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An Integrated Study of the Avocado Root Rot Pathogen Phytophthora cinnamomi


Phytophthora root rot (PRR), caused by Phytophthora cinnamomi (Pc), is the most destructive disease of avocado worldwide. This study described the phenotype of a subset of Pc isolates representing two, previously identified, genetically distinct clades of A2 mating type isolates in California regarding growth rate, optimal growth temperature, virulence, and fungicide sensitivity. Isolates corresponding to the clade I group exhibited higher mycelial growth rate and were more sensitive to higher temperatures than clade II. Isolates that were more virulent in avocado and less sensitive to potassium phosphite were also identified. A detached leaf Pc inoculation method using Nicotiana benthamiana was developed and validated providing an alternative method for assessing the virulence of a large number of isolates.

Seventy-one isolates of Pc collected from avocado growing regions in California were evaluated for their in vitro sensitivities to four new Oomycota fungicides ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin and two registered fungicides mefenoxam and potassium phosphite. The four new fungicides and mefenoxam showed high in vitro toxicities with relatively low mean effective concentrations to inhibit mycelial growth by 50% (EC50) values. Isolates with reduced sensitivity to potassium phosphite were identified. Greenhouse trials were conducted to assess the efficacy of these fungicides for managing PRR on avocado seedlings and rootstocks. Mefenoxam and potassium phosphite were effective treatments, however, oxathiapiprolin, fluopicolide, and mandipropamid were more effective.

RNA sequencing (RNAseq) transcriptome analysis of Pc infected N. benthamiana leaves at 6, 12, 24, 36, and 48 hours post-inoculation was performed. Genes involved in hormone signaling, receptor-like kinases, NBS-LRRs, transcription factors, secondary metabolism, and production of antimicrobial compounds were differentially expressed (DE). Major up-regulated biochemical pathways included the biosynthesis of secondary metabolites and plant-pathogen interactions. Major down-regulated biochemical pathways were primary metabolic pathways and carbon metabolism. Among the DE genes identified, a jasmonic acid responsive gene cytochrome P450 was up-regulated 13.3-fold at 36 hpi, and a gene encoding for a salicylic acid binding protein was down-regulated 10.5-fold. WRKY transcription factor 51, up-regulated by 13.5-fold at 24 hpi, was transiently overexpressed in N. benthamiana and significantly decreased the lesion development on Pc inoculated leaves compared to the GFP control.

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