EFFECTIVENESS OF NOVEL FUNGICIDES ON POWDERY MILDEW DEVELOPMENT AND SPORE PRODUCTION IN WINTER WHEAT UNDER DRY WEATHER CONDITIONS M JAHN Institute for Integrated Plant Protection, Stahnsdorfer Damm 81, D-14532 Kleinmachnow, Germany Background and objectives In the Nineties, novel antifungal compounds have been developed and some of them have been introduced into agricultural practice. The group of strobilurine fungicides seems to become most important to control cereal diseases nowadays. Furthermore, novel agents different from fungicides in their mechanism of action have been discovered. A new generation of crop protection agents is being developed which activates plant defense mechanisms. The first representative of these plant activators is 'Bion'. A lot of information already exists on the mode of action, on physiological side effects, and also on the effectiveness against important plant pathogens. More experience is necessary however, on the effectiveness of these new compounds under different environmental conditions and in cultivars with different resistance background to pathogens like Erysiphe graminis. Also the side of epidemiology following such treatments has not yet been considered. Materials and methods Field trials were made in randomised complete block design. In 1996, 4 cultivars were used with different susceptibility to powdery mildew and different resistance background, namely: Glockner / 5 (mark for varietal susceptibility to mildew, according to Descriptive List of Varieties, resistance gene Pm4b), Kontrast / 4 (Pm5), 'Xanthos' / 3 (Pm5 + 6), Zentos / 3 (no resistance gene). In 1997, only the two cvs. Kontrast and Zentos were grown. Plots were treated with Brio (kresoxim-methyl + fenpropimorph, 1996 only) and Discus (kresoxim-methyl) in GS 37-39. Bion (CGA 245704) was applied in GS 29-31. The disease severity was visually assessed on upper three leaves several times in the vegetation period. Spores were collected using trap plants of the susceptible cv. Kanzler. The trap plants in the first leaf stage were placed into the plots for 24 hours. Spores were sampled three or four times in the season, before and after treatments. The amount of spore production was determined indirectly by assessment of disease severity on the trap plants 7 days after putting them into the plots. Single spore isolates were also produced as well virulence analyses. Results and conclusions In both years, the months March to June were characterized by very dry weather conditions. The rainfall amounted to approximately 50 % of many years´ mean. In the most important month, June, it was as low as 30 % of the long-time mean. In 1996, distinct mildew disease pressure arose. The least development was found with the cultivar with partial resistance (Zentos). Mildew development was near to zero in all plots treated with Brio. Later on in the vegetation period Septoria nodorum also developed. Even though Brio was hardly effective against this disease, the yield increased significantly in all 4 cultivars by up to 20 % to untreated. In 1997, the disease hardly developed. Mildew disease was only detectable with the cv. Kontrast. Treatments with Bion or Discus completely controlled powdery mildew. But there was nearly no effect on Septoria tritici which developed to a remarkable degree. In the cv. Kontrast, the yield was also distinctly increased to untreated by 10 % (Bion) and more than 20 % (Discus). No effect was found on the yield of the cv. Zentos. As expected, mildew spore production corresponded to the disease incidence. In 1996, there was a clear difference in untreated plots between the 3 cultivars with specific resistance on the one hand and the cultivar with partial resistance on the other hand at the beginning of June. After that spore production strongly increased in all cultivars. In plots treated with Brio spore production was significantly reduced in all cultivars until the end of the investigations. In 1997, spore production in both cultivars was relatively low. Nevertheless, the treatments reduced spore production. Discus was highly effective on both cultivars, whereas Bion was distinctly more effective in the cv. Zentos than in the cv. Kontrast. It seems to confirm that the plant activator Bion is a good supplement or support for the partial resistance of cultivars. Field trials of the last years have shown that the new compounds can well be used in winter wheat also under dry weather conditions. Treatments early in the season can reduce spore production of the powdery mildew pathogen Erysiphe graminis and can have influence on the epidemiology of the disease.