It appears that spring is slowly arriving, and with the spring typically comes regular, often frequent rain showers. The upside to this moisture is that it helps thaw the ground and stimulate plants to grow. The downside however can be that this moisture provides a favorable environment for plant diseases to develop.
Damping-Off: If you like to plant early when soils are colder and moisture is high, you may run into problems with damping-off. Damping-off pathogens (e.g., Pythium, Rhizoctonia, Fusarium) are found, at least at some level, in many soils and when combined with wet conditions and young, tender seedlings, death and destruction can be the result. Watch for plants that never emerge (the seed rot or pre-emergence phases of damping-off) or those that do and then fall over onto the soil surface with collapsed lower stems (the post-emergence phase of the disease). You can often avoid damping-off by planting later when soils are warmer and there is slightly drier weather. The warmer soil stimulates plants to grow rapidly out of early stages of growth when they are most susceptible to infection. Using a good seed fungicide treatment (often commercial seeds are pretreated prior to packaging) can also help prevent the disease. Just be sure to handle any fungicide-treated seed according to the directions on the package to minimize any direct exposure to the fungicide.
Root Rots: Root rots are caused by many of the same organisms that cause damping-off including Phytophthora, Pythium, Rhizoctonia and Fusarium. Root rots differ from damping-off however in that affect older herbaceous and woody plants. The pathogens destroy root tissue, thus reducing water uptake, and that can eventually translate into dieback, general plant decline and, in extreme cases, death. Root rot organisms tend to perform better in wet soils, and Pythium and Phytophthora actually reproduce more efficiently under cooler, wetter conditions. So, making sure soils are well drained can be critical for root rot prevention. Adding organic matter to heavier, clay soils to improve drainage prior to establishing a landscape can have a big impact long term on reducing root rot problems. Also, making sure to mulch properly can help moderate soil moisture to a level that makes root rots less likely. I typically recommend using approximately one to two inches of a high quality mulch (e.g., shredded oak bark mulch or red cedar mulch) on heavier, clay soils and roughly three inches of mulch (perhaps up to four inches) on lighter, sandier soils. The mulch should be applied out to at least the dripline of trees and shrubs (i.e., the edge of where the branches extend) and kept away (approximately four to six inches) from tree trunks and crowns of shrubs.
Leaf Spots and Blights: Spring rains can also have a huge effect on the severity of many types of leaf spots and blights like anthracnose, tar spot and apple scab. If extended rainy periods arrive when leaves are first emerging, then numerous infections can occur early and that can translate into severe disease and possibly even defoliation later in the summer. Luckily trees seem to tolerate at least some defoliation, and long term effects due to leaf diseases are often minimal. However, defoliation year after year can stress plants to the point where they become susceptible to more serious diseases (e.g., Armillaria root disease) and insect pests (e.g., two-lined chestnut borer). While we have no control over Mother Nature and the rain she brings in the spring, using other disease management strategies can help lessen the effects of wet spring weather. Careful cleanup and destruction (by burning, burning or hot composting) of plant debris in the fall can significantly reduce leaf pathogen carryover. Proper pruning of trees to promote better air penetration, allowing for more rapid drying of foliage can also help reduce problems with leaf diseases. For certain diseases like apple scab, growing a resistant apple or crabapple variety may be your best option. And finally, in certain situations, use of preventative fungicide treatments may be warranted to keep leaf diseases in check.
So, as you dream of those warm spring rain, dream of them in moderation. As with most things in life, balance is the key. Hope for enough rain to get your plants to grow, but not enough to lead to disease problems. As always, if you have questions about plant diseases and their management, feel free to contact me at (608) 262-2863 or firstname.lastname@example.org.