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

Background and objectives Strawberry is grown in a wide variety of environmental conditions. its fragrant and tasty fruits are highly susceptive to Botrytis cinerea. The suppression of Botrytis spp. sporulation on necrotic tissue is a valid biocontrol strategy [1] when the inoculum is built up inside the crop as demonstrated in strawberry [2]. The ability of the saprophytic fungus Ulocladium atrum to suppress B. ;cinerea on strawberry was studied under field conditions.

Material and methods In a bioassay necrotic sterilized naturally senesced leaflets and dead detached stamens were sprayed with U. ;atrum and B. ;cinerea at 106 conidia/ml, with various intervals between the sprays of U. ;atrum and B. ;cinerea. After incubation at 180C for 14 ;days, sporulation of B. ;cinerea was assessed. Field experiments were carried out from 10 April to 17 July and another from 27 June to 25 August 1997 with 'cold-stored' strawberry plants, cv. Eisanta, as an annual crop. Treatments were: (1) control (untreated); (2)standard treatment (fungicide as recommended in practice); (3) U. ;atrum, weekly spraying from planting till harvest time with a spore suspension of 2X106 conidia/ml; (4) U. ;atrum, weekly spraying at flowering only, with a spore suspension of 2 x 106 conidia/ml. Weather conditions and the plant phonology were monitored. During flowering sporulation of B. ;cinerea on dead leaves was assessed. Harvested fruits were classified as market quality (healthy and big), industry (small and bad shape) and diseased (B. ;cinerea and other pathogens).

Results and conclusions In bioassays, spraying U. ;atrum before, simultaneously or up to 12 ;h after B. ;cinerea reduced the number of conidiophores of B. ;cinerea on the stamens by 70-95%. On necrotic leaves, spraying U. ;atrum 24 ;h or longer before B. ;cinerea reduced sporulation of B. ;cinerea by 30%. Spraying U. ;atrum at the same time as B. ;cinerea or later did not reduce the sporulation of B. ;cinerea on necrotic leaf tissue. In field conditions, sporulation of B. ;cinerea on dead strawberry leaves varied, but at low level, and with only little effect of treatments. Botrytis fruit rot was significantly reduced by spraying U. ;atrum weekly (2.6% rot) and at flowering time only (3.4% rot) compared with the control (5.4% rot) (experiment 1). In field experiment 2, fruit attack by B. ;cinerea was much less than in the first experiment and there was no difference among the treatments. Nevertheless, the sporulation of B. ;cinerea on dead strawberry leaves was higher than in the first experiment. It is supposed that high temperatures during flowering and harvest prevented infection of B. ;cinerea.

Phenological data show that at the beginning of flowering the number of senescing and necrotic leaves increased quickly in both experiments. This might be an important source to build up the inoculum in the field, and thus an important target for antagonist application. Results show that U. ;atrum reduces spore production of B. ;cinerea on necrotic leaves and stamens. Control of strawberry fruit rot was similar to standard fungicide treatment. Knowledge of the occurrence of necrotic tissues in annual strawberry crops may be the basis for a further improvement of timing of antagonist applications.

1. Kohl J, Molhoek VVML, van der Plas CH, Fokkema NJ, 1995. European Journal of Plant Pathology 101, 251-259.
2. Sutton, JC, 1995. Advances in Plant Pathology 11, 174-190.