Lettuce drop caused by inoculum ofsclerotinia sclerotiorum blown-in from a distance

In seven fields free of local viable sclerotia, the part played bySclerotinia sclerotiorum ascospores transported from outside these fields in the incidence of lettuce drop was determined on plants not sprayed against the fungus. Ascospores were continuously deposited on the crop during the growth period and were found in low numbers in five of the fields (average of 0.4-3.5 per plate) and in high numbers in the two other fields (average values of 22 and 62 ascospores per plate) at each exposure in the two fields, respectively. Lettuce drop was 0-0.2% in the fields with the lower inoculum and 1.5 and 2.5% in those with the higher inoculum. Thus, even though plants were not sprayed, transported inoculum caused negligible yield losses.