The epidemiology of purple leaf blotch on leeks in Victoria, Australia

The incidence of purple leaf blotch disease was investigated on seven successive commercial leek crops grown at Cranbourne, Victoria between 1996 and 1997. First symptoms occurred on older leaves, 54-69 days after transplanting. Lesions with typical symptoms were colonised by either Alternaria porri (6%), Stemphylium vesicarium (42%) or mixtures of both pathogens (52%). Purple leaf blotch was caused by a disease complex and was endemic at nobreak Cranbourne due to the continuous cropping of leeks. Disease incidence in all monitored crops increased as plants matured (123-158 days after transplanting) until harvest but never exceeded 11% due to fortnightly applications of mancozeb. Disease levels showed no significant correlation with weekly temperature, precipitation, relative humidity or leaf wetness duration. Disease levels were significantly (P < 0.05) higher on autumn/winter (May/June) 1997 crops when 38 periods of leaf wetness greater than or equal to8 h because of dew and low temperatures (10-13 degreesC). The weekly rate of increase of disease incidence was significantly (P < 0.01) correlated with days after transplantation. nobreak Concentrations of airborne A. porri and S. vesicarium conidia within leek crops showed a diurnal periodicity and maximum numbers were trapped between 11:00 and 15:00 h. The concentration of airborne S. vesicarium conidia was three to six times the concentration of airborne A. porri conidia. Conidia were more abundant during spring/summer (September-February). Ascospores of Pleospora allii were found during May-September. The greater concentrations of airborne S. vesicarium conidia suggest that it may be the dominant pathogen in the purple leaf blight complex. Fungicide sprays were unnecessary until 8-10 weeks after transplanting, and regular protectant sprays curtailed but did not eradicate purple leaf blight. The results indicated that predictive models, based on temperature and the frequency of leaf wetness periods greater than or equal to8 h, will assist in reducing fungicide inputs as plants mature and, in southern Victoria, fungicide applications on leeks should be timed for autumn/winter when infection periods occur.