5.6.2S
USE ATTRIBUTES OF STROBILURIN ANALOGS FOR DISEASE CONTROL

LISE NISTRUP JØRGENSEN

Danish Institute of Agricultural Sciences, Research Center Flakkebjerg, DK-4200 Slagelse

Strobilurin analogs are a novel group of fungicides under development since the early 1980s. These fungicides are derived from secondary metabolites produced by fungi such as Strobilurus tenacellus and Oudemansiella mucida [1]. The naturally occurring strobilurins have been chemically modified to produce analogs which have been optimized for better biological, physical, and environmental properties. Particular problems with photochemical instability and volatility of naturally occurring strobilurins have been overcome in the three announced compounds: azoxystrobin, kresoxim-methyl and metominostrobin.

Their mode of action is inhibition of mitochondrial respiration by blocking electron transfer between cytochrome b and cytochrome c1 [1]. No cross resistance has been found between the synthetic strobilurins and agricultural fungicides already in use. This class of compounds is remarkable for its broad-spectrum activity against many foliar pathogens from the Ascomycetes, Basidiomycetes, Deuteromycetes, and Oomycetes in cereals, rice, pome fruits, grapevine, vegetables, and turfgrass. Their uptake, translocation, residual, and metabolism properties provide protective, curative, and some eradicant disease control.

Individual biokinetic properties of the synthetic strobilurins, such as uptake, transport, and metabolism, can explain some of the differences in selectivity to specific pathogens. Kresoxim-methyl is not very effective on pathogens living internally in plants due to rapid metabolism of the product within leaf tissue. The product has vapour phase activity and is very active on pathogens like Erysiphe graminis living on the plant surface. In contrast, azoxystrobin, which has some foliar uptake, moves systemically in xylem [2]. Therefore it gives better curative control of pathogens living internally in plant tissue such as Puccinia spp. and Stagonospora nodorum. Metominostrobin is reported to provide excellent preventive and curative control of rice blast (Pyricularia grisea) using slow-release, granular formulations in paddy water.

Compared to fungicides already in use, strobilurin analogs have shown potential for enhanced efficacy and yield responses for several diseases/crops. Additional yield increases of 5-10% are quite common in cereals compared to those obtained with traditional EBI fungicides. At Zadoks’ GS75, the Relative Vegetation Index, measured with remote sensors, has been significantly higher for plots treated with strobilurins in comparison with triazoles, indicating a 'greening' effect. This has been seen particularly in winter wheat, where RVI readings relate well to the yields measured.

These benefits are hypothesized to be due to the synthetic strobilurins' potent inhibition of spore germination, which prevents energy losses from the plant due to defense reactions, and a longer residual effect which extends the interval that the compounds afford protection. Protection against S. nodorum was effective for more than 22 days using a half rate of azoxystrobin whereas a half rate of propiconazole only gave approximately 10 days control. Apart from their fungicidal effect, strobilurins induce physiological and developmental alterations. It has been proposed that these are due to inhibition of ethylene biosynthesis, increase in endogenous cytokinins, and reduction in CO2 compensation point. These can also help to explain the retardation of senescence and the 'greening' effect [3].

For several cereal diseases, synthetic strobilurins have shown good potential for use at reduced or appropriate dosages as long as optimal timing in relation to disease development is maintained. Regarding net yields, reduced dosages of both kresoxim-methyl and azoxystrobin in the range of a third to half rate have given best results under Danish conditions.

The use of synthetic strobilurins, either alone or in mixtures, in diverse disease control strategies represents an outstanding potential for minimizing yield losses in a broad range of plant pathogen/cropping systems.

References
1. Clough JM, 1993. Natural Product Reports 1993, 10, 565-574.
2. Godwin JR, Anthony VM, Clough JM, Godfrey CA, 1992. Proceedings Brighton Crop Protection Conference: Pests and Diseases, pp. 435-442.
3. Grossmann K, Retzlaff G, 1997. Pesticide Science 50, 11-20.