Genetic variance against broad mites (Polyphagotarsonemus latus) in Rhododendron simsii hybrid gene pool

In Belgium, the broad mite, Polyphagotarsonemus latus (Banks), is considered as a key pest in ornamental plant production. In pot azalea (Rhododendron simsii Planch hybrids) mites feed on the youngest growth tips, causing malformation of the flower buds and the terminal leaves. The latter become severely stunted and hardened, curl down at the edges, and show brownish or reddish lower surfaces. As in many other ornamental crops, the mite’s feeding behavior on R. simsii results in severe economic damage.

To evaluate the genetic variation against P. latus in pot azalea, 32 accessions of Rhododendron spp. (mainly R. simsii hybrids) were evaluated. For this purpose, plants were artificially infected in a greenhouse by surrounding them with infested Hedera helix L. plants as an infection source. Each week, plants were evaluated by giving a disease score ranging from 0 (no visual damage) to 4 (severely damaged). At certain time points broad mites were also quantified by counting the number of mites present per shoot tip. Results indicate a uniform infection at the initial stage of the experiments. At the end of the screening experiments significant differences in susceptibility between the evaluated accessions were observed. Significant correlations between the average disease score and the number of broad mites were found. Among the accessions tested, ‘Emile De Coninck’ and ‘Mont Blanc’ were rated as the most susceptible ones, while ‘Mistral’ and its sport ‘Elien’ proved to be highly tolerant/resistant towards a broad mite infection.

It is believed that symptoms caused by this pest appear after piercing plants cells and sucking the cell content. Scanning electron microscope (SEM) images of P. latus indicate the presence of two short stylettiform movable digits, fixed on a basal structure. However, based on cross sections of R. simsii leaves it remains unclear how these relatively short movable digits are able to perforate entire epidermal cells to reach subepidermal tissues.