Phenotypic variability of Leptosphaeria lindquistii (anamorph : Phoma macdonaldii), a fungal pathogen of sunflower

Growth of 17 isolates of Phoma macdonaldii, the causal agent of sunflower b lack stem, was investigated for response to pH and temperature, and for mor phology and asexual morphogenesis (pycnidiogenesis and pycnidium size). For all isolates, the optimum pH for growth was between 4 and 5, and the optim um temperature varied between 20 and 30 degrees C and radial growth was slo west at 5 and 35 degrees C. Significant differences in the number and size of pycnidia were observed between isolates. Pycniospore germination was inv estigated under various conditions in five isolates chosen for their geogra phical origins, pigmentation, optimum growth temperature and pycnidiogenesi s. Increasing the concentration from 10(6) to 10(7) pycniospores per mL dec reased the germination rate. The optimum temperature for pycniospore germin ation varied between 15 and 30 degrees C, depending on the isolate, and the optimum and maximum pH values were 5 and 7, respectively. The optimum and minimum relative humidities allowing pycniospore germination were 100 and 9 5%, respectively. Pycniospore germination was photo-independent. An artific ial inoculation method was developed and the aggressiveness of the pathogen was assessed on a susceptible sunflower cultivar, using a 1-9 scale that i ntegrated the percentage of necrotic area on the cotyledon petiole at the s tage when the first pair of leaves was fully developed. Significant differe nces in aggressiveness were observed among the 17 isolates. The parameters investigated clearly suggest the occurrence of a wide phenotypic variabilit y in Phoma macdonaldii.