A. Roustaee et al., Phenotypic variability of Leptosphaeria lindquistii (anamorph : Phoma macdonaldii), a fungal pathogen of sunflower, PLANT PATH, 49(2), 2000, pp. 227-234
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.