Screening for broad mite susceptibility in Rhododendron simsii hybrids

Broad mite Polyphagotarsonemus latus, is a key pest of pot azalea (Rhododendron simsii Planch hybrids). No information on variation in susceptibility or potential tolerance within these plants is available to date. We evaluated a selection of 32 Rhododendron cultivars, mainly R. simsii hybrids,
for susceptibility to broad mite. The plants were artificially infested in a greenhouse by surrounding each azalea with four broad mite-infested English ivy plants (Hedera helix). Broad mite infestation was evaluated by counting the
number of broad mites per shoot tip and assigning a damage rate. Results indicated a comparable infestation rate expressed as the number of mites on all cultivars at the initial stage of the experiments. Correlations between the average damage rate and the number of broad mites per shoot tip on all cultivars
at different time intervals were significantly positive, although low, in each experiment, with R2-values of 0.14 and 0.61. At the end of the experiments significant differences in susceptibility between the evaluated cultivars were
observed. The cultivars, ‘Emil De Coninck’ and ‘Mont Blanc’ were rated as the most susceptible, whereas ‘Mistral’ and its bud sport ‘Elien’ were tolerant towards the broad mite, because damage rates were low and very few broad mites were found.