Alternaria Leaf Spot: How to Identify and Treat Leaf Blight (2023)

Have you heard of Alternaria before? If you’ve done a fair bit of gardening over the course of your life, chances are you’ve come across it at some point. It can affect a massive number of different plant species and is one of the leading causes of crop failure.

Let’s take a look at how to identify and treat it. When you know what you’re looking for, it’s a lot easier to take swift action to save your plants.

What is Alternaria?

It’s most often referred to as “leaf spot” or “leaf blight,” as it almost always affects plants’ leaves rather than stems or roots.

This fungus (Alternaria spp.) is incredibly common in cereal crops but also affects brassicas, nightshades, cucurbits, citrus plants, and cannabis. In fact, there are over 300 species of this fricking fungus and it infects over 4,000 different plants.

Some of the most common species include:

  • Alternaria alternata: Say that three times fast! It can be found on several hundred different plant types, and is one of the main species to cause human allergies and respiratory problems.
  • A. brassicae: If you guessed that this one affects brassica plants, you’re correct. It manifests as leaf spot in mature plants, but can also cause damping-off disease if seeds are sown in affected soil.
  • A. brassicola: Another species that attacks brassicas, it’s extremely aggressive and can easily kill plants completely without swift action.
  • A. citri: This species doesn’t cause sports on leaves, but instead blackens and rots maturing citrus fruits.
  • A. cucumerina: As the name implies, this species attacks cucurbits like zucchini, squash, pumpkin, and cucumber with great enthusiasm.
  • A. dauci: This causes carrot leaf blight, and may also affect wild relatives such as Queen Anne’s lace (Daucus carota).
  • A. radicina: Can cause black rot in carrots, parsnips, celery, celeriac, parsley, and lovage.
  • A. solani: Yup, this one hits the Solanaceae/nightshade family hard, causing bullseye leaf spotting and root or tuber rot in tomatoes, peppers, potatoes, and eggplants.
  • A. triticina: This primarily manifests in leaf blight on durum and bread wheat, but can also affect other cereal crops such as barley, oats, millet, sorghum, triticale, and rye.

There are several others that affect ornamental crops as well, such as A. zinniae and A. dianthi.

How Does Alternaria Manifest?

Alternaria Leaf Spot: How to Identify and Treat Leaf Blight (1)

The first symptoms you’ll see will be on the leaves closest to the plant’s crown. Alternaria first presents as tiny little brown spots with yellow halos. These appear on both the tops and bottoms of the leaves rather than just beneath them like some other fungal infections like Anthracnose.

Alternaria Leaf Spot: How to Identify and Treat Leaf Blight (2)

Those little brown spots grow significantly larger, sometimes developing into bullseye-like ringed patterns. Infected leaves eventually curl up, wither, and fall off.

While those leaves are shriveling and falling, the fungus will spread to the other leaves. The spots enlarge, preventing the leaves from photosynthesizing. As a result, the plants don’t get enough nutrition. More necrotized leaf fall means eventual plant death.

Furthermore, those fallen leaves get blown around disperse fungal spores onto soil everywhere they go, thus affecting other plants on the property. Then the cycle repeats itself.

Conditions in Which it Thrives

Remember that there are hundreds of different Alternaria species, so there will be variations on which spores will thrive where. That said, the majority of them prefer cool, damp conditions.

For example, your plants may suffer an Alternaria bloom after heavy rainfall that lasts more than a few days, especially in spring or fall. This blight thrives when humidity is hovering around 85%, and temperatures are between 55-85°F (that’s 12-29°C). As you can imagine, it thrives gloriously in the Pacific Northwest and New England. And Olde England. And most of Canada and Northern Europe too.

Alternaria also likes to lurk in shady spots around the periphery. Do you have big trees and tall grasses around the edge of your property? Then chances are high that there are some spores nesting in the soil there too. Since it’s killed off by the sun, deep shade is one of its dearest friends.

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How to Treat Alternaria

Like many other fungal issues, prevention is the better part of valor. Remember how Alternaria thrives in cool, damp conditions? You can pretty much kill it off by deep tilling your soil during a hot, dry period. Let the sun obliterate and dissipate the spores over the course of a few days.

Don’t plant seedlings until the soil has warmed significantly after your area’s last frost date. Then use drop irrigation at soil level and avoid overhead watering. During periods of high humidity, cut back overabundant foliage to allow air circulation around leaves.

If Alternaria does show up, remove the infected plant parts immediately and burn them. Then treat surrounding plants with a fungicide that contains potassium bicarbonate, powdered sulfur fungicides, Bacillus subtilis, copper hydroxide, or mancozeb. Then, once this growing season is over, don’t plant crops of the same type in that areas for three to five years.

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Regular crop rotation and deep tilling can prevent a significant number of soil-borne pathogens. Just make sure to do so during dry heat waves, as mentioned, or you’ll risk spreading the fungal spores around rather than destroying them. As mentioned earlier, make sure to clean up (e.g. rake away) leftover garden debris in both spring and autumn, and remove fallen leaves and other detritus from around your plants’ stems as they grow.

Fun Fact: Alternaria Doesn’t Just Affect Plants

So, this is fun: Alternaria isn’t just limited to plants—it can cause skin infections in humans too. There aren’t many plant pathogens that can jump kingdoms, but this fungus is one of them. The issue it can cause is known as Cutaneous Alternariosis.

It’s very rare: only 71 cases have been documented so far, and those have mostly occurred in Europe. Furthermore, it seems to be limited to people who are immunocompromised, and/or have been taking systemic corticosteroids. It can happen when and if the fungus gets into small skin abrasions and manifests as a scaly discoloration.

If you live with autoimmune conditions and/or are on steroid medications, be sure to wear gloves while gardening. Although this fungus won’t cause serious health concerns if it gets under your skin (literally), it is a bit of a pain to get rid of.

Additionally, those who are allergic to mold or mushrooms may also react to this fungus. Should any of the aforementioned symptoms appear, make sure to wear a mask and protective eye gear while treating and disposing of affected plants. Furthermore, avoid rolling around in huge piles of autumn leaves, regardless of how much fun they are to jump into. If your pets partake in such shenanigans, bathe them well afterward so they don’t spread spores all over your home.

Maintain Good Gardening Hygiene

Practice good gardening hygiene and inspect your plants on a daily basis. If you’re able to catch an issue like Alternaria early, you might only lose a few leaves here and there rather than an entire crop. If you find that you’re dealing with recurring blights across plant species, you may have to deep-treat your soil with more aggressive fungicides. Then choose seeds or seedlings that are resistant to fungal infections.

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Remember to clean and sterilize your tools on a regular basis, especially if any fungal issues present themselves. It would be devastating to put a ton of time and effort into treating your plants, only to infect others by transporting spores via spades, trowels, or rakes. Clean your tools in hot, soapy water on a regular basis. Add a bit of bleach to that water food good measure, and then let the tools dry thoroughly in the sun.

All farmers and gardeners will have to deal with pathogens like these at some point. Fortunately, diligence and early detection can mean a world of difference to your plant’s health. Yeah, some days we’re utterly spent and would rather read in bed for another hour than trudge out to the fields at first light, but those few extra minutes can result in a few more jars in the pantry for winter sustenance.

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