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.
Rhododendron Petal Blight
Small water-soaked spots on flower petals that enlarge into soft brown blotches; blooms turn slimy and collapse, then dry and cling to the plant.
Remove and discard infected flowers and fallen petals. Avoid overhead watering. Improve air circulation by thinning. Apply a fungicide labeled for petal blight at bud break in wet springs.
Rhododendron Leaf Spot
Round to irregular brown or purple-bordered spots on leaves, sometimes with a yellow halo; heavy infection causes leaf yellowing and drop.
Rake and destroy fallen leaves. Water at the base to keep foliage dry. Prune for airflow and apply a copper-based fungicide if spotting is severe.
Azalea Lace Bug
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.