The wood decay fungus, Ganoderma adspersum, with no prior record in North America has emerged as a pathogen of almond in the San Joaquin Valley within the past decade. Incidence of the disease appears highest in orchards on ‘Nemaguard’ rootstock with high levels of crown gall, a bacterial disease caused by Agrobacterium tumefaciens. The putative association of G. adspersum with crown gall has been hypothesized, but not formally evaluated in the field. In February 2024, over 6,000 trees were surveyed across two orchards and rated for incidence of each disease. Both orchards were on ‘Nemaguard’ rootstock and included ‘Nonpareil,’ ‘Monterey,’ and ‘Fritz’ varieties. Results of Chi-square test for binomial ratings of two categorical variables indicate a significant association between G. adspersum and crown gall infection on all varieties in both orchards. Furthermore, when the variables were fitted using the generalized linear mixed model, infection with G. adspersum (independent variable) was found to have a significant effect on the association (response variable) for ‘Nonpareil’ variety at one site. For each unit increase in the incidence of crown gall, the probability of infection with G. adspersum increased by more than 1×109 fold on ‘Fritz’ and ‘Monterey’ at both sites, and ‘Nonpareil’ at one site, and by more than 87-fold on ‘Nonpareil’ at the second site. The results support the hypothesis that crown gall predisposes almonds to infection by G. adspersum. Further studies are needed to address the mechanism by which crown gall infection predisposes almond to G. adspersum with the long-term goal of improving the economic lifespan of California almond orchards.
