Disease by Pseudomonas syringae tends to be favored by wet, cool conditions. The optimum temperatures for this disease tends to be in the winter months when temperatures are below freezing. Considering this cold winter I will focus on this pathogen. P. syringae is a very common winter-born bacterium.
Do you sometimes observe black stem canker in Japanese maples? Many mistake this common maple disease for a fungal infection. The pathogen actually is Pseudomonas syringae, a bacterium that should be better understand in order to manage Japanese maples. Chemical application control is possible but costly. A better tactic is to understand the causation of the problem.
1. Pseudomonas is a cold weather disease. The most critical factor is to keep young plants from freezing, be they in nursery containers or in bonsai pots.
2. The bacteria are seed borne, and can be dispersed between plants via rain splashing on soil. Mulching with bark or gravel will reduce the risk. That said, insects and windborne activity can spread it as well.
3. Age of the plant is a factor, leaving tender young maples exposed to below freezing weather is risky.
4. Young plants should be moved under protective cover in early October. Pseudomonas bacterium needs water on the plant structure to be infective. Keeping the plants dry, especially at night is imperative. Cold winter dew can cause damage, so can a humid cold greenhouse.
5. Proper ventilation is a must, and one should take care to water all such greenhouse plants only in the morning, preferably on a sunny day -- so the foliage can dry off before dusk.
6. Pruning or making any such wounds during the winter months should be avoided. When pruning or grafting, the tools should be properly sanitized with alcohol, TSP or even mouthwash. All open wounds provide entry points for this bacterium. If pruned -- or the problem is sighted, Spray with copper spray or lime sulfur. Doing this on a sunny day where the spray can dry off is imperative.
Non-the-less, normal winter freezing and thawing can create cracks in the stem structure of many plants and provide entry points. Copper spray is a control but nothing more. It will not "cure" the plant. Typical symptoms include winter-spring tip blight; blackened stems that are quite different from Verticillium Wilt -- more of a summer disease, where sudden wilting occurs followed by the withering of the leaves and the sudden dieback of shoots.
Prevention requires intelligent purchasing. When you buy these plants from a Nursery avoid such stock. Be critical of the cultural needs of these plants and if you see such plants stump pruned, walk away ... but mention your concern to the Nursery. Once purchased and planted in the garden these plants will provide infection sources for the rest of your garden. Handling of the soil, stems or going from one plant to another can spread it. Planted in the ground, one's footprints can spread it. Pruning should be avoided. If your gardener suggests winter pruning, find another.
Maples are affected by a number of fungal diseases as well. Several are susceptible to Verticillium, which can cause significant damage. This fungus will be present in the soil as well as on plant tissues. Poor pruning techniques contribute to its spread. Death of maples can also be caused by Phytophthora root rot and Ganoderma root decay. Planting too low in ground cloth or putting drip emitters next to the trunk contributes to this. Typically one finds dead tissue with bluish-black stains in the tissue beneath. Ones shoes can also spread these spores.
In late summer and autumn Maples, esp. Vine Maples are commonly disfigured by leaf spotting, caused by Rhytisma fungi and mildew caused by Uncinula and although unattractive these fungi may cause the tree to be, they do not effect the trees' long-term health. Overhead sprinkling contributes to this problem.
I will add a 2009 edit. For years I have suffered from a "Alder allergy" which affected me for over a month every Spring. After stopping my spraying of Copper I have had no such reactions. I was not suffering from allergies -- I was suffering from chemical overdose of what I was spraying for decades.
To put reference to a serious lung disease that my Aunt died of: I will add that "Anthracosis" (not a plant disease) is human based and caused by the the accumulation of carbon particles in the lung. The common culprit being the gulping down of carbon, be it from tobacco, diesel fumes or the burning of late summer plant debris. All is dirt to your lungs and to your body and it never gets rid of it. It stays in your lungs.
(C) secauteur 2003