15 new pathotypes of Synchytrium endobioticum have been identified in the Czech Republic. Starting material for breeding resistant potato varieties, and suitable pathogen's populations for testing resistance of new bred hybrids have been recommended. A new procedure for the study of potato wart disease outbreaks has been suggested, concerning pathotype identification and the resistant potatoes growing in the quarantine territory. Previous information that the pathotype 1 (D1) was not identical on all localities in the Czech Republic has been confirmed.

Yield loss caused by late blight varies among potato cultivars. This thesis reports experiments and models aimed at identifying the plant characteristics that explain the variation. In the experiments, blight did not affect the efficiency with which twenty potato cultivars used intercepted light to produce biomass. The rate of photosynthesis of green parts was not impaired by lesions in other parts. Yield loss thus was fully explained by reduced light interception, i.e. reduced green leaf area. Blight decreased green leaf area mainly by lesion coverage of leaves, but also by accelerating leaf senescence. Simulation analysis showed that the acceleration of senescence accounted for less than 15% of the yield loss in every cultivar, and no genetic variation for this trait was found. The marked genotypic differences observed in foliage area thus were due to differences in lesion coverage, caused by different levels of partial resistance and by maturity class-related differences in foliage growth. It is concluded that breeding for components of partial resistance is more promising than breeding for tolerance, but should be done separately for early and late cultivars. The literature was reviewed and found conflicting about the relation between yield loss and components of resistance to the fungus. The limitations of published epidemiological models as instruments for identification of the most important components are discussed. An improved epidemiological model was constructed and used to identify infection efficiency and radial lesion growth rate as the main determinants of disease progress in potato. Resistance breeding should therefore focus on these components.

The project, which has been a cooperative programme involving the 4 USDA experimental regions and the federal government since 1950, is responsible for the introduction, preservation, classification, distribution and preliminary evaluation of germplasm of tuber-bearing Solanum species. It is largely based at the University of Wisconsin Peninsular Agricultural Research Station at Sturgeon Bay, Wisconsin. Some details are given of IR-1's facilities, operations, germplasm collections (it has received accessions from more than 35 potato collecting expeditions over the past 30 years, although these represent only 120 of the recognised 222 species), maintenance and distribution of germplasm, international collaboration, and recent taxonomic studies and collecting missions. An insert on page 33 gives an outline of wild potato diversity and its disease resistance, crossability and use in breeding.