RELATIVE RESISTANCE TO LATE BLIGHT OF SOLANUM-TUBEROSUM SSP ANDIGENA

Results are reported of late blight resistance in S. tuberosum ssp. andigena (adg) and its use in breeding. Tests were carried out on material of the gene bank in Gross Lusewitz (GLKS). Seedlings were cultivated under glass cover in a 6-years test period. To examine tuber resistance to Phytophthora infestans the following test was set up: 2-5 tubers per idiotype, up to 8 idiotypes per accession and year of test were taken and 1274 different accessions were examined with an average number of 23 idiotypes and 59 tubers from each. Inoculation was made by fresh deep cuts to the pith and by subsequent immersion in a solution of zoospores, 6-9 x 10(4) ml-1 with a mixture of the pathotypes (1.2.3.4.5.7.10.11, 1.2.3.4.7.8.10 and 1.2.3.4) at a temperature of 16-18-degrees-C and at an air humidity of >90%. Evaluation of the presence of aerial mycelium and browning after 7 days allowed the calculation of a resistance score (B(K)): grade 9 (highly resistant) to grade 1 (highly susceptible). The mean values derived from the accessions (Fig. 1) revealed that a medium level of susceptibility is dominating and that they rarely exceed average levels of cultivars. Approximately 1% of the accessions contained greater-than-or-equal-to 25% of idiotypes with B(K) greater-than-or-equal-to 6 and these may be interesting for breeding purposes. Table 1 shows the analysis of accessions with the most favourable mean values. Geographic evaluation did not yield indications for a more purposeful search for. sources of resistance, since detailed reports on find-spots were missing (Table 2). Combination tests with Phytophthora-resistant genes of adg for crosses of adg with S. demissum (dms) and S. stoloniferum (sto) proved to be more useful as they partly showed good heritable resistance to haulm and tuber infection of P. infestans (Table 3). The problem of the possible role of unknown r-genes is discussed. Only a few crosses with ssp. tuberosum (tbr) generated a progeny with sufficient resistance. Selection from such progeny over several years resulted after the second year in rejections, mainly because of susceptibility for viruses, too small tubers and lack of flowering (Table 4). Results of varietal research of the former German Democratic Republic proved that with the use of suitable parents in breeding for nematode resistance, Phytophthora susceptibility from adg could be balanced off (Table 5). The cultivar Auralia is a good example of the importance of adg in achieving a high level of resistance to late blight.