Towards breeding thrips-resistant varieties in Capsicum

Thrips are small insects but a major pest in pepper cultivation worldwide. They are difficult to control because of their high reproductive rate, short life cycle, cryptic behaviour and their wide host range. Protective measures such as chemical and biological control are used to prevent thrips outbreaks in crops, but these are only partially effective. Therefore, host plant resistance is a desired trait for pepper growers. Previously, thrips-resistant pepper accessions were identified. The aim of this thesis is to further characterize the resistance mechanism of the resistant pepper accession CGN16975, and to study its potential as a source for breeding thrips-resistant Capsicum varieties. The gene or genes on chromosome 6 that contribute to thrips resistance were fine mapped and a multidisciplinary approach was used to select candidate genes. Furthermore the role of metabolites in this resistance mechanism was studied. Altogether, the work in this thesis contributes to protection of pepper against thrips, and shows that the resistance mechanism in CGN16975 can be used as a source for breeding thrips-resistant Capsicum varieties

The effect of plant development on thrips resistance in Capsicum

Western flower thrips [Frankliniella occidentalis (Pergande)] is a worldwide pest insect that causes damage in pepper cultivation, so growers would benefit from host plant resistance. The objectives of this study were (1) to evaluate the effect of plant age on thrips resistance using nine Capsicum accessions with different levels of thrips resistance at three different plant ages, and (2) to study the effect of leaf age on thrips resistance in a resistant and a susceptible pepper accession. The fraction of first instar larvae that did not develop into second instar was used as a measure for thrips resistance. Our results show that plants start to develop thrips resistance when they are between 4 and 8 weeks old. This transition was most marked on the resistant accession CGN16975, on which about 50% of the L1 larvae developed into the next stage on 4-week-old plants, whereas none of them developed beyond the L1 stage on 8- or 12-week-old plants. Furthermore, it is shown that youngest fully opened leaves of the resistant accession CGN16975 are significantly more resistant to thrips than older leaves; 89% of the L1 larvae did not develop into the next stage on the youngest leaves, whereas 57% did not develop beyond the L1 stage on the oldest leaves. Young leaves of the susceptible accession CGN17219 are more susceptible than older leaves; 9 versus 52% of the L1 larvae did not develop into the next stage on young and old leaves, respectively. These findings can be used to improve integrated pest management strategies

The effect of plant development on thrips resistance in Capsicum

Western flower thrips [Frankliniella occidentalis (Pergande)] is a worldwide pest insect that causes damage in pepper cultivation, so growers would benefit from host plant resistance. The objectives of this study were (1) to evaluate the effect of plant age on thrips resistance using nine Capsicum accessions with different levels of thrips resistance at three different plant ages, and (2) to study the effect of leaf age on thrips resistance in a resistant and a susceptible pepper accession. The fraction of first instar larvae that did not develop into second instar was used as a measure for thrips resistance. Our results show that plants start to develop thrips resistance when they are between 4 and 8 weeks old. This transition was most marked on the resistant accession CGN16975, on which about 50% of the L1 larvae developed into the next stage on 4-week-old plants, whereas none of them developed beyond the L1 stage on 8- or 12-week-old plants. Furthermore, it is shown that youngest fully opened leaves of the resistant accession CGN16975 are significantly more resistant to thrips than older leaves; 89% of the L1 larvae did not develop into the next stage on the youngest leaves, whereas 57% did not develop beyond the L1 stage on the oldest leaves. Young leaves of the susceptible accession CGN17219 are more susceptible than older leaves; 9 versus 52% of the L1 larvae did not develop into the next stage on young and old leaves, respectively. These findings can be used to improve integrated pest management strategies.</p

The effect of a thrips resistance QTL in different Capsicum backgrounds

Thrips are a worldwide threat to Capsicum because they damage fruits, flowers and leaves directly by feeding, and indirectly by vectoring tospoviruses such as tomato spotted wilt virus. Therefore, growers would benefit from thrips-resistant varieties. Previously, a quantitative trait locus (QTL) that provides resistance to Frankliniella occidentalis has been identified. Here we explore the potential of this QTL for breeding thrips-resistant varieties by studying its effect on two thrips species (i.e. F. occidentalis and Thrips tabaci) in four different Capsicum annuum backgrounds. We observed differences in thrips resistance between different genetic backgrounds, both in plants that have the resistance allele for the QTL region in homozygous state as well as in plants with the susceptibility allele in homozygous state. This suggests the presence of factors in these backgrounds that either increase or reduce thrips resistance. Altogether, we confirmed the major effect of the QTL on thrips resistance in all four genetic backgrounds to both F. occidentalis and T. tabaci, thus showing its general applicability as a source for breeding thrips-resistant Capsicum varieties