5.6.1
EFFECT OF THE STROBILURIN FUNGICIDE AZOXYSTROBIN ON LEAF SAPROPHYTES, HOST PHYSIOLOGY AND YIELD OF WHEAT CROPS

JR BERTELSEN and V SMEDEGAARD-PETERSEN

Dept. Plant Biology, Royal Veterinary and Agricultural University, 1871 Frederiksberg C, Denmark

Background and objectives
Research at ZENECA Agrochemicals has led to the development of azoxystrobin, a new systemic fungicide belonging to the strobilurin group of fungicides. It is active against Ascomycetes, Basidiomycetes, Deuteromycetes and Oomycetes [1]. Azoxystrobin provides excellent disease control in cereals and occasionally can produce yield increases beyond that explained by its visible disease controlling capacity in comparison to triazole-based cereal fungicides. A prolongation of the maintenance of green leaf area is also observed with azoxystrobin over triazoles. Strobilurins are known to exert their fungicidal effects by interfering with the aerobic metabolism of the fungi, thereby having a potent effect to prevent spores from germinating with a lesser effect against hyphae. This is opposed to most other currently used cereal fungicides, which act only on hyphal growth. It is hypothesised, that the ability of strobilurins to prevent spore germination and penetration attempts of fungal pathogens and filamentous leaf saprophytes explains the better persistence of green leaf area, leading to higher yield even in crops free of visible disease. Filamentous leaf saprophytes have previously been demonstrated as a yield reducing factor in barley [2].
This project aims at an understanding of the yield increases achieved in winter wheat with azoxystrobin. The experiments are performed by comparing the action of ‘Amistar’* (azoxystrobin) and a fungicide with a different mode of action, ‘Opus’* (epoxiconazole) to untreated control plants.

Materials and methods
Selected saprophytes were tested for their ability to elicit host responses in the presence of ‘Amistar’ or ‘Opus’ at a range of concentrations. Studies also examined whether the differences observed microscopically were reflected in the energy expenditure of whole leaves. For this purpose, carbon dioxide efflux measurements were performed with an infra red gas analyser on consecutive days, following the pattern of leaf respiration after inoculation.

Field experiments have been carried out to examine the effects of these fungicides on saprophytes, and the impact of saprophytes on wheat yield. To achieve this, it was essential to keep the field as free of disease as possible. Therefore, the experimental plots were placed in fields where wheat had not been grown for several years, and barley was cultivated around the edge. ‘Amistar’, ‘Opus’ and control plots were randomized. The fungicides were applied at stem extension, flag leaf and ear spray timings. The development of necrotic leaf area and disease coverage on the wheat plants was estimated for each leaf layer during the growth season. The saprophytic colonisation of the green portion of the leaves was assessed and yield parameters were recorded for each plot. To support the field data, wheat plants were grown in a glasshouse to maturity and harvested. The plants were inoculated several times with saprophytes and received the same fungicide treatments as the field plots.

Results and conclusions
‘Amistar’ demonstrated a better ability to inhibit spore germination and hyphal growth of saprophytic fungi than ‘Opus’. The ability of the fungi to elicit host responses in the presence of fungicides was impaired. In field trials, the ‘Amistar’-treated leaves had the lowest density of saprophytes and both fungicide treatments resulted in significantly less saprophytic growth on the leaves than in control plots. The maintenance of green leaf area was better in fungicide treated- than in control leaves and the yields were significantly higher. Further investigations are continuing.

References
1. Godwin JR, Anthony VM, Clough JM, Godfrey CRA, 1992. Proceedings Brighton Crop Protection Conference: Pests and Diseases, pp.435-442.
2. Smedegaard-Petersen V, Tolstrup K, 1985. Annual Review of Phytopathology 23, 475-490.

* Amistar and Opus are trade marks belonging to Zeneca Ltd and BASF, respectively.