Is it a Mold or Mildew Problem in Your Garden?

By Diane Young
Published: June 11, 2020 | Last updated: April 23, 2021 03:23:46
Key Takeaways

Got M&M's? No, not that kind. We’re talking about molds and mildews, an unappetizing subject and a bane to growers. M&Ms are fungal diseases (oomycete microbes) that most growers have to face at one time or another. Molds and mildews thrive on one thing: moisture. Powdery and downy mildews are the two culprits that most frequently plague gardeners.

Both mold and mildew thrive in humid conditions and can be tough to get eliminate. Let's determine which one you have and how you can get rid of it.


Types of Mildew

Powdery Mildew

Powdery mildew produces white spots on plant leaves. It can affect plants outdoors, but it’s more common indoors and in greenhouses. Indoor powdery mildew spreads rapidly when the temperature is around 70˚F. Poor air circulation and low light put out the welcome mat for powdery mildew and, unlike outdoor powdery mildew, it thrives in drier conditions. Powdery mildew is an aggressive, contagious disease.

The most common plants affected are ornamentals—begonias, African violets, kalanchoe, ivy and jade. The white powder rubs off easily with your finger. Don't hose off affected plants. Instead, water from the bottom to avoid wetting the leaves. Space plants to give them elbow room to grow and to enhance airflow.


Indoors, you can direct a small fan on a low setting toward them. Exhaust fans are a must in greenhouses. If you find powdery mildew on a plant, it’s time to take quick action. The good news is that powdery mildew can be cleared up successfully with products like neem oil from your local hydro store.

Neem and Tea Tree Oil Sprays for Fighting Plant Pathogens

Neem oil is pressed from an evergreen tree native to India and Pakistan. Tea tree oil is from the Malaleuca tree, which grows in southern California and similar climates. Add 2-3 drops of liquid dish soap in a small amount of warm water in a 1 qt. spray bottle, swirl to mix, then slowly add 1 tsp. of either oil (organic and cold-pressed) and finish filling the bottom with more warm (not hot) water, swirling vigorously to mix well.

Use organic soap as many commercial soaps contain harsh chemicals that can harm your plants. Spray the oil fungicide every three days or once a week in the morning before the sun gets too hot as it can burn oil-treated leaves. Spray sparingly only on the affected areas, don’t soak the foliage and don’t use oil spray on new plants or recently transplanted seedlings. If the leaves start to wilt after any of the sprays, there’s too much soap—dilute with more water. Both neem and tea tree oil are available in ready-to-use sprays.


Read More: Neem Oil - The Answer to a Pest-Free Organic Garden

Downy Mildew

For starters, downy mildew is a particular problem for crucifers—everything in the cabbage family including radishes, grapes, cresses and wasabi. It doesn’t survive year-round in Canada or the northern United State, but it winters over in the Gulf Coast states. In the spring it migrates north in time for cucurbit crops—pumpkins, melons, squash, cantaloupes, some gourds and anything else that grows on vines.


Downy mildew only targets plant leaves. Large, yellow patches appear on the upper leaf surfaces. As the young leaves mature, the spots spread rapidly and turn brown, and the undersides of the leaves look water-soaked. If you look closely or through a magnifying glass, you’ll see a purple-brown mold. With a 10X lens, you’ll also see small football-shaped spores among the mold. Downy thrives during cool, dewy nights, spreading rapidly, leaving you with dead leaves hanging from the stems.

To control downy mildew, eliminate all the moisture you can around affected plants, improvedrainage and tie or train your vine plants onto trellises or other upright supports. Water from below with a drip hose to avoid splashing the leaves and do selective leaf pruning to improve air circulation. Reduce the humidity with vent and window fans to pull still, moist air outside. Since downy mildew appears in late summer, select early season varieties to shortcut the problem.

While one group of downy mildew only attacks cucurbits, others zero in on a single plant type, such as basil, grapes, hops and soybeans. When working with downy mildew-infected plants, wear safety glasses, long sleeves and a face mask to keep the spores from your eyes, nose, throat, lungs and skin.

Wash your hands and fingernails carefully afterward. Keep your growing area free of plant debris. If all else fails, try a commercial fungicide to kill the spores to prevent spreading. Spray outdoors or in a well-ventilated greenhouse. To give nature a break, select products that are non-toxic to birds, plant pollinating-butterflies and our dwindling honeybees.

Types of Mold

Molds are small organisms that are found almost everywhere. They can be black, white, orange, green or purple. Like mildews, they reproduce by tiny, seed-like spores that travel in the air. They start to grow as soon as they land on a damp spot. Various rot and rusts are also in the mold family. Follow the same precautions to avoid molds as for mildews. It's all about moisture, poor air movement and shade.

Gray Mold

Gray mold is another unwelcome fungus that affects a wide range of plants. It especially likes herbaceous annuals and perennials. This is the baddie that ruins your strawberries! It moves quickly during damp, cool, mild weather. You can recognize gray mold by gray mushy spots on flowers, leaves, stems and produce. It develops first on wilted flowers causing plants to shrivel and rot.

Be sure to remove these dead flowers daily. It needs high humidity—over 93%—and temperatures between 45 and 60˚F. Cuttings are especially susceptible to gray mold. To help prevent it from spreading, disinfect pruning shears, any other tools used and work surfaces. Copper and sulfur-based organic fungicides help control gray mold.

As an aside, blossom-end rot on tomatoes looks like black mold, but it’s actually caused by calcium deficiency. Correct this with a calcium spray or with amino acids, which contain calcium, into your soil. If you find any moldy growth you’re not sure of, take some affected leaves or fruit to your agricultural extension service for identification and correct treatment.

Slime Mold

There are more than 900 species of slime molds that flourish in moist shade. Yuck! They’re all harmless, not a threat to gardens, and only eat bacteria, yeasts and other fungi on rich soil, dead leaves, the forest floor and rotting raised-bed timbers. In urban areas, slime molds are found in mulch, compost, leaf mold in gutters, and even in air conditioners with clogged drains—a good reason to clean the drain regularly in air-conditioner units in shady windows. They were once considered fungi because they grow from spores, but are now in the Protista family because they morph into complex life cycle organisms.

Read More: Grow Organically with Compost Teas & Nutrient Mush

Wolf's Milk Slime

This is also known as Toothpaste Slime. It looks like small, round reddish-pink balls that release a pinkish-gray paste when popped. It grows peacefully on dead wood and big logs.

Black Slime Mold

This slime mold is chilling to look at and worse to step in. Fortunately it prefers lawns and is rarely seen by urban gardeners.

Dog Vomit Slime Mold

This is the one that always grosses people out and is seldom correctly identified. It looks like dog barf and people have complained about the neighbor's dog vomiting in their yard. It can be bright yellow or peanut butter-colored, fluffy like scrambled eggs, foamy, slimy, soft like a pancake, sometimes bleeding a dark red fluid, or a repulsive gelatinous mass that spreads like an alien life form, capable of reaching the size of a 14-in. pizza! Gross! If you break it up, the pieces will pull themselves back together. The blobs can navigate, go around obstacles and if a food source is nearby, the mold senses it and heads straight for it at snail speed.

Dog vomit slime mold usually goes through its life cycle in five to seven days, getting hard and crusty before releasing its spores for the next generation. If you can't stand looking at it that long, gently shovel or scoop it into a plastic bag, close tightly and toss in the trash. Don't try to hose it away if it's in the hard, crusty stage or you'll make the spores burst out like a small cloud of smoke. To prevent an unwanted return, work neem oil into the spot, which kills any fungus in the soil.

Just remember that M&Ms are easier to prevent than they are to cure. I hope all your M&Ms are the chocolate kind!

Read Next: Battle of the Bugs - A Guide to Beneficial Insects


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Written by Diane Young

Profile Picture of Diane Young
Diane Young has had her food and gardening articles published in the Burlington Free Press, The Boston Globe, The Christian Science Monitor, Country Journal and other regional publications. Soon after, Troy-Bilt commissioned her book, Community Gardening in North America. A New England gardener, Diane lives in Maine and Mexico.

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