See:
Kalmia (Mountain Laurel) - Leaf Spots
Rhododendron - Ramorum Leaf Blight and Shoot Dieback
Cause Phytophthora syringae, a fungus-like microorganism. Disease develops in winter and early spring during cold, rainy weather. Rain-splashed inoculum begins the infections generally on the lower leaves. Infected leaves fall off the plant and sporangia produced on these leaves during cold, rainy weather can spread the disease. Oospores in the leafy debris and soil allow the pathogen to survive until the following winter. This Phytophthora species only infects the aboveground parts of plants.
Symptoms Irregular necrotic spots develop on the leaves. They often affect the leaf tips. Symptoms usually begin on the lowest leaves, nearest the ground, then progress upwards, leading to defoliated stems. Stems may also become black and necrotic. New buds and shoots become infected and defoliate, leading to branch dieback. Symptoms are similar to those caused by other foliar-infecting Phytophthora species but Phytophthora blight will occur during the winter and early spring.
Cultural control
- Rake up and destroy dead leaves around the base of the plant.
- Prune off infected leaves and branches.
- Mulch to prevent inoculum in soil from being splashed onto the lower leaves.
- Avoid reusing pots from a previous crop for propagation. If pots must be reused then wash off all debris and soak in a sanitizing solution or treat with aerated steam for 30 min.
- Prevent potted plants in nurseries from falling over allowing foliage to touch the ground.
Chemical control Use fungicides as preventative treatments. Tank-mix and/or alternate products from different groups with different modes of action to prevent the buildup of resistant fungi.
- According to IR-4 data, Aliette is safe on this crop. This product provides good control of this disease on many other crops. Do not use with adjuvants. Group P7 fungicide. 24-hr reentry.
- Daconil Weather Stik at 1.38 pints/100 gal water. Use preventively only. Group M5 fungicide. 12-hr reentry.
- Insignia SC at 6 to 12 fl oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventively only. Group 11 fungicide. 12-hr reentry.
- Mancozeb-based products can be used as mixing partners and provide some protection. Group M3 fungicides. 24-hr reentry.
- Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
Reference Jones, R.K., and Benson, D.M. (eds.) 2001. Diseases of woody ornamentals and trees in nurseries. St. Paul, MN: APS Press.