In general, most fungal leaf spots are not threatening to the health of the plant, although under severe conditions some defoliation can occur. The symptoms usually include discrete spots with tan to brown centers surrounded by a darker border. The most common fungal leaf spot pathogens of Rhododendron spp. are Botrytis cinerea, Pestalotia sydowiana, P. rhododendri, Septoria azaleae, Colletotrichum azaleae (Glomerella cingulata), Cercospora handelii, Phyllosticta cunninghamii, and P. rhododendri.
- Leaf spots caused by fungi often can be distinguished by their fruiting structures and pattern of lesion development.
- There are numerous leaf spotting diseases that occur on shrubs, but few are lethal.
- Most established shrubs produce more leaves than they need for normal growth. Unless severe leaf defoliation takes place, enough leaves are usually left for healthy growth.
- On young shrubs or newly planted shrubs leaf loss is more detrimental.
- When a shrub loses most of its leaves its food reserves are depleted which may cause dieback, decline, and/or death.
- The majority of leaf spotting diseases are favored by cool, wet, spring weather.
- Shrubs that are prone to leaf spotting diseases include aucuba, mountain laurel, photinia, roses (cercospora and black spot), junipers, firethorn, leucothoe, and rhododendron.
Pseudocercospora leaf spot (Pseudocercospora kalmiae) on mountain laurel. Photo: Elizabeth Bush, Virginia Polytechnic Institute and State University, Bugwood.org
Entomosporium leaf spot (Diplocarpon mespili) on chokeberry (Photinia x fraseri). Photo: Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org
- In most cases, leaf spotting diseases will not threaten the health of the shrub.
- The best management practices for most leaf spotting diseases involve pruning and removal of infected leaves and dead twigs during the winter or dry summer months.
- Give plants plenty of space to allow good air circulation, minimize water on the foliage by not wetting the leaves when watering.
- Rake and remove infected fallen leaves in the fall followed by applications of fresh mulch.
- Plant resistant varieties when available.
- For plants such as roses, where leaf spots can be detrimental, select resistant cultivars and apply fungicides when necessary.
- Fungicides are not curative and will not help leaves that already have spots on them. For them to be most effective they need to be applied before the leaves become infected.
- Although fungicides are registered for disease control, they are not necessary for most situations.