RELATIVE RESISTANCE TO LATE BLIGHT OF SOL ANUM-TUBEROSUM SSP ANDIGENA

Results are reported of late blight resistance in S. tuberosum ssp. an digena (adg) and its use in breeding. Tests were carried out on materi al of the gene bank in Gross Lusewitz (GLKS). Seedlings were cultivate d under glass cover in a 6-years test period. To examine tuber resista nce to Phytophthora infestans the following test was set up: 2-5 tuber s per idiotype, up to 8 idiotypes per accession and year of test were taken and 1274 different accessions were examined with an average numb er of 23 idiotypes and 59 tubers from each. Inoculation was made by fr esh 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-degr ees-C and at an air humidity of >90%. Evaluation of the presence of ae rial mycelium and browning after 7 days allowed the calculation of a r esistance score (B(K)): grade 9 (highly resistant) to grade 1 (highly susceptible). The mean values derived from the accessions (Fig. 1) rev ealed that a medium level of susceptibility is dominating and that the y rarely exceed average levels of cultivars. Approximately 1% of the a ccessions 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 favo urable mean values. Geographic evaluation did not yield indications fo r a more purposeful search for. sources of resistance, since detailed reports on find-spots were missing (Table 2). Combination tests with P hytophthora-resistant genes of adg for crosses of adg with S. demissum (dms) and S. stoloniferum (sto) proved to be more useful as they part ly showed good heritable resistance to haulm and tuber infection of P. infestans (Table 3). The problem of the possible role of unknown r-ge nes is discussed. Only a few crosses with ssp. tuberosum (tbr) generat ed a progeny with sufficient resistance. Selection from such progeny o ver several years resulted after the second year in rejections, mainly because of susceptibility for viruses, too small tubers and lack of f lowering (Table 4). Results of varietal research of the former German Democratic Republic proved that with the use of suitable parents in br eeding for nematode resistance, Phytophthora susceptibility from adg c ould 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 l ate blight.