California is the major state for producing almonds, pistachios, and walnuts, with a total market value of $1.6 billion. Both domestic and export markets of these nuts presently allow a maximum level of aflatoxin B1 contamination in the edible nuts to be 20 ppb. Even very low degrees of infection of the nuts by A. flavus can result in aflatoxin levels above the mandatory standards. Biological control to reduce the population of and to inhibit the biosynthesis of A. flavus in orchards may be useful to decrease infection and thus aflatoxin content in the edible nuts. Certain saprophytic yeasts were shown to effectively compete with postharvest fungal pathogens such as Penicillium expansum and Botrytis cinerea. The potential of saprophytic yeasts to reduce aflatoxin contamination in tree nuts has not been hitherto extensively explored. A safe visual bioassay for screening yeasts antagonistic to A. flavus has been developed. The nor mutant of A. flavus has a defective norsolorinic acid reductase and blocks the aflatoxin biosynthetic pathway, resulting in the accumulation of norsolorinic acid, a bright red-orange pigment. We used the nor mutant in the assay to screen yeasts strains for their ability to inhibit aflatoxin production by visually scoring the accumulation of this pigment as well as the growth and sporulation of the fungus. Yeast strains that reduced the red-orange pigment accumulation in the nor mutant were identified and shown to inhibit aflatoxin biosynthesis of several toxigenic strains of A. flavus.
S.-S.T. Hua, J.L. Baker, and M. Flores-Espiritu
Thomas Gradziel, Noreen Mahoney, and Ashraf Abdallah
Genetic differences were observed in levels of aflatoxin production following controlled inoculations of California almonds [Prunus dulcis (Mill.) D.A. Webb, syn. P. amygdalus, Batsch.; P. communis (L.) Arcangeli, non Huds.]. Genetic variation was also observed in kernel oil composition, and in susceptibility to Aspergillus flavus Speare as indicated by rate of mold expansion on the surface of cut kernels. Several almond lines resulting from the introgression of peach [P. persica (L.) Batsch] germplasm had very low aflatoxin levels relative to commercial cultivars tested. Peach-derived almond breeding lines and cultivars also produced some of the highest oil quality as determined by the proportion of oleic acid, and by the oleic to linoleic acid balance. The proportion of linoleic acid to total oil ranged from 16% to almost 30%. No correlations were detected between aflatoxin production in inoculated almond kernels and either kernel oil composition or mold growth rate on injured kernel tissue.
Evan Elford, Jim Todd, Peter White, Rachel Riddle, John O’Sullivan, and Rene Van Acker
analyzed at a moisture content of 0%. Previous studies have shown tigernut grown outside of North America can be contaminated with aflatoxins, a naturally occurring and toxic group of secondary metabolites produced by Aspergillus fungi ( Sebastià et al
Thomas M. Gradziel and Dechun Wang
Colonization and sporulation of aflatoxigenic Aspergillus flavus Link on intact and injured seed was evaluated for a selection of almond [Prunus dulcis (Mill.) D.A. Webb] cultivars. Barriers to fungal development were identified at the intact seedcoat and at the seed cotyledon tissue. The seedcoat barrier was expressed as a delay in fungal colonization for up to 3 days following the inoculation of intact seed. Seedcoat resistance was uniformly high for all cultivars tested. Cotyledon resistance, which was expressed as a lower rate of disease development was identified only in the cultivars Ne Plus Ultra, Ruby, and Carrion.
F. Dicenta, P. Martínez-Gómez, E. Martínez-Pato, and T.M. Gradziel
Aspergillus flavus Link. is a filamentous fungus affecting almond [Prunus dulcis (Mill.) D.A. Webb] kernels in the field and during storage. This fungus can produce afla-toxins (carcinogenic and immunosuppressive mycotoxins), which prevent the marketability of almond kernels. Aspergillus flavus resistance has not been an objective in conventional almond breeding programs. Because the importance of this disease is increasing, evaluations of cultivar susceptibility are being performed. In this study, the screening for A. flavus resistance in 40 almond genotypes has been carried out in controlled inoculation conditions at 26 °C. Eighteen days after the inoculation, kernels of all the almond cultivars assayed showed susceptibility to A. flavus. Nevertheless, differences among cultivars in the percentage of kernel surface colonized by the fungus were observed. The Spanish cultivar Ramillette was the least susceptible. Susceptibility was not related to the geographic origin of the cultivar.
Craig E. Kallsen and Dan E. Parfitt
‘Gumdrop’ is a new female pistachio (Pistacia vera L.) cultivar for California. It matures earlier than all commercial cultivars with equivalent yield and nut quality to ‘Kerman’. ‘Gumdrop’ can be harvested about 10–12 days before ‘Golden Hills’ pistachio () and 24 days before ‘Kerman’, the standard pistachio cultivar grown in California (). ‘Gumdrop’ has very good yield, nut quality, and processed nut appearance similar to ‘Golden Hills’ and ‘Kerman’. ‘Gumdrop’ blooms about 5 days before ‘Golden Hills’ and 10–11 days before ‘Kerman’. ‘Gumdrop’, ‘Golden Hills’, and ‘Kerman’ comprise a harvest series, maturing over a 24–30 day period. The early nut maturity of ‘Gumdrop’ will permit pistachio growers to extend their harvest period. The earlier maturing date of ‘Gumdrop’ also makes it less susceptible to insect damage from navel orangeworm, a major pest of pistachio implicated in the occurrence of aflatoxin contamination. An application for a U.S. Plant Patent was submitted on 4 Apr. 2016.
Dan E. Parfitt, Siov B. Ly, Ali A. Almehdi, Helen Chan, and Sui-Sheng T. Hua
Aspergillus flavus produces aflatoxin, a cancer-causing contaminant of pistachio in many production areas. A superior yeast strain of Pichia anomala has been demonstrated to inhibit the growth of A. flavus in the laboratory. It was selected for further study and potential release based on tests of durability and ability to inhibit A. flavus growth. This strain has been tested in the field for the past two years to evaluate its ability to survive in a field environment and to inhibit A. flavus production. The yeast was evaluated in the field to determine if: 1) the biocontrol yeast can survive in pistachio orchards; 2) the yeast has no phytotoxic effects on the pistachio trees or nuts; and 3) the yeast can be demonstrated to control A. flavus in the field. Studies during 2003 were conducted using a replicated experiment with three yeast concentrations and a water control. Treatments applied later in the season were found to be most effective. Highest yeast concentrations were observed just prior to harvest. Three spray concentrations and a water control were applied to evaluate possible phytotoxic effects on pistachio during 2004. No differences in leaf or nut appearance, in nut percent splits, or dry weight were observed for any of the treatments when compared to the water control. Artificial wounding experiments were also conducted during 2003 and 2004 to simulate the occurrence of early split nuts, the primary repository for A. flavus contamination. A 5× reduction in A. flavus colonization was observed on treated wounded nuts vs. untreated wounded nuts. A 5× reduction in A. flavus sporulation was also observed on treated wounded nuts vs. untreated wounded nuts.
Francisco Vargas, Miguel Romero, Joan Clavé, Jaume Vergés, Josef Santos, and Ignasi Batlle
-kernel nuts. The almond shells are hard, reducing worm damage (and thus preventing aflatoxin contamination) and also avoiding bird damage. They are well suited to the European industry based on hard-shelled cultivars. The four cultivars are easily trained and
Ed Stover, Malli Aradhya, Louise Ferguson, and Carlos H. Crisosto
requiring pollination. Aflatoxin, a mycotoxin produced during A. flavus infection, is sometimes detected in dried figs. This compound is a potent carcinogen and is subject to monitoring and product rejection at very low thresholds by the Food and Drug
Dan E. Parfitt, Craig E. Kallsen, Brent Holtz, and Joseph Maranto
optimize the use of labor and harvest equipment. Navel orangeworm (NOW) infestation ( Amyelois transitella Walker) has been shown to be associated with high levels of Aspergillus flavus Link, the causal agent for aflatoxin contamination ( Doster and