Infection of onion leaves by Alternaria porri and Stemphylium vesicarium and disease development in controlled environments

Infection of onion by Alternaria porri and Stemphylium vesicarium was investigated under a range of controlled temperatures (4-25 degrees C) and leaf wetness periods (0-24 h). Conidia of A. porri and S. vesicarium germinated within 2 h when incubated at 4 degrees C. Terminal and intercalary appressoria were produced at similar frequencies at or above 10 degrees C. The maximum number of appressoria was produced after 24 h at 25 degrees C. Penetration of leaves by both pathogens was via the epidermis and stomata, but the frequency of stomatal penetration exceeded that of epidermal penetration. There was a strong correlation (R-2 > 90%) between appressorium formation and total penetrations at all temperatures. Infection of onion leaves occurred after 16 h of leaf wetness at 15 degrees C and 8 h of leaf wetness at 10-25 degrees C, and infection increased with increasing leaf wetness duration to 24 h at all temperatures. Interruption of a single or double leaf wetness period by a dry period of 4-24 h had little effect on lesion numbers. Conidia of A. porri and S. vesicarium separately or in mixtures caused similar numbers of lesions. Alternaria porri and S. vesicarium are both potentially important pathogens in winter-grown Allium crops and purple leaf blotch symptoms were considered to be a complex caused by both pathogens.