Institute for Integrated Plant Protection, Stahnsdorfer Damm 81, D-14532 Kleinmachnow, Germany

Background and objectives
The complex responses of plants to abiotic stressors are described under the heading general adaptation syndrome (GAS). In previous experiments it was demonstrated that responses of plants to biotic stressors fundamentally follow the GAS. These responses of plants comprise not only resistance responses but also tolerance responses, including increasing productivity of the plant, with the aim of adaptation to the stressor [1]. In studies carried out on compatible host-parasite systems, it had been shown that during the adaptation processes, the plants not only compensate for the effects caused by pests organisms, but their productivity is also stimulated beyond the level of performance achieved by healthy plants. This means that not only the individual organs but the whole organism is temporarily raised to a higher level of productivity, as is shown clearly by 15N-tracer experiments and drawing up a 'balance sheet' for the whole plant [2]. A special case of tolerance was observed in the host-parasite relationship, wheat-Microdochium nivale. Here, the increased productivity was maintained to maturity. By inoculation at the heading phase, stimulation of physiological processes (nitrogen and carbohydrate metabolism, green leaf area) and dry matter production resulted in higher yields (6-33%) [1,2]. A close dependence of occurrence and intensity of stimulation responses on plant ontogeny and disease severity was shown in previous studies [1]. If it is possible to utilize this phenomenon of enhanced productivity in infected plants, it would be of some economic and environmental interest. However, further knowledge of the external and internal conditions under which it occurs is needed. Thus, the results of further studies on the dependence of stimulation reactions in our wheat-snow mould disease model system on site of inoculation, abiotic stressors (temperature, light) or antifungal compounds (Phenylpyrroles (Saphire), Strobilurines (Diskus), Bion) are presented.

Results and conclusions
All experiments were carried out using a special screening system for tolerance responses on whole plant level, based on a 15N-tracer technique [2]. Plants were grown in growth chambers (influences of abiotic stressors) or under roof under open air conditions. No any stimulation was observed after inoculation of second leaves from above and dry matter and yields were diminished after inoculation of stems. However, inoculation of other parts of the plant resulted in stimulation reactions. Additional abiotic stressors mostly reduced the yields. Low temperature and/or short-term darkness are better tolerated by our model system than higher temperatures and long light periods.

All compounds investigated influenced the pathogen directly and reduced disease severity. Saphire and Bion applied without inoculation influenced the yield directly (temporary and manifest enhancement) compared with the uninoculated, untreated control. Diskus had no any direct significant effect on yield. At maturity, plants treated with Saphire, Diskus or Bion and inoculated with M. ;nivale had significantly higher yields (+17.9%, +26.02% or + 22.28%, respectively) than the healthy, untreated plants. However, compared with inoculated, untreated plants, the yields in the plants with inoculation and fungicide application were temporarily significantly reduced (EC 83 Saphire, -1 1,2%; EC 75 Bion, -25.5%, Diskus: -31.2 %). Thus, despite the fact that Saphire and Bion had positive effects on yield and all three compounds reduced the severity, not the occurrence, of disease, the intensity of stimulation processes caused by M. ;nivale was reduced. This suggests that the tolerance potential of the plant is influenced by these compounds. Further investigation is urgently needed.

1. Seidel P, 1996. In: Lyr H, Russel PE, Sisler HD, eds. Modern Fungicides and Antifungal Compounds. Andover: Intercept, pp. 273-280.
2. Seidel P, 1 996, Plant Research and Development 44, 81-99.