Abstracts of Presentations at the 2024 APS Pacific Division Meeting

In 2021, Opuntia littoralis (Coastal prickly pear) plants grown at a San Diego County native nursery for restoration use were observed with brown and gray lesions.Subsequent surveys in 2022 at 11 restoration sites in the county found 5-25% symptom prevalence.Alternaria spp.were recovered from 81% of symptomatic samples processed (n=21) from these sites.Using spore suspensions, detached cladodes inoculated with the three most common Alternaria spp.recovered (A.alternata, A. tenuissima, A. infectoria) exhibited significantly larger lesions than non-inoculated cladodes (P < 0.01), with A. alternata-inoculated cladodes exhibiting the largest lesions (6.5 mm).We completed Koch's postulates and recovered the inoculated species from 88% of the inoculated cladodes, suggesting that these fungi are new pathogens of O. littoralis in California.To confirm pathogenicity and determine the effects of environmental conditions on pathogenicity in rooted cladodes, we set a randomized complete block design experiment in the greenhouse, varying humidity and irrigation levels.Plants inoculated with A. alternata had significantly larger lesions than noninoculated plants (P < 0.01).Further, under higher humidity, plants with sufficient water had significantly larger lesions than plants with low water (P < 0.05).Identifying factors that exacerbate the pathogenicity of A. alternata on O. littoralis will inform best management practices to prevent further spread of disease.