Etiology of Pine Ghost Canker in Southern California Urban Forests

Pine ghost canker is a recently described disease affecting multiple pine species in urban forests of Southern California. Symptoms include wedged cankers with irregular margins and cryptic discoloration on cross sections of branches, which can lead to severe dieback and potentially tree death. In this study, we identified and characterized five Neofusicoccum species (N. luteum, N. mediterraneum, N. parvum, N. stellenboschianum, and N. vitifusiforme) as the primary etiological agents of pine ghost canker. These pathogens were consistently isolated from multiple symptomatic pine samples (n = 41) and identified by morphology and phylogenetic analyses using four DNA barcodes (rDNA ITS, tef1, tub2, and rpb2). Pathogenicity was confirmed on healthy branches of 15-year-old Monterey pines, where the five Neofusicoccum species caused vascular lesions that were not significantly different in length. Secondary fungi (Diaporthe, Diplodia, Neopestalotiopsis, and Pestalotiopsis spp.) were also recovered from symptomatic tissues but did not cause vascular lesions in pathogenicity tests. The five Neofusicoccum species showed significant differences in mycelial growth rate at 20, 25, and 30°C. The optimal temperature for mycelial growth of N. luteum and N. parvum was 30°C, whereas for N. mediterraneum, N. stellenboschianum, and N. vitifusiforme, it was 25°C. All five species were able to resume growth at 20°C after showing no growth during a 7-day exposure to 5 and 40°C. This study also constitutes the first report of N. luteum, N. stellenboschianum, and N. vitifusiforme causing pine ghost canker in California. Environmental factors such as warmer temperatures, irrigation, and pest infestations are discussed as drivers of disease expression in pine trees. Management practices are also proposed.