Biological control of Botrytis cinerea in roses by the antagonist Ulocladium atrum

Botrytis cinerea is a troublesome pathogen in roses. Pot-roses are produced in a cycle of less than three months. Cuttings are rooted, four per pot. After three weeks they are pruned, and this is repeated three weeks later. They ramify and produce flowers within six weeks after the last pruning. During rooting and after pruning, wounds and senescing leaves are available as a substratum for B. cinerea. Infection may lead to death of cuttings and abundant sporulation of B. cinerea on killed plantlets and dropped leaves. The production of conidia on leaf residues enhances the danger of contamination of cuttings and latent flower infection in the greenhouse. The possible role of the saprophytic fungus Ulocladium atrum as a competitor in colonisation of necrotic tissue of pot-roses has been examined in two trials with cultivar Moonlight. A few days after each pruning plants were sprayed until run-off with a spore suspension (1000000 conidia per ml) of U. atrum in water with 0.01/ Tween 80 in comparison to a control and fungicide treatment. In the second trial mixtures of a low dose of fungicide and U. atrum were also applied. There were four replicates of 28 pots for each treatment. Just before the second pruning, foci of B. cinerea on dead leaves on the plant and on the soil of the pot were counted for each pot. The second assessment was at a few days before sale of the flowering plants. Intensity of sporulation of B. cinerea on dead leaves, mostly on the soil, was scored. In the first trial only U. atrum was effective in reducing the sporulation of B. cinerea with over 50/ compared to the control. Death of cuttings by B. cinerea was negligible. In the second trial U. atrum provided 60/ reduction of sporulation of B. cinerea, and the fungicide was highly effective. The reduction of sporulation by U. atrum or the fungicide will reduce sporeload of B. cinerea in the air of the greenhouse. High numbers of B. cinerea conidia in the air are responsible for infection of pruning wounds and contamination of flowers, which may result in infection and rotting of flowers under disease promoting conditions. The present trials also point to a possible role of U. atrum in resolving a problem in cut-roses, where some batches are so strongly contaminated by conidia of B. cinerea originating from decomposing pruning material and senescing leaves on the soil that flowers may develop post-harvest grey mould symptoms.