5.1.20
USE OF POTATO GENOTYPE MIXTURES FOR DEPLOYMENT OF RESISTANCE AGAINST PHYTOPHTHORA INFESTANS: EFFICACY MAY VARY GEOGRAPHICALLY WITH THE LEVEL OF OUTSIDE INOCULUM
KA GARRETT1,2, CC MUNDT1, GA FORBES2, RJ NELSON3, LN ZUNIGA4 and E RONCAL5

1Department of Botany and Plant Pathology, Oregon State University, Corvallis, OR, 97331-2902, USA; 2Centro Internacional de la Papa, Apartado 17-21-1977, Quito, Ecuador; 3Centro Internacional de la Papa, Apartado 1558, Lima 100, Peru; 4Instituto Nacional de Investigaciones Agrarias, Huancayo, Peru; 5Instituto Nacional de Investigaciones Agrarias, Cajamarca, Peru

Background and objectives
Phytophthora infestans, the causal agent of late blight, is the most important pathogen of potato. Current management strategies are dependent on intensive use of fungicides, though this is changing as potato varieties with better resistance to late blight are being developed. One means of deploying this resistance would be through use of potato genotype mixtures. Genotype mixtures have proven useful for managing such diseases as mildews and rusts of small grains, but have not been as widely tested for other disease systems. We developed studies to examine the effect of genotype mixtures on late blight. Our first objective was to estimate the size of mixture effects in field studies in Ecuador, Peru, and the USA. Second, the field studies in Ecuador and Peru were designed to allow comparison of different mixtures with varying levels and types of resistance. Third, the field study in the USA was designed to estimate the impact of the pattern of initial inoculum, aggregated or dispersed, on mixture effects.

Materials and methods
In Ecuador, near Quito, experimental plots of eight different mixtures of potato cultivars were planted, representing a range of overall resistance including both horizontal and vertical resistance to late blight. In Peru, several mixtures varying in level of resistance were compared at two sites, Huancayo and Cajamarca. In the USA, near Corvallis, Oregon, one mixture of resistant and susceptible varieties was inoculated with P. infestans either evenly throughout the plots or, using the same amount of total inoculum, in only one corner of each plot. Mixture effects were measured by comparing AUDPCs of a mixture to the weighted average of the appropriate monocultures at the same site. Sites were selected to represent a latitude gradient with Quito on the equator, Cajamarca at 7 south, Huancayo at 12 south and Corvallis at over 40 north.

Results and conclusions
Estimated mixture effects (negative and positive) from the first season in Quito were small; there were only two statistically significant, both positive. The mixture with the largest estimated effect was composed of one cultivar with vertical resistance, one cultivar with high horizontal resistance, and one susceptible cultivar. For this mixture, the observed AUDPC was 57 compared to a predicted AUDPC of 86 based on observations in monocultures. The first readings from the second season at Quito suggest similarly small mixture effects. The first season in Peru is under way and initial disease evaluations suggest that there may be greater mixture effects at the Peruvian sites than near Quito. This would be consistent with the hypothesis that mixture efficacy increases where potato growing is more seasonal and thus the amount of outside inoculum arriving at fields may be lower. The estimated mixture effect from the first season at Corvallis was larger than in Quito but, contrary to our expectations, the estimate was greater for general inoculum than for aggregated inoculum. Therefore, the role of inoculum pattern is unclear. We are using models to further examine the impact of outside inoculum and inoculum pattern on mixture efficacy.