Zeneca Agrochemicals, Jeatott's Hill Research Station, Bracknell, Berkshire, RG42 6ET, UK

Background and objectives
In vivo and in planta light and cryoscanning electron microscopy studies were undertaken to examine the effects of preventative, curative or systemic applications of azoxystrobin on infection development of Plasmopara viticola, the causal pathogen of grapevine downy mildew. A study of this nature is important because if the stage in the fungal life cycle at which a compound is active can be determined this can help explain the most effective application timing for disease control. The chemicals chosen for the study have different biochemical modes of action, thus, ft was anticipated that they would affect the fungus in different ways. Azoxystrobin is a novel, highly active strobiludn fungicide which gives excellent control of both downy and powdery mildews of grapevine. Its biochemical mode of action is the inhibition of fungal mitochondrial electron transfer between cytochrome b and cytochrome cl[1].

Results and conclusions
Only 30% of sporangia of P. vfticoia released their zoospores on grapevine leaves treated preventatively with azoxystrobin (0.04 mg ai/l). Metalaxyl and mancozeb had no consistent effect on zoospore release at rates of up to 625 mg ai/1 and 125 mg aill respectively. Azoxystrobin, metalaxyl and mancozeb completely inhibited zoospore motility in vivo and zoospore encystment in planta when applied preventatively. However, the rates required to cause these effects varied greatly according to the chemical applied (azoxystrobin 0.04 mg aill, metalaxyl 625 mg aill, mancozeb 5 mg aill). Complete control of grapevine downy mildew was achieved at the same rates in a parallel disease development study. As curative treatments, applied 2 days after inoculation, azoxystrobin (250 mg ai/l) and metataxyl (200 mg ai/l) caused the collapse of mycelium within the plant, thus preventing further colonisation and disease development. Mancozeb (2600 mg aill) had no effect on the development of the pathogen when applied curatively. Treating vine leaves with a band of droplets just above the petiole and inoculating the same leaves 48 hours later with a suspension of P. viticoia sporangia showed azoxystrobin (250 mg adi) to reduce markedly zoospore encystment both in the area of chemical application and in the untreated area towards the leaf tip. Mancozeb (2800 mg ai/l) had a similar effect to azoxystrobin in the treated area, but no effect in the untreated area.

The results from these studies correlate well with what is known about the biochemical modes of action and physico-chemical attributes of these three fungicides. Azoxystrobin is a strobilurin fungicide with preventative and curative action. It is most potent when applied preventatively because its biochemical mode of action is to disrupt energy production within the fungus; zoospore motility is a high energy demanding process. Azoxystrobin exhibited curative action by causing the collapse of mycelium within the leaf, but the rate required to give good disease control was much higher than when applied preventatively. There was evidence that, following preventative application, azoxystrobin redistributed in the leaf and diffused out onto the surface to reduce zoospore encystment, probably by inhibiting zoospore motility. Metalaxyl is a systemic, curative phenylamide fungicide which inhibits ribosomal RNA synthesis, thus interfering with protein synthesis. As in previous studies [2], metalaxyl was most effective against the later stages of P. viticoia infection causing the collapse of mycelium within the leaf because ft is most effective when de novo protein synthesis is necessary. Mancozeb is a non-systemic ethylenebisdfthiocarbamate fungicide, thus it was effective as a preventative treatment inhibiting zoospore motility and reducing zoospore encystment, but showed neither an effect on mycelial growth within the grapevine nor redistribution within the leaf to reduce zoospore encystment in untreated areas.

1. Godwin JR, Anthony VM, Clough JM, Godfrey CRA, 1992. Proceedings 1992 Brighton Crop Protection Conference - Pests and diseases, 1, 435-442.
2. Staub TH, Dahmen H, Schwinn FJ, 1979. Journal of Plant Disease and Protection, 87, 83-91.