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 responding to what nature presents.

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 occur in susceptible plants when they are 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 the stems are pinched back and sprayed with fungicide regularly (see the label for the recommended reapplication frequency).

Wilting

While it may seem counterintuitive that excessive rain can lead to wilting, it is a common phenomenon that can be caused by several factors.

First, and most benign, is that leaves may droop when sunny conditions return after an extended period of cloudy, rainy weather. When the sun returns, the roots are briefly unable to supply water as fast as the leaves are losing it (transpiring). This can be due to a temporary lack of oxygen around the roots, caused by soil oversaturation. This usually only lasts a day or two 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 reason for wilting is root disease. In areas with poor drainage (including containers with blocked drainage holes), soil can remain saturated for extended periods. 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 area of the plant and the soil is dark and soggy and has a rotten odor, this is your problem.

Treatment: Mild root rot can be treated by reopening blocked drainage holes in containers. 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 the label directions for application. If possible, pinch back or prune the plant to reduce the number of leaves demanding water. Once wilting stops and the plant begins to show signs of new growth, apply organic fertilizer.

Severe root rot often causes the plant to die. 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, requiring different treatments (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 exhibit signs of nitrogen deficiency 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 overapply as excess nitrogen can cause problems with diseases or insects.

Photo Credits
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