Research at the SVP on resistance to Phytophthora infestans focuses on:

1. 1) the development of efficient methods to evaluate the level of race nonspecific haulm resistance to P. infestans and to distinguish between different components of resistance;

2. 2) the detection of genotypes with race nonspecific resistance to P. infestans;

3. 3) the study of resistance mechanisms and the inheritance of the resistance; In this paper these research topics are highlighted and future research plans are mentioned.

DEVELOPMENT OF TEST METHODS

A field test has been developed at the SVP to assess leaf resistance to P. infestans. Clones to be tested are planted between spreader rows of a susceptible variety, in replicated microplots of six plants each. Standard varieties are included for comparison. All plants are inoculated artificially with a zoospore suspension. The percentage of diseased leaf area is assessed seven times per clone at weekly intervals, starting one week after inoculation. These seven scores are transformed to a weighted mean (WM) value using a formula (Tazelaar 1981). The WM values of 30 varieties lacking major (R) genes corresponded well with the known scores for resistance to P. infestans in the Dutch variety list (r = -0.85). Moreover the results of this test appeared to be highly reproducible.

However, correlation analyses showed that there are high positive coefficients of correlation between: