Spider mite symptomatology in a segregating F₂ population in Fragaria ×ananassa Duch.

Research in the field of susceptibility, tolerance as well as resistance towards the two-spotted spider mite (Tetranychus urticae Koch) is very complex in Fragaria L. and therefore not completely understood. The goal of this study was to reveal factors behind the symptomatology of a F₂ segregation population after wild species introgression under defined spider mite inoculation. F. ×ananassa 'Senga Sengana' was used as a susceptible genotype crossed with a highly tolerant wild species accession F. chiloensis ssp. lucida USA 2. The resulting population of about 350 plants was selected for the most favourite genotype (P-90999) which was used for self-pollination subsequently. The resulting F₂ model population contained 103 randomly selected seedlings propagated by runners later on. Two-spotted spider mites from in-house rearing on Fragaria seedlings were used for inoculation of the model population in insect proof cages under greenhouse conditions. The experiment was repeated eight times in total at three periods. The symptomatology was recorded using an adequate score system of nine precisely defined levels. The evaluation showed that the infection occurred in five unique colonization patterns depending on the stage of infection and the genotype. Cluster analysis was used to find the different groups. Each cluster could be described by means of their pathological development. The expressed symptoms, the climatic conditions and the development of the spider mites showed positive correlation. The susceptibility to spider mites segregates from tolerant over medium to very susceptible genotypes. The obtained results are valuable for resistance breeding in strawberry.