Factors influencing infection of onion leaves by Alternaria porri and subsequent lesion expansion

Alternaria porri conidia deposited on onion leaves formed one to several germination tubes and appressoria and often penetrated at more than one locus under conditions favorable for infection. After 3 h in the dew chamber at 24 degrees C following inoculation of onion leaves, 73% of conidia had germinated and 5% had formed appressoria. Infection hyphae were not observed until 6 h following inoculation, at which time 2% of conidia had formed infection hyphae and 0.5% of conidia had caused visible lesions. Length of dew period was significantly positively correlated with lesion numbers (r = 0.784) but not with lesion size. There were two types of lesions: expanding and nonexpanding (flecks of less than or equal to 2 mm in diameter). Expanding lesions resulted even when plants inoculated with dry conidia of A. porri were incubated in a growth chamber under conditions not conducive to infection for 4 days prior to being placed in a dew chamber for 24 h under conditions conducive to infection, indicating that conidia survived well under these conditions. Flecks formed concurrently with expanding lesions. Germinated conidia were inevitably found near the centers of flecks, but, for unknown reasons, these flecks did not continue to expand.