Do your rhododendron leaves look silvered and pale as in the photo above?
If so, then your rhododendrons are suffering from thrip damage. These sap-sucking insects can cause considerable damage to rhododendrons and many other plants, particularly in hot dry conditions. Thrips feed on leaves by piercing or scraping the surface of the leaf cells and sucking up the contents, leaving a characteristic silvery appearance. Adult thrips can fly and an infestation can appear over a short period. They overwinter in the soil and other cracks and crevices, so in spring they can often appear again on the plants affected the previous year.
Some rhododendron species are resistant to thrip damage; the species that have a 'furry' layer on the undersides of their leaves tend to be protected. Susceptible species should be sprayed with PLANThealth Spectrum on the undersides of leaves. Protection is best carried out in spring before an infestation takes hold and while thrips are juvenile and do not fly. Apply again at 14 day intervals if insects are still found under leaves. Application of LawnPro Protect soil insecticide in spring can also control early instars (juvenile stages) in the soil and leaf litter.
In hot dry conditions when the plants are stressed, water them thoroughly around the roots and foliage but do not let them stand in water for more than a day.
In autumn, carry out a plant 'clean up' to reduce overwintering of thrip and other pests. Spray with a PLANThealth Copper Fungicide Liquid followed by Organic Super Spraying Oil. Then a month later apply Organic Super Sulphur.