Resistance to Phytophthora infestans in Solanum tuberosum and wild Solanum species = Resistentie tegen Phytophthora infestans in S. tuberosum en wilde Solanum - soorten

Resistance to <em>Phytophthora infestans</em> , the causal agent of late blight, is present in the potato, <em>Solanum tuberosum</em> , and in many wild relatives of this crop. The resistance of <em>S. tuberosum</em> is partial and, though helpful in reducing the use of fungicides, is not sufficient to fully solve the problem. It also has the disadvantage of being associated with late maturity. Since 1910, other genes for resistance have been introduced into the potato genepool from the Mexican species <em>S. demissum</em> . These genes, designated R genes, initially supplied complete protection from the disease, but the pathogen soon managed to overcome the resistance of these genes, which is no longer useful. This thesis describes several other wild <em>Solanum</em> species as potential donors of resistance to <em>P. infestans.</em> Firstly, the partial resistance of a number of old cultivars of <em>S. tuberosum</em> , was studied, and appeared to be durable. In the South American. species <em>S.</em><em>berthaultii, S</em> . <em>circaeifolium</em> , <em>S. microdontum</em> and <em>S. venturii,</em> both partial and (nearly) complete resistance were found, while <em>S. arnezii x</em><em>hondelmannii, S. leptophyes, S</em> . sparsipilum and <em>S. vernei</em> carried promising partial resistances. The Mexican species <em>S. verrucosum</em> was also found to be highly resistant. Crosses with susceptible diploid <em>S. tuberosum</em> revealed the presence of major genes in <em>S.</em><em>microdontum</em> , <em></em>while <em></em>minor genes appeared to be important in <em>S. arnezii x</em><em>hondelmannii.</em> Complementary <em></em>action of major genes from one of the susceptible <em>S. tuberosum</em> parents was also found. A wide genetic variation was found with these species for the components of resistance infection efficiency (IE), lesion growth rate (LGR) and sporulation capacity (SC), while latent period (LP) was found to vary less. In <em>S. tuberosum</em> , IE, LGR and SC and in <em>S</em> . <em>microdontum</em> IE, LGR and LP appeared intercorrelated. Results supported the suggested role of glandular trichomes in the resistance of <em>S</em> . <em>berthaultii.</em> The <em></em>resistance of some <em>S. tuberosum</em> cultivars appeared to be lost at short photoperiods. A similar, though much smaller loss was observed for the partial resistance of <em>S.</em><em>microdontum.</em> The resistance of the nonhost species <em>S. nigrum</em> , evolved in India, was found to be of a pre-haustorial nature, and associated with a hypersensitive reaction. This resistance was transferred to sexual hybrids with <em>S. tuberosum</em> and <em>S.</em><em>demissum</em> .