I have a native Dogwood tree that is suffering from Dogwood anthracnose. It is situated all by itself in a mulched bed that gets full sun all day. The leaves and flowers developed brown spots and blotches and eventually shrivelled up. I pruned off all affected parts and now the tree is beginning to replace all the lost foliage with fresh shoots. What is the most environmentally friendly treatment for this disease? I've heard that certain treatments harm not only the pathogen but nontarget organisms as well, and I'd rather refrain from using highly toxic sprays.
Quoted from " home and garden pest management guide for BC, 2001" page 134. " Management: ... Rake up and burn or dispose of any fallen leaves during the growing season and autumn. Prune out dead or diseased twigs and branches. If trees are small, fungicide applications may be worth while. Spray trees thouroughly at bud break and 10 to 14 days later with Benomyl 50% WP at 1 ml/L of water. ( * Benomyl is no longer available for retail purchase where existing supplies have been depleted, an alternative is copper spray or bordeaux mixture, check label for rates and application timing * ). If rainy periods occur later in the season, additional sprays may be required. Severly affected trees may need to be removed."
Dogwood anthracnose (Discula destructiva) is an interesting disease. A couple of decades ago, the disease hit the Pacific Northwest with a vengeance. Although urban trees were affected most severely (undoubtedly because of greater disease pressure and other stresses), trees in undisturbed native habitats were also diseased. So severe and far-reaching was the anthracnose that many people assumed we would lose all dogwoods in the area. Indeed, the disease causes much mortality in eastern North America on Cornus florida (eastern dogwood). Luckily, the pathogen that causes dogwood anthracnose does not appear to be so virulent in the Pacific Northwest that native trees here cannot recover. Weather is probably the deciding factor in infection. A cool, wet period seems to be most conducive to infection, and such factors probably have to coincide with a specific time of tissue susceptibility. In other words, the conditions have to be "just right" for the disease to take off and become established. However, it is well documented that stress predisposes plants to disease susceptibility. Stressors for Cornus nuttallii (Pacific dogwood) would include compacted soil, poor drainage, full exposure with overly dry soil (C. nuttallii is adapted to a summer drought regime, but let's be reasonable!) and wet soil in summer (e.g., irrigated soil -- see previous comment). Nearly all the local anthracnose-affected dogwoods recovered, including the wild natives and the even some of the more severely affected C. florida (eastern dogwood). Anthracnose has visited us subsequently, but mostly only on C. florida and urban C. nuttallii. This suggests that the there isn't much anyone can do to prevent the disease from occurring and that as along as trees aren't overly stressed, they will eventually recover. Avoid spraying the foliage or developing flowers with water. The application of fungicides is probably a waste of money and also likely counter-productive, particularly with a systemic such a benomyl, which will kill most of the good fungi, but probably not the target pathogen. Common fungal pathogens frequently develop resistance to this fungicide (nearly all introductory pest management classes at local colleges have a laboratory session that investigates just how common resistance is -- I'm surprised that the Ministry of Agriculture persists in this recommendation). If it makes you feel better to pick off diseased leaves, go ahead, but remember that any green you remove is photosynthetic tissue that contributes strength to the plant. Better just to leave it alone (take away the dead leaves on the ground -- this is potential inoculum -- but limit foot traffic on the root zone) and wait out the disease. Good luck.
