Pathogenicity of three Botryosphaeriaceae fungi, Diplodia scrobiculata, Diplodia mutila, and Dothiorella californica, isolated from coast redwood (Sequoia sempervirens) in California

Three Botryosphaeriaceae species, not previously recorded on coast redwood, were detected from native redwood stands in Humboldt Co., CA during general surveys between 2014 and 2017. Pathogenicity experiments with three of these fungi, Diplodia scrobiculata, Diplodia mutila, and Dothiorella californica, were performed on 2‐year‐old redwood seedlings to fulfil Koch's postulates and prove pathogenicity. Only D. scrobiculata produced lesions significantly longer than control lesions, although they were generally smaller than those of more virulent Botryosphaeriaceae pathogens in previous studies. In a separate pathogenicity study of D. scrobiculata on younger seedlings without secondary growth, inoculations on wounded stems resulted in black lesions that were much more externally obvious than the wound inoculations on lignified stems after 90 days. However, no lesions developed on non‐wounded stems of younger seedlings. Lesion lengths caused by D. scrobiculata were consistent between the two studies. On one wound‐inoculated, non‐woody seedling, D. scrobiculata was recovered from asymptomatic stem tissue 2 cm below the inoculation site, indicating possible pathogen latency. These experiments indicate that there are more Botryosphaeriaceae fungi involved in coast redwood dieback than previously known and confirm the pathogenicity of D. scrobiculata on primary and secondary tissue of coast redwood.