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

Background and objectives
The fungal saprophyte Ulocladium atrum is a potential antagonist suppressing the colonization of necrotic plant parts by Botrytis spp. and the subsequent sporulation of such pathogens. For the optimization of field applications of the antagonist, information on the effect of environmental factors on growth, survival and antagonistic activity is needed. It is known that U. ;atrum is resistant to interruptions of leaf wetness periods [1] and that propagules of U. ;atrum have a high potential to survive on leaf surfaces in the field [2]. No information in literature is available on the temperature requirements of this fungus. The objective of our study was to investigate the effect of temperature on conidial germination, mycelial growth and antagonistic potential of U. ;atrum under controlled conditions.

Materials and methods
The speed of conidial germination of U. ;atrum at temperatures between 3C and 36C was assessed on water agar. Mycelial growth rates of U. ;atrum and Botrytis spp. at temperatures between 3C and 36C were determined on malt extract agar. Bioassays were carried out on dead onion leaf segments in moist chambers. Conidial suspensions of B. ;cinerea (1X103 or 1X104 conidia/ml) or, B. ;aciada (1X105 conidia/ml) were sprayed. After leaves had been incubated moist for 24 ;h at 18C to allow the establishment of the pathogens in the substrate, U. ;atrum (1X106 conidia/ml) was sprayed on the leaf segments, which were subsequently incubated at temperatures between 6C and 24C. The sporulation intensity of Botrytis spp. was assessed after 3 ;weeks.

Results and conclusions
The optimum temperature of U. ;atrum for conidial germination was between 27C and 30C . To reach germination rates of 50%, conidia of U. ;atrum needed only 2.6 ;h at optimum temperature, less than 6 ;h at temperatures above 15C and only 18 ;h at 6C . Mycelial growth rate of U. ;atrum was almost similar in the range between 18C and 30C, with an optimum at 27C, and there was no sharp decease of growth rate at lower temperatures. Although U. ;atrum was more thermophilic than Botrytis spp., the temperature requirements of this antagonist for mycelial growth almost overlapped with those of Botrytis spp.

Sporulation of B. ;cinerea and B. ;aciada in bioassays was significantly suppressed by U. ;atrum more than 85% at all temperatures. There was no effect of temperature on the outcome of the antagonistic interaction. In the field, the competitive colonization of necrotic leaf tissues by Botrytis spp. and U. ;atrum depends on leaf wetness periods (LWP). Under Dutch growing conditions, the mean air temperature during LWP in onion fields ranged between 6C and 23C, with an average of 15C.

It can be concluded that the high germination speed of conidia of U. ;atrum in the entire range of pertinent temperatures allows the fungus to colonize necrotic leaf tissues in the field rapidly during the limited periods of leaf wetness. Since strong antagonistic activity against Botrytis spp. was found in the entire relevant temperature range, it can be expected that the antagonist also suppresses substrate colonization by Botrytis spp. and sporulation of the pathogens under field conditions.

1. Kohl J, van der Plas CH, Molhoek WML, Fokkema NJ, 1995. European Journal of Plant Pathology 101, 627-637.
2. Elmer PEG, Kohl J, 1998. European Journal of Plant Pathology, in press.