5.5.12
EFFECT OF AZOXYSTROBIN ON THE OOSPORES OF PLASMOPARA VITICOLA

A VERCESI1, A VAVASSORI1, F FAORO2 and M BISIACH1

1Istituto Patologia Vegetale, UniversitÓ di Milano and 2Centro Miglioramento Sanitario Colture Agrarie, CNR, Milano, Italy

Background and objectives
Azoxystrobin (AZ), a new fungicide active against Plasmopara viticola [1], the causal agent of grapevine downy mildew, proved to be very effective in reducing the disease level in vineyard and demonstrated both preventive and curative activity [2]. It is known that AZ inhibits sporangium germination, zoospore motility and mycelial growth; however no information is available up to now on its effects on the oospores of P. viticola. The aim of this work was to investigate the influence of AZ on the oospore formation and germination.

Materials and methods
AZ effects on the oospore formation have been studied in vineyard on naturally infected leaves treated twice at the end of the season (end of August-middle of September). Leaves showing mosaic symptoms have been collected in the middle of October in both treated and untreated plots. The leaf areas containing numerous oospores were excised, put in nylon bags (pore size = 100mm) and overwintered at the surface of the vineyard soil. The number of oospores per square cm was evaluated by extracting oospores from leaf tissues and referring their total number to the fragment area, assessed by image analysis. Oospores were also processed for transmission electron microscopy. Germination assays were carried out at 20░ C on water agar monitoring at least 400 oospores for 14 days.

Results and conclusions
The number of oospores per square cm strongly decreased (up to 75%) in treated tissues in comparison with the untreated one. Moreover, oospores differentiated in treated leaves showed a reduction in wall thickness and disorganisation of the protoplasm. Germination was also affected and the sporangium formation was often delayed. These results suggest that AZ significantly reduces the overwintering inoculum of P.viticola, thus decreasing the number of primary infections, lowering and/or delaying the epidemic development of grapevine downy mildew in the next spring.

References
1. Goodwin JR, Anthony VM, Clough JM, Godfrey CRA, 1992. Proceedings Brighton Crop Protection Conference: Pest and Diseases, 499-519.