5.6.3
THE IMPACT OF STROBILURINS ON THE PLANT PHYSIOLOGY OF WHEAT

J HABERMEYER, M GERHARD and V ZINKERNAGEL

Technical University of Munich, Freising-Weihenstephan, Germany

Background and objectives
The new strobilurin containing fungicides with the active ingredients Kresoxim-Methyl and Azoxystrobin are well known to carry out good fungicidal performance. Beside these fungicidal effects some physiological side effects concerning plant growth and senescence are discussed. Therefore a research program was initiated to get an insight in these non-target effects of strobilurins on cereals. As physiological experiments with greenhouse plants are sometimes contradictory and artifically field experiments with disease free wheat plant were carried out.

Materials and methods
In 1996 and 1997 wheat plants cvs Astron, Batis, Bussard, were sprayed three times with non-strobilurin fungicides at the locations Roggenstein (near Munich) and Weihenstephan. Additionally we added kresoxim-methyl resp. azoxystrobin at five different growth stages in fungicide concentrations from 50 to 800% of the recommended dose. Plants treated in such a manner were used for physiological studies.

The possibly altered phytohormone level was measured with the parameter ethylene production by aid of gas chromatography. As Grossmann and Retzlaff [1] proposed for controlled conditions, a change in the carbon dioxide compensation point after applying kresoxim-methyl gas exchange and chlorophyll fluorescence measurements were carried out in the field. The possibly changed photosynthetic activity on biomass production and stress tolerance of wheat plants should be investigated. The above-ground biomass production after strobilurin use was assessed weekly. The nitrogen and carbon content as well as translocation proceedings were studied by using an elementary analyser and mass spectrometer.

Results and conclusions
Strobilurin treated wheat plants appeared darker green dependent on the applied strobilurin intensity. In 1997 wheat flag leaf size increased significantly in cv. Astron and only in tendency in cv. Batis in kresoxim-methyl and azoxystrobin treated plots. These fungicides significantly decreased ethylene formation of draught stressed flag leaves correlating to senescence retardation and necrotic leaf area. Compared to untreated and convential sprayed plants strobilurin treated leaves reached the same level of ethylene formation (as a marker for relative senescence) about two resp. 1 week later.

No significant correlation between the intensity of ethylene production in GS 75 on flag leaves and the timing of strobilurins (varying between GS 30 and GS 59) was remarkable, i.e. ethylene formation was reduced (compared to convential sprayed plants) to the same level independant from application timing. This 'memory-effect' resulted in corresponding yield responses.

Chlorophyll fluorescence studies showed greater stress tolerance of strobilurin treated plants especially concerning draught stress. Stomatal regulation is significantly changed in strobilurin plots over a whole day measuring period.

In conclusion, kresoxim-methyl or azoxystrobin induce severe physiological changes in field wheat plants. In combination they seem to enable plants for better adaption to environment. In most cases this leeds to an average yield increase up to 5-10%.

References
1. Grossmann K, Retzlaff G, 1997. Pesticide Science 50.