Preventing Disease After Heavy Rains

Too much of a good thing?
What to do when heavy rains bring diseases to your garden

During the summer, we see it all—blistering hot sun, then lingering rains, and even hail and high winds. All of it takes a toll on your garden. Although it seems that droughts get much more attention, the truth is that heavy, soaking rains can cause a variety of diseases in gardens, including nasty, fungal diseases that thrive in warm, humid, wet conditions. Many plant diseases require moisture to infect and spread, which makes everything worse after a heavy rainfall. Plants that wash away or rot in soggy soil are obvious problems, but there are more subtle, lingering issues that often can occur after a lengthy period of rainfall. 

Injured roots

Some plants have almost no tolerance for soaked soil and can die very quickly from oversaturation. Lavender, azaleas, carnations, and dianthus rarely survive a “super soaking” event and much prefer drought to drowning. Others, such as daylilies or iris may not die, but they suffer from root rot. If it isn’t addressed, they might survive in the short term, but in a few months, they’ll die off. Many needled evergreens are also vulnerable, but don’t turn brown and drop needles for months after the rain event. 

So, what do you do about injured roots? Unfortunately, not much. Here are some tips going forward:

  • Select plants that are more tolerant of wet conditions.
  • Improve drainage by adding organic matter and compost when you plant.
  • Use raised plant beds instead. 
  • Be careful not to walk on soggy soil, which compacts it even further.


Too much rain can lead to spotting, streaking, and browning of leaves. This is caused by the fungi that love these warm, wet spells of weather, but the good news is, many times plants will just grow through this wet phase and ultimately, return to normal. Keeping plants covered in fungicide is an option, although it’s challenging. New growth might not be covered, and even just a little rainfall can wash most of the fungicide off a plant. When the sunshine and heat return, the fungi will likely die off on their own. 

What to do:

  • Don’t assume that if a plant has dropped its leaves, or if a perennial has yellow or brown leaves, that it’s dead. 
  • Wait until spring to see if the plant has new outgrowth.
  • Rake and remove fallen, diseased leaves and make sure they don’t go in the compost pile. Throw them in the trash. This way, fungal spores do not combine with your compost to reinfect your plants. 

Molds and mushrooms 

Look out at your lawn. Are there mushrooms and mold out there? Wet lawns often produce lots of different mushroom-looking growths, and mulched beds may have bulb-like growths called slime mold. They start out yellowish and turn black. There’s no need to spray, and no need to worry about these. 

What to do:

Nothing. They’re harmless to pets and people. If you don’t like them, just rake them away. 

Yellow or brown leaves

It’s possible that leaves of plants turn yellow or brown because of lack of nutrients, not disease. Often, after a long wet spell, the microorganisms that break down root-feeding nutrients in the soil have in effect, drowned from the rain. 

What to do:
This problem often corrects itself when the soil dries out again. 

If not, give the plants a fertilizer treatment. Make sure it contains nitrogen and iron, as those are generally very effective at restoring nutritional balance to plants that have been overwatered.

Pro tips:
Check for soil that has been eroded after a heavy rain. Don’t allow tree and shrub roots to be exposed—recover them as soon as possible, or they’ll die. 

If you find the washed-out soil has been deposited elsewhere, try to return it to its original spot so you don’t have to buy more to replace it. 

Don’t allow soil or mulch to stay piled up against tree trunks after a storm as it can deprive the tree of oxygen and kill it.

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