Genotypic Characterization of Phytophthora cinnamomi from Ornamental Crops in North Carolina

Abstract

Forty-two Phytophthora cinnamomi isolates from Camellia spp., Ilex spp., Juniperus spp., and Rhododendron spp. were characterized for mating type, mefenoxam fungicide sensitivity, and aggressiveness on Rhododendron ‘Hino de Giri’. Isolates collected from Camellia spp. were of the A1 mating type, while isolates from the other host plants were A2. All isolates were sensitive to mefenoxam at 100 ppm and all but one was sensitive at 1 ppm. Isolates from Rhododendron spp. scored higher average foliar disease and root rot ratings, while A1 isolates from Camellia spp. had the lowest average foliar disease and root rot ratings. The population sample of 42 isolates was also examined for DNA sequence polymorphisms in two nuclear loci, beta-tubulin (Btu) and a portion of the intergenic spacer (IGS) region of the nuclear rDNA repeat, and one mitochondrial DNA locus, cytochrome c oxidase subunit 1 (COX 1). Six base substitutions were found among the 42 isolates with a multi-locus data set. Isolates grouped into four haplotypes. Haplotype grouping corresponded to isolate mating type, plant host, and heterozygosity in the Btu locus. Our inferred multilocus rooted gene genealogy revealed a putative ancestral lineage representing the most frequently sampled haplotype in the population. This haplotype contained A2 isolates collected from Ilex spp., Juniperus spp., and Rhododendron spp.. Isolates of the A1 mating type diverged more recently in the genealogy. There is an increase in heterozygosity at the Btu locus that coincides with the appearance of the A1 mating type. These findings increase our understanding of the population structure of P. cinnamomi.