Genetic determinism of lemon resistance to the two-spotted spider mite attack

Among the huge number of both open-field and protected crops threatened by the two-spotted spider mite (Tetranychus urticae Koch), lemon (Citrus limon) is one of the most affected. By feeding, the spider mite causes leaf spot chlorosis and photosynthesis impairment, while russeting on fruit compromise their marketability. Mite infestations are difficult to control even with the use of pesticides since it easily develops resistance against acaricides. With the aim of a more sustainable pest control management, research is focusing on the release of natural enemies and on the application of natural compounds with toxic or repellent/deterrent activity against mites. Since host resistance could represent a sustainable strategy for spider mite control, hormonal, metabolomic and transcriptomic studies were recently carried out on several citrus genotypes to unlock the mechanism of host resistance. In the present study, a marker-trait association analysis was performed on a lemon segregating population (lemon ‘Interdonato’ ♀ × lemon ‘Femminello Siracusano 2kr’ ♂, 125 individuals) for understanding the genetic determinism of resistance to the two-spotted spider mite attack. Phenotyping was performed through a detached-leaf system using a modified Huffaker cell, where four females of spider mite were released for seven days. Then the percentage of damaged area was estimated through the digital analysis of leaves, and mite survival was also assessed. Genotypes showed a percentage of diseased area ranging from 0.36 to 9.67%. Thanks to the recent de novo sequencing of lemon, a functional genomic study was performed on the same genotypes through the SPET (single primer enrichment technology). Afterwards, a QTL analysis was performed on selected SNPs and genomic regions significantly associated with resistance to the two-spotted spider mite attack were identified. Hopefully in the near future, the molecular breeding strategy applied using the lemon segregating population will make available new molecular markers applicable for MAS (marker assisted selection) and for the genetic improvement of citrus against biotic stresses.