Plant Fungus: Identify & Treat Garden Fungal Growth

Updated March 9, 2022
Plumeria Leaf with Fungus

Every gardener deals with it eventually, damaging the leaves of their prized roses or decimating their beloved tomatoes: plant fungus. Aside from looking unsightly, it can cause lasting damage to plants, and even persist from year to year if it isn't treated properly. The good news is, there's a way to fix nearly every fungal issue you might be forced to deal with in your garden.

Identifying Garden Plant Fungus

Usually, you'll know at a glance that something is amiss with your plants. Fungus is usually noticeable on the leaves and stems of plants, and leaves having spots or being wilted will be your first clue that there might be a fungal problem.

Plant Fungus Diseases Infographic

By knowing what to look for, and being able to identify the most common types of plant fungus you're likely to come across, you'll be that much closer to getting your plants the help they need. With that in mind, here are eight of the most common garden fungus issues.

Powdery Mildew

Mildew on plant leaf

Powdery mildew is one of the easiest to figure out in terms of garden fungus identification, mostly because it looks exactly how you'd expect: like someone has coated the leaves of your plant with a fine dusting of powder. Eventually, the leaves will start to turn grayish brown, dry up, and fall off.

If left alone, the fungus will spread to the stems of the plant, and eventually there will be so much leaf die-off that the plant won't be able to sustain itself. While most fungal issues are less of an issue in dry weather, this is one that will still continue to thrive, even in non-humid conditions.

Plants Likely to Get Powdery Mildew

Lilacs, squashes, cucumbers, monarda, apples, grapes, phlox, zinnias, peonies, and roses are especially susceptible to powdery mildew.

Getting Rid of Powdery Mildew

To get rid of powdery mildew, remove any affected leaves from the plant as soon as you notice them, and then destroy (don't compost) them. Keep an eye on the plant to make sure it's not spreading. If you keep a close eye on it, it likely won't ever get to the point where it will damage your plant, and by removing affected leaves as soon as you see them, you'll keep your plant looking nice as well.

There are several homemade remedies for powdery mildew, usually incorporating baking soda, oil, and soap, but these usually aren't effective, can sometimes cause more harm than good, and can result in a sodium buildup in the soil, which can affect overall plant health. In general, just remove affected leaves, and that's all that's needed.

If it seems like powdery mildew is spreading faster than you can keep up with it, you can try a commercially available fungicide for powdery mildew, or try this DIY recipe:

  • One gallon of water
  • ½ teaspoon of liquid soap
  • 1 tablespoon potassium bicarbonate (NOT baking soda/sodium bicarbonate)

Mix the ingredients, and then spray onto the leaves daily. Potassium bicarbonate has been shown to kill mildew spores, so this should keep it from spreading.

It's worth noting that you can't "undo" fungal issues, so once a leaf has it, the best thing you can do is remove the leaf.

Downy Mildew

Cucumber leaves affected by downy mildew

Downy mildew is most commonly an issue in cool, damp environments that don't have adequate airflow. It's most likely to affect young plants that haven't fully established themselves in the garden, as well as unhealthy or stressed plants.

Downy mildew causes the upper side of the leaf to develop a white or yellow, blotchy appearance. The undersides of the affected leaves will develop a gray, fuzzy-looking mildew. Eventually, the spots on the upper sides of the leaves will turn gray or brown, and the leaf will get dry and fall off.

Temperatures below 65 degrees Fahrenheit are where downy mildew thrives, so you'll want to be more on the lookout for this issue during the cooler parts of the growing season.

Plants Likely to Get Downy Mildew

Downy mildew can affect both edible and ornamental plants. In the vegetable garden, you'll want to keep an eye on:

Ornamental plants such as pansies, snapdragons, salvias, and columbine can also be affected.

How to Get Rid of Downy Mildew

There is no fungicide or spray that will get rid of downy mildew in your garden. Your best bet is to remove and destroy affected leaves, or, if it's affected an entire plant, pull the plant completely.

The spores of downy mildew can overwinter in soil, mulch, and other plant debris, so thoroughly cleaning up any area of your garden where you've had this issue is a must, as well as not planting plants that are susceptible to downy mildew in that spot for a year or two.

Black Spot

Rose bush with disease. Brown spot on leaves

If you grow roses, black spot is the most common plant fungus you'll likely deal with. If you start noticing gray, brown, or black spots forming on the leaves, and then start seeing leaf drop as a result, there's a very good chance you're dealing with black spot.

Black spot is most often an issue in cool, damp weather, or in areas where there isn't good air circulation around the plant, allowing fungus to flourish. Black spot usually isn't likely to kill your plant, but the loss of leaves (if there's a lot of leaf drop) can eventually weaken the plant overall, which is practically an invitation to other pests and diseases.

Plants Likely to Get Black Spot

Roses are the plants that have the most frequent issues with black spot, but other plants affected by this fungal disease include:

  • Dahlias
  • Anemone
  • Gardenia
  • Ranunculus
  • Hibiscus

However, nearly any deciduous plant can be affected by black spot if damp, cool conditions are present.

How to Get Rid of Black Spot

Black spot is another fungal issue that can overwinter in soil and fallen plant debris, so your best bet for protecting your plants is to quickly remove and destroy any affected leaves, both those on the plant and those that have already dropped to the ground.

If the plant isn't getting good airflow, you might want to consider pruning out some of the branches to open the plant up a bit. It's also a good idea to mulch the soil around any susceptible plants to keep infected soil from splashing back up onto the leaves.

And, finally, a fungicide formulated for the specific plant (roses are the most common) might be a good idea. It will keep the black spot from spreading and save your plant from further health issues.

Rust

Rust fungus on leaf

Rust is another plant fungus that looks exactly as you'd expect it to: it looks like orange-brown rust is forming on the leaves of your plants. This is actually a large, varied group of fungi, with over 5,000 different types of fungus. However, the most common type of rust you'll deal with in your garden is Phragmidium.

Rust thrives in areas that are in partial to full shade, as well as moist, warm conditions. If you grow any of the commonly affected plants in these conditions, it pays to inspect your plants regularly in the hopes of catching any rust issues as soon as possible. Newly forming rust will be present on the undersides of the leaves, and will look whitish in color and have a slightly lumpy texture. Eventually, they'll turn that familiar rust color, and then yellow, and eventually the leaf will fall off.

Rust spots are small, and can be scattered all over the leaf and stem. If it isn't caught early on, it can eventually cause the plant to drop most or all of its leaves, which can stunt growth, or even kill the plant outright.

Plants Likely to Get Rust

Some of the most popular garden plants can be affected by rust. Keep an eye out for this fungus on these plants, especially if they're growing in a shadier spot:

How to Get Rid of Rust

The first step in controlling rust in your garden is to be vigilant. Check any of the commonly affected plants you might grow and remove any leaves that start showing signs of rust.

Be sure to clean up any leaves that fall from these plants and destroy (don't compost).

There are commercial fungicides available, and these are a good choice if you're seeing a large amount of rust on your plants.

There are also rust-resistant varieties of most of these commonly affected plants, so if you've found rust to be a persistent issue in your garden, it might be worthwhile to plant some resistant varieties.

Fusarium Wilt

Chili disease viral diseases

Fusarium wilt affects both edible and ornamental plants. It's most commonly seen during the heat of summer. Basically, your plant just starts looking sad: drooping, wilted leaves that no amount of water can revive, along with (eventually) a black, rotted stem and rotted roots.

The fungus that causes fusarium lives in the soil and is carried through the roots into the rest of the plant.

Plants Likely to Get Fusarium Wilt

Luckily, fusarium wilt isn't a fungus that affects a wide variety of plants. If you start noticing the above symptoms with any of the following plants, there's a good chance you're dealing with fusarium wilt:

  • Asparagus
  • Beans
  • Eggplant
  • Peas
  • Peppers
  • Tomatoes
  • Dianthus

It's most commonly an issue for tomatoes, but it pays to be on the lookout for it if you grow any of these plants.

How to Get Rid of Fusarium Wilt

The fungal spores that cause fusarium wilt can live in soil for five to 10 years, so if you find some of your plants affected by it, it's important to make note of that fact and ensure that you're not planting types of plants that are also susceptible to it in that spot for the next few years.

There isn't any spray or other treatment to get rid of fusarium wilt. Your best bet is to remove any plants. Don't add these plants to your compost pile; dispose of them in the trash instead.

You can often kill the fungus in the soil by covering the area with black plastic and leaving it for several weeks (or an entire growing season, if you want to be really safe.)

You can also find varieties of most of these plants that are resistant to fusarium, so that would be a great option if this is an issue that seems to pop up frequently in your garden.

Verticillium Wilt

blackcurrant leaves damaged by fungal disease Fusarium or Verticillium wilt

Like fusarium wilt, verticillium is a pathogen that lives in the soil and is brought up into the plant via its roots. However, unlike fusarium which attacks a fairly focused number of plants, verticillium can affect just about any plant if conditions are right and the fungus is present in the soil.

And because it eventually clogs up the vascular system of the plants, which moves nutrients, water, and carbon dioxide through the roots, stems, and leaves, it can quite often spell death for the plant if left untreated.

Verticillium wilt makes the leaves wilt, shrivel, and fall off, sometimes all on one side of the plant. Eventually, the stems, branches, and roots will also rot.

Plants That Can Get Verticillium Wilt

Over 300 types of plants are susceptible to verticillium, including trees, shrubs, groundcovers, annuals, perennials, and vegetable plants.

How to Get Rid of Verticillium Wilt

Again, like fusarium, there is no treatment for verticillium wilt. Your best option is to remove any affected plant, and then either leave the area unplanted or solarize the soil (cover it with black plastic so the heat of the sun will kill the fungus).

Verticillium wilt can last in the soil for up to ten years. Solarizing is your best option, and with be sure to destroy (not compost) any affected plants so it doesn't spread to other areas of your garden.

Blight

Tomatoes get sick by late blight. Phytophthora infestans.

The most common blight gardeners come up against affects potato and tomato plants. Blights are spread via spores, either being splashed up onto plant leaves from the soil, or (even worse) being carried on the wind.

Blight first appears as papery, light brown splotches on the leaves of plants, and eventually the entire leaf shrivels and falls off. Blight spreads the most readily in warm, humid conditions, and can persist in the soil via fallen leaves or even infected tubers (in the case of potato blight).

Plants That Can Get Blight

Blight is most commonly associated with tomato and potato plants.

How to Get Rid of Blight

Once you notice blight, there's not much you can do to stop it. If you're growing potatoes, your best option to get a crop of edible potatoes is to cut off any stems and foliage all the way back to the ground and dig up any potatoes you can. For tomatoes, you can let the plants keep growing in the hopes that you might get some harvest-size fruits.

All leaves, stems, and roots should be removed from the garden and destroyed. Don't compost them, since you'll risk spreading blight to your other garden beds.

There are blight-resistant varieties of potatoes and tomatoes available, and if you commonly have this issue in your garden, it's a good idea to seek out those varieties.

Anthracnose

Walnut anthracnose or walnut black spot

Anthracnose causes small gray or brown spots on plant leaves and stems, as if someone has sprinkled them with a fungal version of glitter. It's also known as leaf blight or twig blight. This fungus thrives in cool, wet conditions, so it generally tends to show up in spring. It affects a wide variety of garden plants, but the good news is that although anthracnose isn't the prettiest look for your garden, it's rarely fatal, and chances are good that your plants will bounce back just fine.

Plants That Can Get Anthracnose

Anthracnose can affect a variety of plants, including shrubs, trees, perennials, annuals, and succulents.

How to Get Rid of Anthracnose

Once you notice the telltale spots on the leaves of your plants, clip them off and dispose of them in the trash. Also, be sure to rake up and remove any stems or fallen leaves, and consider mulching the soil around your plants to keep any fungus in the soil from splashing up onto your plants. There really isn't any spray or other treatment for anthracnose.

Garden Hygiene and Pruning to Prevent Fungus

Once you've identified what type of fungus you're dealing with, you can also work on preventing it in the future. Many fungal issues are encouraged by poor airflow, so consider pruning and thinning your plants, as well as mulching the soil and making sure you rake up any leaves from around diseased plants. A little vigilance, some preventative measures, and a touch of garden tidiness will go a long way toward having fewer fungal issues to deal with.

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Plant Fungus: Identify & Treat Garden Fungal Growth