A set of putative marker genes to study plant defense responses against Polyphagotarsonemus latus, a key pest in the production of Rhododendron simsii hybrids, was selected and validated. Genes belonged to the biosynthetic pathway of phytohormones jasmonic acid (JA) (RsLOX, RsAOS, RsAOC, RsOPR3 and RsJMT) and salicylic acid (SA) (RsPAL and RsICS). Furthermore, RsPPO, a putative marker gene for oxidative stress response was successfully cloned from R. simsii. A CTAB-based extraction protocol was optimized to assure excellent RNA quality for subsequent RT-qPCR analysis. The RT-qPCR protocol was extensively tested and RsRG7 and RsRG14 were selected as reference genes from a geNorm pilot study. Validation of the marker genes was done after application with elicitors [methyl jasmonate (MeJA), coronatine, beta-aminobutyric acid and acibenzolar-Smethyl] or wounding. Both 100 mu M MeJA and 0.1 lM coronatine had a significant effect on the expression of all marker genes. Foliar application of MeJA on the shoots resulted in a significantly earlier response when compared to root application and subsequent sampling of the shoots. Expression patterns after MeJA treatment were generally the same in six R. simsii genotypes: 'Nordlicht', 'Elien', 'Aiko Pink', ' Michelle Marie', 'Mevrouw Gerard Kint' and 'Sachsenstern'. Wounding resulted in the same expression patterns as MeJA treatment except for RsJMT. None of the genotypes showed a significant induction of the latter gene 6 h upon wounding. Findings of these experiments indicated that the tolerant genotype 'Elien' has low basal expression levels of RsPPO. This might be the first step towards the breeding of mite-tolerant genotypes

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 R 2 -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.

The broad mite, Polyphagotarsonemus latus (Banks), is one of the major pests causing severe economic damage in Rhododendron simsii Planch hybrid production in Belgium. In order to optimize biological control programs and to parameterize warning programs, we studied the effect of environmental temperature on the development of P. latus on R. simsii leaves. In combination with a photoperiod of 16:8 h (L:D) and a relative humidity of 80 ± 5 %, six constant temperatures (15, 17, 20, 25, 30 and 33 ± 1 °C), were studied. Total developmental times of 13.3, 10.5, 6.6, 4.2, 3.5 and 4.0 days were measured, respective to each of the aforementioned temperatures. Development of females took significantly longer than that of males at 15, 17, 20 and 30 °C. Survival rates observed between 17 and 30 °C varied between 43.5 and 96.9 %. Lower survival rates were found at 15 and 33 °C, i.e. 31.8 and 23.6 %, respectively. The lower, optimal and upper developmental threshold (t min , t opt and t max , respectively) and thermal constant (K) of the pest were estimated for each life stage by a linear and two non-linear models. Based on measurements of total development of P. latus thermal thresholds of 10.0, 30.1 and 36.0 °C were calculated for t min , t opt and t max , respectively. The number of degree-days needed to complete immature development when feeding on R. simsii was 66.7.

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The cold hardiness of the broad mite, Polyphagotarsonemus latus, a key pest in Rhododendron simsii hybrid production in northwestern Europe, was investigated in the laboratory. Survival of eggs, larvae and female adults and reproduction capacity of female P. latus were evaluated following cold exposure at 7 °C. Adult females were also exposed to temperatures of 2 and -3 °C. Further, the supercooling point and lower lethal times of adult females were determined. No eggs survived exposure to 7 °C for 17 or more days. Larval survival upon the cold treatment decreased from 53 to 13% when exposed to 7 °C for 14 and 49 days, respectively. Two-day-old adult females exposed to 7 °C for up to 42 days did not suffer significant mortality, but when returned to 25 °C their oviposition rates were lower than those of mites maintained at 25 °C. Less than 40% of females exposed for 13 days to 2 °C survived; only 20% of these females was able to reproduce upon recovery. Subzero temperatures dramatically decreased survival and reproduction capacity of adult females. The supercooling point of female adults was -16.5 °C. Median lethal times averaged 61.2 h and 9.3 days at -3 and 2 °C, respectively. In conclusion, a long term exposure (up to 6 weeks) of R. simsii plants infested with P. latus to a temperature of 7 °C, which is required for breaking dormancy of the flowers, is not expected to have detrimental effects on the survival and reproductive performance of the female mites.

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