Water molds spread via "swimming" spores that move easily through water. That means frequent rain and the resulting saturated soils both improve conditions for the pathogens. The movement of water above ground helps to spread the molds to new plants, and perpetually wet or damp soils allow them to thrive.
Phytopthora spp. is a set of water molds responsible for several very serious tree diseases, including Sudden Oak Death and Beech Bleeding Canker. It also affects Maples and other hardwood trees and shrubs, primarily in their root systems. Due to the especially wet weather this year, we've seen a lot of Phytopthora root rot in the landscape. In most cases, symptoms of decline above ground (small leaves, stunted growth, dead twigs) are traced back to the root system (where there is often discoloration, noticeable rotting, and sometimes lesions on thicker roots and even at the base of the stem).
To preserve an infected plant, treatment for Phytopthora is critical. In addition to applying controls for the disease, making environmental changes that improve drainage and keep the root system of a plant from being saturated with water is incredibly important. (In fact, sprinkler systems can be just as damaging as heavy rains in this respect -- just one example of why it's important to take all of the factors in a plant's environment into consideration.)
Another water mold, Pythium spp. causes a number of diseases in turf-grass, including Pythium blight and Pythium root rot. Both of these diseases spread quickly and create irregular patches of brown grass on a lawn. With the blight, grass will often be wet or greasy first, turning later to a more dried out brown. You may also see fungal threads growing above ground on turn infected with Pythium blight. The root rot, on the other hand, is less obvious above ground, presenting as dead brown patches. The roots, however, are obviously rotten and discolored.
Ryegrass is especially prone to Pythium problems, as are bluegrasses and fescues. As with the Phytopthora, controls are available, but fixing drainage and irrigation problems is just as, if not more, important. Certain practices, like refraining from mowing grass when it's wet, will also help prevent the spread of these diseases.
- Ken Almstead, Arborist in Riverdale & Lower Westchester NY