CHANGES IN SUSCEPTIBILITY OF BEAN-LEAVES (PHASEOLUS-VULGARIS) TO SCLEROTINIA-SCLEROTIORUM AND BOTRYTIS-CINEREA BY PRE-INOCULATIVE OZONE EXPOSURES

The effects of ozone on the susceptibility of leaves of Phaseolus vulg aris to Sclerotinia sclerotiorum and Botrytis cinerea have been invest igated. Seedlings of one ozone-sensitive ('Pros') and five relatively ozone-insensitive cultivars ('Gamin', 'Precores', 'Groffy', 'Narda', ' Bema') were exposed to different ozone concentrations (0, 120, 180 and 270 mug m-3) for 8 h. One day after the exposures, primary leaves wer e detached and immediately inoculated with spores of either pathogen s uspended in water or in a 62.5 mM KH2PO4 (Pi) solution. Visible ozone injury differed between the cultivars and increased with increasing oz one concentration. On the leaves of non-exposed plants, spores of the pathogens suspended in water caused very few lesions, whereas fungal p athogenicity was stimulated by addition of Pi to the inoculum. Ozone-i njured leaves of all cultivars exhibited lesions after inoculation of the leaves with the pathogens suspended in water, and the number of le sions was positively correlated with the level of ozone injury for eit her pathogen and cultivar. The increase in susceptibility of bean leav es in response to increasing ozone concentrations was greater for B. c inerea than for S. sclerotiorum when spores were suspended in water, b ut was similar when the spores were suspended in Pi. In general, the n umber of lesions following inoculation with spores in Pi increased wit h increasing ozone concentration. However, the number of lesions in th e ozone-insensitive 'Groffy' was reduced by an exposure to 120 mug m-3 but increased with higher concentrations. This pattern of susceptibil ity response to the pathogens was not found in the other ozone-insensi tive cultivars and, thus, did not appear to be related to the inherent ozone insensitivity in bean.