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

Background and objectives
Microbial antagonists can be aimed at different targets in the life cycle of foliar pathogens and applications may result in (1) the reduction of survival structures, (2) the suppression of spore production, or (3) the prevention of infection. The choice of the strategy depends on the intensity of the interaction between pathogen and antagonist, but also on the duration of their interaction. Suppression of sporulation of the pathogen will reduce the progression of epidemics. This approach allows a longer interaction period between pathogen and antagonist as compared to interactions during the infection process and is successfully applied in the control of biotrophic pathogens such as powdery mildews. The exploitation of this approach was studied for the biological control of necrotrophic pathogens such as Botrytis spp., which exclusively sporulate on necrotic tissues.

Material and methods
Saprophytic fungi isolated from necrotic leaves were screened in bioassays on dead onion leaves for their efficiency in suppressing sporulation of Botrytis spp. The effect of interruption of leaf wetness period, temperature and host tissues was subsequently tested for selected antagonists. Antagonists were also tested on dead leaves exposed to field conditions for their ability to survive and to compete with naturally occurring saprophytic fungi including Botrytis spp. [1]. Conidial suspensions of Ulocladium atrum were applied in field experiments with onion and strawberry and in cyclamen grown in commercial greenhouses. The effect of such treatments on inoculum production of Botrytis spp., and disease development was assessed. The survival of U. ;atrum propagules on green and dead leaves was studied.

Results and conclusions
Among Alternaria spp., Gliociadium spp., Trichoderma spp. and other isolated saprophytes, Ulocladium atrum 385 were found to meet the selection criteria best: U. ;atrum efficiently suppressed sporulation of B. ;cinerea, B. ;aclada, and B. ;elliptica in bioassays. High antagonistic efficacy of this antagonist was found in a broad temperature range (6-24C), on dead leaf tissues of various hosts and even after interruptions of the leaf wetness period. Dead lily leaves exposed to field conditions were colonized by the antagonist and colonization of such leaves by B. ;cinerea was consistently reduced by more than 90% [1]. The antagonism of U. ;atrum is most likely based on nutrient competition [2]. In field-grown onion, sporulation of Botrytis spp. on dead leaf tissue was reduced by weekly applications of U. ;atrum. Significantly fewer leaf spots caused by Botrytis spp. were found at the end of the growing season compared with the control. The effect of the antagonist did not differ from the standard fungicide treatment. In strawberry, weekly applications of U. ;atrum significantly reduced fruit losses caused by B. ;cinerea as efficiently as standard fungicide treatments in several field experiments. U. ;atrum was also tested on cyclamen in commercial greenhouses for its potential to prevent leaf rot caused by B. ;cinerea. Three to five applications of U. ;atrum at intervals of 4 ;weeks resulted in significant reductions of disease severity in several experiments. Applications of fungicides by the growers did not result in better disease control than obtained with U. ;atrum applications. The antagonist colonized dead tissues of the different crops under field and greenhouse conditions. Propagules of the antagonist survived on green leaves for at least 10 ;weeks under greenhouse conditions and for at least 3 ;weeks in the open field.

U. ;atrum is an attractive candidate for biocontrol of Botrytis spp. and possibly other necrotrophic pathogens in various crops.

1. Kohl J, Molhoek WML, van der Plas CH, Fokkema NJ, 1995. Phytopathology 85, 393-401.
2. Kohl J, Belanger RR, Fokkema NJ, 1997. Phytopathology 87, 634-642.