Conidial persistence and competitive ability of the antagonist Ulocladium atrum on strawberry leaves

The persistence of conidia and competitive ability of Ulocladium atrum, an antagonist of Botrytis cinerea, were studied in Weld plots of strawberry in The Netherlands. Single application of conidial suspensions (2 +/- 10(6) c onidia 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 199 7 and 1998. Leaf samples were taken approximately weekly in the summer crop s and monthly in the overwintering crops up to 30-120 days after applicatio n, to assess total conidial density, conidial viability, and competitive co lonization of necrotic leaf tissue by U. atrum in relation to naturally occ urring B. cinerea and other necrotrophic fungi. The total density of U. atr um conidia on green strawberry leaves declined exponentially after applicat ion by an average relative rate of -0.10/day. Variation in the rate of decr ease between individual sprays was large and partly associated with the ave rage daily precipitation during the first week after application. For poole d data, percentage viability of U. atrum conidia declined linearly at a rat e of 1%/day after application. Colonization of necrotic strawberry leaf tis sue by U. atrum increased with total conidial density and, at densities >15 00 conidia cm 22, leaf colonization by naturally occurring B. cinerea was c onsistently 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 lea ves become necrotic. Further research needs to focus on enhanced rain-fastn ess of deposited conidia of U. atrum to reduce spore loss.