Rhododendrons are the large-leaved cousins of azaleas — evergreen shrubs with enormous trusses of bloom in spring that can be spectacular at scale. The catawba rhododendron and its hybrids are reliably hardy in Zone 4–8 and among the best for the Hudson Valley. They need the same conditions as azaleas: acidic, humus-rich soil, dappled shade, and consistent moisture.
Like azaleas, rhododendrons are ornamental shrubs rather than cutting-garden plants. Their value is in the landscape — as large-scale structural evergreens with extraordinary spring bloom — not in vase arrangements.
Crown Rot
Lower leaves yellow and wilt; the base of the rosette turns brown and soft, sometimes with white fungal threads at the soil line. Plants collapse in wet conditions.
Remove and destroy affected plants. Improve drainage and avoid overhead watering. Do not mulch directly against the crown. Space plants for airflow and avoid replanting in the same wet spot.
Powdery mildew
White or grey powdery coating on leaves — usually starting on older growth in humid conditions or when nights cool.
Improve air circulation by thinning plants. Apply neem oil or potassium bicarbonate spray at first sign. Avoid overhead watering.
Petal blight
Flowers develop small watery spots that rapidly enlarge into soft brown blotches, collapsing the bloom into a slimy mass that clings to the plant instead of dropping cleanly.
Remove and destroy infected blooms and fallen petals, and clear old mulch beneath the plant where the fungus overwinters. Avoid overhead watering; in wet springs a preventive fungicide at bud break helps where it recurs.
Fungal leaf spot
Brown or purplish spots on the leaves, often ringed with a yellow halo, that spread and merge in warm, humid, or wet weather; heavily spotted leaves yellow and drop early.
Pick off spotted leaves and clear fallen debris to remove overwintering spores. Improve air circulation by thinning, and water at the base so foliage dries quickly. If it persists, apply a copper- or sulfur-based fungicide at the first sign.
Azalea Lace Bug
Stephanitis pyrioides →Leaf upper surfaces show stippled, silvery-white speckling; undersides carry dark spots of excrement and small clear-winged insects. Heavy feeding bronzes foliage.
Spray leaf undersides with insecticidal soap or horticultural oil when nymphs appear in late spring. Repeat through the season. Site plants in shade, since stressed sun-grown shrubs are hit hardest.