2.10.3
THE SURVIVAL AND SAPROPHYTIC COMPETITIVE ABILITY OF THE BOTRYTIS SPP ANTAGONIST ULOCLADIUM ATRUM IN LILY CANOPIES

THE SURVIVAL AND SAPROPHYTIC COMPETITIVE ABILITY OF THE BOTRYTIS SPP. ANTAGONIST ULOCLADIUM ATRUM IN LILY CANOPIES

PAG ELMER1and J KÖHL2

1 HortResearch, Private Bag 3123, Hamilton, New Zealand; 2 DLO Research Institute for Plant Protection (IPO-DLO), P.O. Box 9060, 6700 GW Wageningen, The Netherlands

Background and objectives

The Botrytis spp. antagonist, Ulocladium atrum has consistently reduced B. cinerea sporulation on necrotic lily (Lillium spp) leaves in the field by up to 90% [1] and therefore has the potential to be an effective biological control agent. However, when U. atrum conidia are sprayed onto a crop, the majority of antagonist inoculum is deposited onto healthy green tissues, but the primary target is necrotic leaf parts [2]. We hypothesised that if sufficient viable antagonist inoculum was present on green tissues just prior to the onset of necrosis then rapid colonisation by the antagonist may exclude Botrytis spp. and or other competitive saprophytes. Therefore, the level of viable antagonist inoculum on green leaves may be important where biological control is aimed at colonisation of necrotic tissues. Our objective was to investigate the ability of U. atrum to survive, colonise artificially induced necrotic leaf tissues and compete against B. elliptica and naturally occurring saprophytes.

Materials and methods

Conidia of U. atrum were applied (2 X 106 ml -1) to green lily leaves in the field in 1995. At intervals (maximum=21 days) after U. atrum application, green leaves were sampled from each of three canopy levels (top, middle, bottom). Conidia density and field germination (the percentage of conidia that had germinated on the leaf surface) was measured after half leaf sections were stained with Calcofluor and examined with a Fluorescence microscope. The other half of each leaf section was incubated in optimal conditions for spore germination for 18h and the percent spore germination, termed germination potential (GP), determined as described above. A method was developed, based on the desiccant herbicide, paraquat, which induced localised necrosis on lily. Tagged leaves treated with paraquat were sampled at intervals (maximum=21 days) after the initial U. atrum application. Fourteen days after each paraquat treatment, necrotic leaves were sampled and the area of necrotic tissue with sporulating U. atrum and other commonly occurring saprophytes measured after incubation in humidity chambers.

Results and conclusions

U. atrum conidia density cm-2 at the top and middle canopy levels was not significantly different immediately following antagonist application, but significantly fewer conidia were deposited onto leaves in the lower canopy level. There was a significant (P < 0.001) linear decline of U. atrum conidia over time. Conidial viability, measured as germination potential (GP), declined slightly (100 % to 88 %) after seven days and thereafter there were no further changes in GP, even after 21 days of field exposure. U. atrum colonisation of necrotic tissues, artificially induced with paraquat, was consistently highest on necrotic leaves at the top level of the canopy and consistently lower on leaves from the bottom canopy level. Necrotic leaf colonisation by U. atrum decreased over time from 51 % to 21 % when necrosis was induced 21 days after antagonist application. There was a significant (P < 0.001) linear relationship (R2 = 0.713) between colonisation of necrotic tissues and conidia density prior to induction of necrosis. When necrosis was induced immediately after antagonist application, U. atrum outcompeted commonly occurring saprophytic Alternaria spp. and Cladosporium spp. Botrytis spp. was not detected in these field experiments due to prolonged dry conditions. We conclude that U. atrum has the ability to survive and persist in the phyllosphere for up to 21 days in the field and provided further evidence that U. atrum has the necessary survival characteristics to be a successful biological control agent of Botrytis spp. in various crops.

References

1. Köhl J, Molhoek WML, van der Plas CH and Fokkema NJ 1995. Phytopathology 85: 393-401

2. Elmer PAG, Köhl J 1998. European Journal of Plant Pathol (In press)