Characterization of Phytophthora cinnamomi from ornamental crops in South Carolina

Phytophthora cinnamomi is a plant pathogen infects over 900 hosts. Little is known about variability among isolates of P. cinnamomi that attack woody ornamental crops in South Carolina. 142 isolates of P. cinnamomi from diseased plant samples submitted to the Clemson University Plant Problem Clinic from 1996-2011 were characterized for growth rate, mycelium growth habit, mefenoxam sensitivity, and mating type. Average growth on PARPH-V8 selective medium was 60 mm in 72 h at 25°C. Mycelium growth habit on PARPH-V8 was classified as aerial, sparse, dwarf, or appressed, and 85% of isolates had aerial mycelium. All isolates were sensitive to the fungicide mefenoxam at 100 ppm. The population was composed of 129 A2 and 13 A1 isolates. The ITS 1 and 2 loci were sequenced, and this region had low diversity with only two genotypes that were different from the majority of the population. One of these genotypes consisted of an isolate matching P. cinnamomi var. parvispora, and the other genotype included four morphologically diverse isolates. Consequently, there was a high degree of genetic uniformity in the ITS region among these 142 isolates. Host-pathogen relationships for this population were compared to reports in the literature, and 33 new associations were found