisture (wb), and microwave vacuum dried at nominal rates of 4, 8, 16, and 32 times the normal rate of conventional wagon drying. Harvest group, microwave treatment level, and order of processing were configured in a Latin Square design. Concurrent with each microwave run (MV), a separate portion of the shelled peanuts was deep bed dried (CS) and another portion was similarly dried but within shell (CH). Analyses of variance were performed to determine the significance of treatment type, microwave level, harvest group, and processing order differences on splitting and skin slippage tendencies, mold growth, and germination potential. No significant (p> .05) differences were observed among treatment types for splitting and skin slippage potential, though slight (p< .05) differences existed among microwave treatment levels (damage increasing with increasing microwave drying rate), and drying order within microwave treatment (damage decreasing with increasing drying order). Larger (p< .01) splitting and skin slippage differences existed among harvest groups. MV treatments had significantly (p< .01) higher presence of A. flavus than the CS and CH treatments though aflatoxin was not detected in any sample. The percentage of normal strong germinated kernels from MV treatments was significantly (p<.01) lower than from the CS and CH treatments, with germination decreasing with increasing microwave process rate.