INRA Centre de Rennes Plant Pathology Station, Domaine de la Motte, BP 29, F-35653 Le Rheu Cedex, France

Background and objectives
Cultivar associations combining resistant and susceptible plants in various spatial patterns have been used successfully to control airborne diseases, particularly in cereals, but their usefulness in other plant pathosystems has been little explored. Associations are simple to design and operate and their 'bimering' action with respect to yields could prove particularly suitable in sustainable agricultural schemes, both in developed and developing countries. We therefore decided to investigate their applicability to control potato late blight caused by Phytophthora infestans, a disease currently controlled primarily by intensive spraying schedules with fungicides. The objective of the present work was therefore to determine whether some mixture effect could be observed in the potato-P. ;infestans pathosystem in terms of disease progress and yield following natural infection.

Materials and methods
Because current breeding effort for late blight resistance focuses on high levels of partial race-nonspecific resistance, the experiments relied on one susceptible cultivar (Bintje, B ) and two partially resistant cultivars (Charlotte, Ch, and Claustar, Cl). In addition to their different levels of late blight resistance, the cultivars were chosen for their similar earliness. Field plots (12 rows of 20 tubers each) were established with each of the three cultivars alone. or with alternate rows (B-Ch-Cl, B-Ch-Ch or B-B-Ch). No fungicide application was applied to the plots in order to let natural infections develop. Every plant in every plot was scored for disease severity on a 1-9 scale [1]. Five consecutive scorings, starting with the first observation of symptoms and spaced at regular intervals chosen according to the speed of disease development were performed. Plots were harvested by hand and yield per row was determined.

Results and conclusions
Late blight developed in only two (1993 and 1997) of the five experimental years, with disease progressing much more rapidly in 1997 than in 1993. In both years, the epidemics started sooner and developed faster in pure 'Bintje' than in any other plot. Disease progression on Bintje plants was slower in mixed than in pure plots; mean disease severity on that cultivar was 30-50% lower in alternate plots than in pure plots at the final scoring date. A slight, but nonsignificant increase in disease severity was observed on the partially resistant cultivars in the alternate plantings. The disease remained focal in all plots at all scoring dates. A significant yield increase (35-62%) was observed in 'Bintje' grown in association in the pure plots. No significant yield differences were observed in 1993 for the partially resistant cultivars in pure and alternate plots, but a consistent yield increase (13-48%) was observed in alternate plantings in 1997.

The effectiveness of mixtures is usually attributed to three potential mechanisms: a reduction in the available inoculum owing to the presence of resistant plants, a reduction of inoculum spread between susceptible plants owing to the physical barrier constituted by resistant plants and preinoculation of susceptible plants following challenge by avirulent pathotypes [2]. Mixtures are thus most likely to work with complete race-specific resistance in the host (with pathogens inducing small, non-expanding lesions) and random distribution of resistant and susceptible genotypes within the plots. Although our experimental system was far from these optimal conditions (partial resistance, large expanding lesions, alternate rows) significant benefits in both disease reduction and yield could be observed in the 2 ;years where epidemics developed. Statistical analysis of the spatio-temporal patterns of epidemic progress in the various plots is underway to better understand the mechanisms that contributed to the performance of cultivar associations.

1. Cruickshank G,, Stewart BE, Wastie RL, 1982. Potato Research 25, 213-214.
2. Wolfe MS, 1985. Annual Review of Phytopathology 23, 251-273.