Period of susceptibility of almonds to aflatoxin contamination during development in the orchard

Authors: PICOT, ADELINE - University Of California; ORTEGA-BELTRAN, ALEJANDRO - University Of California; PUCKETT, RYAN - University Of California; Siegel, Joel; MICHAILIDES, THEMIS - University Of California

Submitted to: European Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/17/2016
Publication Date: 12/1/2016
Citation: Picot, A., Ortega-Beltran, A., Puckett, R.D., Siegel, J.P., Michailides, T.J. 2016. Period of susceptibility of almonds to aflatoxin contamination during development in the orchard. European Journal of Plant Pathology. 148(3):521-531. doi: 10.1007/s10658-016-1108-2.

Interpretive Summary: Almonds are the largest tree nut crop grown in California, and there are currently one million acres planted in California. Crop quality is essential to ensure a healthy domestic and export market. One of the greatest quality threats to all tree nuts, and other commodities such as peanuts, is aflatoxin contamination. Aflatoxins are poisons produced by two species of fungi, Aspergillus flavus and Aspergillus parasiticus, that colonize almonds in the tree once the almond hull has split, and then develop and produce aflatoxins in the nuts before harvest. Aflatoxins are frequently associated with nut damage by the navel orangeworm, which is the principal insect pest of almonds in California. The purpose of this study was to determine if there is a specific time that nuts are most vulnerable to aflatoxin contamination after hull split, and whether navel orangeworm damage is always necessary in order for almonds to become colonized by the two species of Aspergillus. Aspergillus fungi were able to successfully develop in all stages of the almonds tested, and become established even in undamaged nuts. However, aflatoxin contamination was eleven times higher when navel orangeworm fed on the nuts than the levels on nuts that were undamaged, and the levels of aflatoxin were highest when the fungi had the longest opportunity to develop in the field. This study established that almonds are susceptible to Aspergillus infection from hull split until harvest, and it is important to control navel orangeworm throughout this period. This information underscores the necessity of insect control to ensure high nut quality.

Technical Abstract: Almonds can be contaminated by aflatoxins, mainly produced by Aspergillus flavus and A. parasiticus. Infection by Aspergillus species can be facilitated by insect damage to the kernel during hull split, which occurs 4 to 6 weeks before harvest. Within this period of time, it is unknown which kernel stages are the most susceptible to aflatoxin contamination. Beginning at hull split, Nonpareil almonds were infested weekly with navel orangeworm (NOW) eggs and then inoculated with one isolate of A. flavus or A. parasiticus for five weeks and then collected at harvest. Aflatoxin levels were quantified using HPLC. The aflatoxin contamination was always much higher in NOW-damaged kernels but aflatoxins were also found in undamaged kernels throughout the five week inoculation period. While insect injury is not required for kernel infection, it is an important risk factor for aflatoxin contamination. Laboratory inoculations were also performed on kernels collected during this same time period. Aflatoxin levels were significantly lower on dried almonds (water activity (aw)<0.75) but the inhibition effect was suppressed when almonds were incubated under high humidity conditions or when the Aspergillus species were inoculated on almond meal agar amended with ground kernels. This study demonstrated that aflatoxins can accumulate in both damaged and undamaged kernels with an initial low aw, at any time after hull split, should the moisture conditions in the field rise to a level favorable for aflatoxin production.