5.2.31
BIOLOGICAL CONTROL OF BOTRYTIS CINEREA IN COMMERCIALLY GROWN CYCLAMEN BY ULOCLADIUM ATRUM

J KOHL, G GRIT, M GERLAGH and BH DE HAAS

DLO-Research Institute for Plant Protection (IPO-DLO), PO Box 9060, 6700 GW Wageningen, The Netherlands

Background and objectives
Botrytis cinerea can cause severe damage of leaves and flowers of cyclamen (Cyclamen persicum), reducing its ornamental value. The use of fungicides for disease control is limited by possible side effects of fungicides on plant development and fungicide resistance of the pathogen. Because of the high resistance of healthy leaves against B. cinerea infection, only a high inoculum pressure of the pathogen leads to successful infections. Naturally senesced leaves within the canopy are a prerequisite for disease initiation. B. cinerea can colonize such leaves saprophytically and subsequently infect adjacent healthy petioles or leaves. The fungal saprophyte Ulocladium atrum has been selected for its strong competitive ability during the colonization of necrotic leaf tissues and its high resistance to environmental stress such as interruptions of leaf wetness periods [1]. Experiments under controlled conditions have shown that U. atrum can colonize necrotic cyclamen leaves leading to an exclusion of B. cinerea from the substrate and that propagules of the antagonist can survive on dry surfaces of cyclamen leaves for at least 10 weeks under greenhouse conditions [2].

The objective of our study was to investigate the potential of U. atrum to protect cyclamen plants from B. cinerea damage under commercial greenhouse conditions.

Material and methods
In total, 10 experiments were carried out in six different greenhouses during spring and autumn of 1997. Varieties and growing conditions differed considerably between growers, e.g. water was provided via wet mats from the bottom in some greenhouses and plants were irrigated overhead up to three times per week in others. U. atrum conidia were produced on oat kernels and conidial suspensions (1x106 conidia/ml) were prepared. Experiments started with 17 to 20-week-old plants. The treatments, each with four replications consisting of 24 plants, were: (i) two U. atrum applications at the beginning of the experiment at an interval of four weeks; (ii) three to five U. atrum applications at intervals of four weeks from the beginning of the experiment until four weeks before marketing; (iii) water applications at four-week intervals; (iv) no applications. In six experiments, fungicides were applied as a fifth treatment, according to the grower's standards. The disease severity (DS) resulting from natural infection of plants at marketable age was assessed as the number of petioles or leaves per plant showing B. cinerea sporulation.

Results and conclusions
In seven experiments, DS ranged between 0.6 and 3.6 in the untreated control. Two applications of U. atrum reduced DS significantly by 56 to 83% in four out of the seven experiments. Three to five applications of U. atrum resulted in significant reduction of DS by 39 to 80%, in all cases. Applications of fungicide by the growers did not result in better disease control than obtained with U. atrum applications. Three experiments were conducted in the same greenhouse with conditions very favourable for B. cinerea development, resulting in DSs between 7.1 and 1 1.0 in the control treatment. Treatments with U. atrum or two applications of iprodione did not reduce DS significantly. The application of water alone did not affect DS. Overhead irrigation seemed not to have a detrimental effect on the performance of the antagonist.

It can be concluded that U. atrum controls B. cinerea in cyclamen under practical growing conditions to a similar extent to standard fungicides.

References
1. Kohl J, Molhoek VVML, van der Plas CH, Fokkema NJ, 1995. Phytopathology 85, 393-401.
2. Kohl J, Geriagh M, de Haas BH, Krijger MC, 1998. Phytopathology (in press).