Spatial and temporal population dynamics of Aspergillus flavus in commercial pistachio orchards in Arizona

Authors: CHING'ANDA, CONNEL - University Of Arizona; ATEHNKENG, JOSEPH - International Institute Of Tropical Agriculture (IITA); BANDYOPADHYAY, RANAJIT - International Institute Of Tropical Agriculture (IITA); Callicott, Kenneth; ORBACH, MARC - University Of Arizona; Cotty, Peter; Mehl, Hillary

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2022
Publication Date: 1/12/2022
Citation: Ching'Anda, C., Atehnkeng, J., Bandyopadhyay, R., Callicott, K.A., Orbach, M., Cotty, P.J., Mehl, H.L. 2022. Spatial and temporal population dynamics of Aspergillus flavus in commercial pistachio orchards in Arizona. Plant Health Progress. 23:272-280. https://doi.org/10.1094/PHP-10-21-0128-RS.
DOI: https://doi.org/10.1094/PHP-10-21-0128-RS

Interpretive Summary: Aflatoxins are toxic fungal metabolites that can cause liver cancer, reduce human development and may cause death. Occurrence of these toxins greatly reduces the markets in which crops can be sold. Aspergillus flavus is a primary cause of aflatoxin contamination in several crops. Biological control using non-aflatoxigenic strains of A. flavus to displace toxigenic strains is one of the most effective techniques for controlling aflatoxins in crops. In tree nuts, the success of the applied biocontrol is dependent on the extent to which the applied biocontrol can move into the upper canopy. To determine movement of the applied biocontrol strain in pistachio orchards, populations of A. flavus at three canopy heights were characterized over two growing seasons. The applied biocontrol strain was detected at all canopy heights. Aspergillus flavus population density was greatest during periods of high temperature and rainfall, which was also a critical period of nut development and maturation. June to August was the period during which A. flavus inoculum increased in Arizona pistachio orchards, and aflatoxin biocontrol products are likely to be most effective when applied prior to establishment of aflatoxin-producing fungi in the tree canopy.

Technical Abstract: Aspergillus flavus infects a wide range of crops, including pistachios, and subsequent aflatoxin contamination results in significant economic losses. Application of biocontrol products based on non-aflatoxigenic (atoxigenic) strains of A. flavus is one of the most effective tactics for controlling aflatoxins in crops. Both risk of aflatoxin contamination and effectiveness of biocontrol are influenced by the extent to which A. flavus spores move into pistachio tree canopies during periods of nut development. Thus, the purpose of this study was to evaluate spatial and temporal population dynamics of A. flavus, including the applied biocontrol strain AF36, in canopies of pistachio orchards in Arizona. Propagule densities of A. flavus were quantified on leaf samples collected from lower, middle, and upper canopies from spring through harvest in 2018 and 2019. Aspergillus flavus propagule densities peaked during periods of high temperature and rainfall in 2018 (up to 600 CFU/g) and 2019 (up to 23 CFU/g), which coincided with nut development and maturation. The applied biocontrol strain AF36 was detected at all canopy heights, but overall propagule densities were greater in the upper and middle canopy (mean = 70 CFU/g) compared to the lower canopy (mean = 47 CFU/g). Results suggest June to August is the period during which A. flavus inoculum increases in Arizona pistachio orchards, and to most effectively displace aflatoxin-producing fungi in tree canopies, biocontrol applications should precede this period. In addition, this study demonstrates that soil-applied biocontrol strains can successfully disperse throughout the canopies of commercial tree nut orchards.