HISTOLOGICAL CHARACTERIZATION OF RESISTANCE IN LACTUCA-SALIGNA TO LETTUCE DOWNY MILDEW (BREMIA-LACTUCAE)

Lactuca saligna L. possesses resistance to Bremia lactucae Regel (lettuce downy mildew) of potential value to lettuce breeders. All the tested accessions of this species were highly or completely resistant to all of the races of B. lactucae used. The nature of this resistance was investigated quantitatively in a microscopical study of the earliest events (24 and 48 h after inoculation) of the infection process of B. lactucae in adult plant leaf discs of accessions of Lactuca sativa L., L. saligna and interspecific F1 hybrids. At least two resistance mechanisms are present in L. saligna. Race specific resistance was found in L. saligna CGN 5147. A different resistance mechanism, resulting in highly reduced levels of sporulation in leaf disc tests, was found in four other L. saligna accessions. The histological results showed that the latter resistance is very different from both race-specific and the partial (field) resistance of lettuce cv. Iceberg. The development of B. lactucae is affected at a later stage of the infection cycle than with race-specific or partial resistance. However, the ultimate level of resistance, as measured by sporulation intensity, is much higher than that of cv. Iceberg. In contrast to race specific resistance, the resistance of L. saligna was recessive in interspecific crosses. For lettuce breeding this means that a new resistance mechanism, which provides a high level of resistance to B. lactucae, is available. Because of its different mechanism, the resistance in L. saligna may be more durable than race-specific resistance. © 1994 Academic Press Limited.