Different Mechanisms of Whitefly Resistance in Cabbage and its Wild Relatives

Brassica crops, in particular Brussels sprouts, kale and Savoy cabbage suffer from the cabbage whitefly (Aleyrodes proletella). As a first step to identify resistant material we screened a collection of 432 accessions, including wild material and landraces of Brassica oleracea as well as wild relatives for whitefly resistance in no-choice field and greenhouse experiments. Resistant accessions were identified among B. oleracea var. capitata (heading cabbage) landraces and some of the wild relatives, notably B. incana and B. villosa. While in heading cabbage resistance is only expressed in plants of at least twelve weeks old, some wild relatives were already starting to express resistance at a the age of six weeks. QTL mapping was used to identify chromosomal regions involved in whitefly resistance. Within the heading cabbages, QTLs were found for whitefly adult survival, oviposition rate and morphological traits possibly related to the resistance. However, none of the measured morphological traits co-localized with the whitefly resistance QTLs. Analysis of the probing behaviour of whitefly adults revealed that the resistance trait affects A. proletella at the phloem level. Another population that we studied was obtained from a cross between a fully whitefly resistant plant of B. incana, which is densely covered with trichomes, and a susceptible B. oleracea cultivar. In this population we mapped whitefly resistance to a single locus. At the same locus we also mapped the presence/absence of trichomes. We conclude that several mechanisms for whitefly resistance exist in the Brassica genepool, which may be exploited in breeding.