Aspergillus flavus is a ubiquitous saprophyte that is able to produce the most potent natural carcinogenic compound known as aflatoxin B1 (AFB1). This toxin frequently contaminates crops including corn, cotton, peanuts, and tree nuts causing substantial economic loss worldwide. Consequently, more than 100 countries have strict regulations limiting AFB1 in foodstuffs and feedstuffs. Plants and microbes are able to produce volatile compounds that act as a defense mechanism against other organisms. Pichia anomala strain WRL-076 is a biocontrol yeast currently being tested to reduce AF contamination of tree nuts in California. We used the SPME-GC/MS analysis and identified the major volatile compound produced by this strain to be 2-phenylethanol (2-PE). It inhibited spore germination and AF production of A. flavus. Inhibition of AF formation by 2-PE was correlated with significant down regulation of clustering AF biosynthesis genes as evidenced by several to greater than 10,000-fold decrease in gene expression. In a time-course analysis we found that 2-PE also altered the expression patterns of chromatin modifying genes, MYST1, MYST2, MYST3, gcn5, hdaA and rpdA. The biocontrol capacity of P. anomala can be attributed to the production of 2-PE, which affects spore germination, growth, toxin production, and gene expression in A. flavus.

Nonpareil almonds, Prunus dulcis , account for the largest percentage of almond varieties grown in the Central Valley of California. Several studies have investigated the various nonvolatile and volatile components of various plant parts; however, the volatile organic compound (VOC) emission of almonds from a single cultivar has not been studied over the course of a growing season. This aspect is particularly relevant to research concerning the navel orangeworm (NOW), a major insect pest of almonds and other tree nuts. Despite the continued presence of NOW, the identification of particular VOCs and their relationship to NOW have not been addressed. The VOC emission of Nonpareil almonds was collected in situ over the course of a growing season by solid-phase microextraction (SPME). The VOCs (Z)-hex-3-enyl acetate, (Z)-hex-3-enyl butyrate, undecan-2-ol, beta-bourbonene, and tetradecane were present for the majority of the days investigated. Several VOCs exhibited positive electroantennographic signals from male and/or female NOW moths.

Aflatoxigenic aspergilli inflict major economic damage to the tree nut industry of California, with the highest negative impact to almonds. Aspergilli and fungi in general are known to emit volatiles in varying quantity and composition dependent upon their growth media. The goal of the study was to determine the volatile emission of whole and blanched almonds that had been picked out and labeled as inedible by processors. The aflatoxin content and number of colony forming units of each sample were also determined. A total of 23 compounds were consistently detected and identified. Several volatiles from the blanched almonds demonstrated significant increases when compared to the emissions of whole almonds. Several of these volatiles are considered fatty acid decomposition products and included hexanal, heptanal, octanal, nonanal, 3-octen-2-one, tetramethylpyrazine, and decanal. The almond samples investigated were characteristic of a typical postharvest environment and illustrative of potential contamination within a stockpile or transport container. Volatiles indicative of fatty acid decomposition were predominant in the samples that underwent some form of blanching. The emission amounts of hexanal, heptanal, octanal, and hexanoic acid increased 3-fold in samples contaminated with aflatoxin; however, due to variability between samples they could not be considered as indicator volatiles for aflatoxin content. The emission profile of volatiles from almond kernels contaminated with naturally occurring aspergilli and associated fungi is heretofore unreported.

The spiroketal (E)-conophthorin has recently been reported as a semiochemical of the navel orangeworm moth, a major insect pest of California pistachios and almonds. Conophthorin and the isomeric spiroketal chalcogran are most commonly known as semiochemicals of several scolytid beetles. Conophthorin is both an insect- and plant-produced semiochemical widely recognized as a nonhost plant volatile from the bark of several angiosperm species. Chalcogran is the principal aggregation pheromone component of the six-spined spruce bark beetle. Recent research has shown conophthorin is produced by almonds undergoing hull-split, and both spiroketals are produced by mechanically damaged almonds. To better understand the origin of these spiroketals, the volatile emissions of orchard fungal spores on fatty acids common to both pistachios and almonds were evaluated. The volatile emission for the first 13 days of spores placed on a fatty acid was monitored. The spores investigated were Aspergillus flavus (atoxigenic), A. flavus (toxigenic), Aspergillus niger, Aspergillus parasiticus, Penicillium glabrum, and Rhizopus stolonifer. The fatty acids used as growth media were palmitic, oleic, linoleic, and linolenic. Spores on linoleic acid produced both spiroketals, those on linolenic acid produced only chalcogran, and those on palmitic and oleic acid did not produce either spiroketal. This is the first report of the spiroketals conophthorin and chalcogran from a fungal source.