Resistance to Phytophthora infestans in diploid and tetraploid potato families. 2. Resistance in tubers

The tuber reaction to Phytophthora infestans was evaluated in individual genotypes of four diploid and four tetraploid potato families, their parents and standard cultivars. The genotypes were tested several times. Two testing methods were used, based on evaluation of tuber slices or whole tubers, respectively. All inoculations were done with MP 245, a fungus isolate with a broad virulence spectrum. If the reaction of tuber slices was evaluated, the distribution of genotypes in several families deviated considerably from a normal one. In some distributions more than one peak was detectable, indicating segregation of major genes. Genotypes transgressive in resistance were found in four families. If the reaction of whole tubers was evaluated, the distribution of genotypes did not deviate significantly from a normal one. Genotypes transgressive in resistance were found in one family. The tuber reaction to P. infestans depended to a large extent on testing conditions

Resistance to Phytophthora infestans in diploid and tetraploid potato families. 3. Correlations between characters

In four diploid and four tetraploid families correlations were evaluated between resistance to Phytophthora infestans of leaflets, tuber slices and whole tubers as well as correlations between three aspects of resistance and maturity or tuber shape regularity. A virulent P. infestans isolate was used for inoculations. Only in the family D4 a significant positive correlation was found between all the three aspects of resistance. Genotypes with susceptible leaflets and resistant tuber slices were not identified in any family. In some families genotypes with resistant whole tubers and susceptible tuber slices were found; the family D5 was found to have genotypes in which resistance of leaflets was associated with susceptibility of tuber slices. The resistance to P. infestans of leaflets or tuber slices tended to be negatively correlated with both early maturity and regularity of tuber shape

Resistance to Phytophthora infestans in diploid and tetraploid potato families. 1. Resistance in detached leaflets

Potato families, segregating for resistance to Phytophthora infestans, were tested repeatedly to evaluate the distribution of resistance to P. infestans, the repeatability of testing results and correlation between resistance and some other characters. Four diploid and four tetraploid families were evaluated together with their parents and with potato cultivars used as standard. For all inoculations a virulent fungus isolate MP 245 was used. Leaflets were collected from plants growing in the field (summer tests) or in a glasshouse (autumn and spring tests). Segregation of major genes determining resistance was detectable in most families. In families originating from mating a resistant parent with a susceptible partner some progeny genotypes with resistance level of the resistant parent could be identified. In families originating from two parents showing only some resistance, a transgression of resistance could be found in the progeny. The expression of resistance depended on testing conditions. Some genotypes were consistently superior or inferior in resistance under all testing conditions, but often repeated evaluations of genotypes did not provide consistent results and significant interactions genotypes x tests were detectable