Infection without wounding and symptomless shoot colonization of Pinus radiata by Fusarium circinatum, the cause of pitch canker

Fusarium circinatum is widely regarded as a necrotrophic pathogen of pines that infects shoot tissue through mechanical or insect-mediated wounds, causing girdling lesions that result in death of infected branches. However, in the study reported here, F.circinatum colonized 100% of seedling stems and 70% of mature tree branches that were not wounded. Hyphae were observed beneath the epidermis of non-wounded shoot tissue within 14days following inoculation. In both seedlings and branches, most infections of non-wounded tissue did not induce symptoms for the duration of the trial (seven to eight weeks). In surveys of native Pinus radiata stands in California, F.circinatum was recovered from 7.5% to 100% of healthy-looking seedlings (2-3years old) across three of the four stands surveyed. These results suggest that wounding is not a requirement for shoot infection under all circumstances and that F.circinatum can grow within shoot tissue without causing symptoms.