Effect of environmental and host factors on Botrytis leaf blights of onion

The effect of host and environmental factors on the infection on onions by Botrytis squamosa was studied in controlled environment. Leaf dieback and sporulation on infected leaves was also examined. At temperatures ranging from 9 to 24°C, no lesions developed on inoculated onions after 6 h leaf wetness. Lesion numbers increased as leaf wetness duration increased from 9 to 48 h except at 24°C where lesion numbers did not increase at leaf wetness durations >12 h. Of the temperatures examined, 18°C was the optimum for lesion production. At 15 and 21°C, lesion frequency increased as inoculum concentration was increased from 1,000 to 10,000 spores/ml. Increasing the inoculum concentration to 20,000 spores/ml did not increase lesion frequency. Lesion frequency was similar whether a total inoculum dose of 300,000 spores/ml was applied at one time or split into 3 applications within a period of 6 days. Exposure of spores on leaves to dry air for 4 h after 4 or 8 h wetness was sufficient to reduce lesion frequency. Germination of spores incubated in water on watch glasses for 2, 4 or 6 h was halted by 1 h of dryness. Preinoculation wetness of 0, 1 or 24 h did not affect lesion frequency. Partial removal of onion leaf wax did increase lesion frequency but only on plants not subjected to concomitant preinoculation wetness. Infection of lower leaves did not affect the susceptibility of upper leaves to subsequent infection. Preinoculation moisture stress, if sufficient to cause wilting, reduced lesion frequency on inoculated onions but increased sporulation intensity. Young onion plants (<6-8 weeks old) grown from seed were more susceptible to infection by B. squamosa than were older plants. On onions with <4 leaves lesion frequency was greater on older than on younger leaves. Plant age did not affect sporulation intensity.Under the controlled environment conditions used in these studies, leaf dieback did not differ significantly from that on noninoculated onions.The data were used to revise the "infection values" assigned to the various combinations of temperature and leaf wetness duration that affect infection of onion by B. squamosa. Results also indicate that weather data to be used for predicting leaf blight development should be collected starting at the time the onions emerge from the soil.