EFFECTS OF VARIOUS OZONE EXPOSURES ON THE SUSCEPTIBILITY OF BEAN-LEAVES (PHASEOLUS-VULGARIS L) TO BOTRYTIS-CINEREA

The effects of various ozone exposures in predisposing bean leaves (Ph aseolus vulgaris L.) to Botrytis cinerea have been investigated under laboratory conditions. Seedlings of two bean cultivars were exposed to incremental ozone concentrations (120, 180 and 270 mug m-3 for 8-h da y-1) for five days and primary leaves were subsequently inoculated wit h conidia suspended in water or in an inorganic phosphate solution (Pi ), and with mycelium. Ozone injury increased with increasing ozone con centration and was much higher in the ozone-sensitive cultivar 'Pros' than in the ozone-insensitive 'Groffy'. Ozone only increased the numbe r of lesions on leaves of Pros after inoculation with either of the co nidial suspensions. The Pi-stimulated infection in Groffy was reduced by the lower ozone concentrations. Ozone decreased lesion expansion af ter inoculation with mycelium. In a chronic fumigation experiment, pla nts of the two cultivars were exposed to 90 mug m-3 (7-h day-1) and th e primary and the oldest three trifoliate leaves were inoculated after five and seven weeks of exposure. Ozone enhanced the senescence-relat ed injury only in Pros. The number of lesions was not influenced by oz one for either cultivar, conidial suspension or inoculation date. Lesi on expansion after inoculation with mycelium was generally reduced in exposed plants. Thus, contrasting effects of ozone on the susceptibili ty of bean leaves to B. cinerea were observed depending on the cultiva r, the conidial suspension, the disease parameter and the ozone exposu re pattern. In extrapolating the laboratory results to the field, it i s suggested that episodic and chronic exposures to ambient ozone are o f minor importance in increasing the susceptibility of bean leaves to B. cinerea.