One of the constants of gardening is that you never have a "perfect" season. It will be too hot or too cool, too dry or too wet. Every season requires adapting and reacting to what nature throws at us.
This season has started on the wet side. Who knows what the entirety of the season will bring? But if it remains soggy, these are some wet weather effects to watch for in the garden.
Lower Leaf Drop
This can happen in susceptible plants when planted close together in beds. The lack of air circulation means that after rains, the lower leaves can take a long time to dry, making conditions favorable for fungus to develop.
Leaves develop small brown or black spots that quickly spread until the entire leaf turns brown/black and drops off. The disease can progress very rapidly during periods of wet weather, going from few symptoms to leaf drop in just a few days. Susceptible plants include blue salvia, verbena, vinca, and zinnias.
Treatment: Apply a broad spectrum fungicide like Funginoil or Infuse to protect remaining leaves. If the stems are not affected by the disease, some annuals will sprout healthy new leaves if stems are pinched back and are sprayed with fungicide regularly (see label for how often you should reapply).
While it seems strange that too much rain can lead to wilting, it is not uncommon and can be caused by a couple of things.
First, and most benign, is that leaves may droop when sunny conditions return after an extended period of cloudy weather. In cloudy conditions, leaves grow faster than roots. When the sun returns, the roots are briefly unable to supply enough water to support the new leaves, until root growth catches up. This usually only takes a few days and will correct itself as long as you don't mistake this wilting as a need for water and apply more. Always check the soil moisture!
A more serious serious reason for wilting is root disease. In areas with poor drainage (including containers with blocked drainage holes), soil can become waterlogged. Plant roots need oxygen and soggy soil has no space for air. Without oxygen, plant roots begin to suffocate and die. Soil diseases that flourish in oxygen-deprived soils can move in and kill more roots. Without enough healthy roots to take up water, the plant wilts. If you dig around the root are of the plant and the soil is dark and soggy and has a rotten odor, this your problem.
Treatment: Mild root rot can be treated as follows: in containers, reopen blocked drainage holes. If you are using a saucer underneath a pot outdoors, remove it so the pot doesn't sit in water. Use a fungicide labeled for soil drench application like Exel and follow label direction for application. If possible, pinch back or prune the plant to reduce the amount of leaves demanding water. Once wilting stops and plant begins to show signs of new growth, apply organic fertilizer.
Severe root rot often kills the plant. If replacing a plant that died of root rot in the same area be sure to amend the soil well to improve drainage or replace the plant with one tolerant of wet soils. If you replant in a container that held a plant with root rot, be sure to sterilize the pot with a 10% bleach solution and use new potting soil. (It's actually a good idea to sterilize all reused pots for this reason.)
White "Powder" on Leaves
This is powdery mildew, a plant fungus that thrives in humid conditions. Many ornamental plants and vegetables are prone to developing powdery mildew. While it rarely kills plants outright, it can cause leaves to yellow or even turn black in severe cases. Plus, it just looks ugly. Good air circulation can help control powdery mildew.
Treatment: Use a broad spectrum fungicide like Funginoil or Infuse as soon as you spot powdery mildew. Or, apply to susceptible plants when wet weather is forecast. Susceptible plants include garden phlox, hollyhocks, squashes, monarda, lilac, and roses. (Check the label to determine how often you can use it.) Prune or pinch plants to allow good air circulation.
Leaf Spots on trees & shrubs)
Wet weather encourages many different leaf spot diseases. Every ornamental plant is susceptible to infection by at least one type of leaf spot. Spots can range in size from pinpoints to covering the entire leaf.
Minor leaf spots are rarely more than a cosmetic problem on otherwise healthy plants, and treatment is not always necessary when few spots are seen or the spread is very slow. Rapidly spreading leaf spots should be treated as soon as possible.
Treatment: It's usually a good idea to look up leaf spots to try to identify them. Some are fungal and some are bacterial, and require different treatment (or no treatment). You can apply a broad spectrum fungicide containing chlorothalonil, copper, or mancozeb when spots are seen, but as noted it is not always necessary or worth the time and expense. The best management practices are put in place before the spots are observed: collect all fallen leaves in fall and discard (do not compost) to reduce the amount of spores present the following season. On susceptible plants or ones frequently affected, apply a protective fungicide (Fungonil) in spring just as leaves begin to unfold and again 14 days after that. If the season is unusually rainy, additional applications may be needed.
Leaves turn pale or yellow
Many plant nutrients are water-soluble and can be washed away in the soil when it rains heavily. Nitrogen, the “N” in N-P-K fertilizer, is very susceptible to being washed away, especially in container plantings. (That’s why slow release or granular fertilizers are better than liquid for long-term greening: liquid fertilizers can wash away quickly.) Even established landscape plants can show signs of low nitrogen in rainy weather.
Treatment: Apply a balanced liquid fertilizer for quick greening, then follow in a week or two with a slow release or granular fertilizer for long-term fertilizing. Don't over-apply as excess nitrogen can cause problems with diseases or insects.
Leaf drop by New Garden Landscaping & Nursery
Wilting Impatiens by Forest & Kim Starr [CC BY 3.0], via Wikimedia Commons
Powdery Mildew by Dollymoon (Own work) [GFDL or CC BY-SA 3.0], via Wikimedia Commons
(cercospora) Leaf Spot by David B. Langston, University of Georgia, Bugwood.org [CC BY 3.0 us], via Wikimedia Commons
Chlorotic Strawberries Public domain by I.Sáček, senior (Own work) [CC0], via Wikimedia Commons