The persistence of conidia and competitive ability of Ulocladium atrum, an antagonist of Botrytis cinerea, were studied in field plots of strawberry in The Netherlands. Single application of conidial suspensions (2 ± 106 conidia ml-1) of U. atrum were sprayed on the canopy and repeated 14 times, each in a different plot during the spring, summer or early winter of 1997 and 1998. Leaf samples were taken approximately weekly in the summer crops and monthly in the overwintering crops up to 30-120 days after application, to assess total conidial density, conidial viability, and competitive colonization of necrotic leaf tissue by U. atrum in relation to naturally occurring B. cinerea and other necrotrophic fungi. The total density of U. atrum conidia on green strawberry leaves declined exponentially after application by an average relative rate of -0.10/day. Variation in the rate of decrease between individual sprays was large and partly associated with the average daily precipitation during the first week after application. For pooled data, percentage viability of U. atrum conidia declined linearly at a rate of 1ýay after application. Colonization of necrotic strawberry leaf tissue by U. atrum increased with total conidial density and, at densities > 1500 conidia cm-2, leaf colonization by naturally occurring B. cinerea was consistently reduced. With the exponential decline in total conidial density of U. atrum, effective suppression of B. cinerea on strawberry leaves can only be expected when U. atrum is applied less than one week before the leaves become necrotic. Further research needs to focus on enhanced rain-fastness of deposited conidia of U. atrum to reduce spore loss.