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Tomato early blight, the dreaded Alternaria fungus: alternatives to beat it

Black necrosis on leaves with a yellow border are the marks of alternaria, or early blight, on tomato plants

This blight is a true plight! Early blight covers tomato plants with black spots, and even makes the fruits inedible. From trying to avoid in the first place, to treating it with natural treatments, discover how to deal with this disease while using only completely organic techniques!

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What is early blight?

Early blight is a fungal disease. This means it’s caused by a type of mushroom. The culprits are: Alternaria solani (for the tomato specifically) and Alternaria alternata ! As is the case for all fungus, they tend to appear when the air is warm and moist. Although this disease most often infects tomato plants, it can also impact potato, red beet, fruit trees, cabbage, carrot and eggplant. These particular mushrooms stay dormant in the soil, and they easily survive winter colds. They spread through various means: rain, wind, contact with contaminated soil, sick plants or infected seeds. When the air become loaded with warm moisture, spore germinate and quickly infect tomato plants.

What are the symptoms?

Symptoms of early blight on tomato include brown and black stemsBlack spots in the shape of round rings form on stems, leaves and fruits. On the fruits, these spots lead to rot: they’re not edible anymore. Early blight is distinct from downy mildew! In the case of mildew, yellow spots appear that then fade to brown. Keep an eye on the weather forecast, since this disease takes hold during warm spring days. If it rains a lot, risk of developing the disease increases, but sometimes simply morning dew followed by a warm afternoon is enough to trigger early blight.

Preventive measures

  • Grow your tomatoes under the cover of a greenhouse or tunnel to avoid both morning dew and rainfall.
  • Space plants further apart from each other to reduce the moist air “oasis” effect.
  • Tomato fruits aren't edible anymore when infected by early blightAvoid wetting the leaves when watering. A good idea is to use a cut plastic bottle as a funnel, stuck into the ground near the foot of the plant. That way you can water without getting water on the leaves.
  • Practice crop rotation.
  • Don’t grow plants that are vulnerable to the same diseases near each other.
  • Select certified disease-free seeds, and never collect seeds from plants that have been infected.
  • Choose robust varieties that resist disease.
  • Spread a layer of mulch on the ground to ensure fruits don’t touch wet soil.


Natural preventive treatments

Still more prevention is possible to offset the disease’s appearance: fermented tea, weed decoctions and plant infusions will reinforce plant immune systems, help them develop strong roots, and will boost growth and fruit formation.

Horsetail tea

This natural fungicide is sprayed on vulnerable plants. Never use it pure: you’ve got to dilute it to a 10% mix (9 volumes water for 1 volume pure tea). Use it to water the plants, once or twice a month. It’s easy to prepare yourself, but you can also find it in garden stores nowadays.

Garlic infusion

Garlic has both antifungal and antibacterial properties. Use it to prevent fungal diseases such as early blight. Prepare a batch of garlic infusion yourself at home, all you need is a large cooking pot.

  1. Chopped garlic works wonders against tomato early blightThinly chop a pound of garlic (500g).
  2. Toss these tiny bits into 5 quarts of water (5l).
  3. Bring to boiling point, then let simmer for 30 minutes.
  4. Turn the heat off and cover the pot.
  5. Let it sit for 12 hours.
  6. Filter your mix, pour into bottles or jugs.
  7. That’s it! Ready to use (but remember to dilute it!) with a sprayer.

Bacillus subtilis

This bacteria is used to colonize space on leaves and thus keep Alternaria alternata fungus from taking hold. It’s a bacteria that also secretes anti-fungal compounds that block spore germination.

Curative treatments that are organic

There are no curative treatments against Alternaria solani fungus, the one that impacts tomato. Regarding the other species – tomato, red beet, and citrus species, there is a treatment that helps eradicate Alternaria alternata. It’s actually another fungus called Aureobasidium pullulans that competes directly with Alternaria for space and nutrients. As for infected tomato plants, there’s no choice but to eliminate diseased specimens and carefully apply all the preventive measures to keep the disease from spreading any further.

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  • Angela Dyson wrote on 1 June 2024 at 16 h 01 min

    I definitely have early blight on many of my garden plants. It seems to be spreading quickly to different species. It first started last year with all the rain we had in spring and early-mid summer. My tomatoes and aubergines contracted it first, I then noticed it was starting to spread to my clematis and a couple of other bin related species. This year it has smothered both my wonderful hydrangeas, has spread to my delphiniums, lupins, strawberry plants, my rosemary bush, margarum, also my alstroemeria and possibly even my hemerocalis (excuse the spelling… I’m neurodivergent.) Will the Aureobasidium pullulans be sufficient to treat these plants before it spreads further? Where can you get it?

    Please help me save my precious plants that I’ve nurtured from young, grown from seed or spent a long time loving. 😔 I’m usually giving gardening advice but I have to say this early blight has stumped me and I’ve not had the health or energy last year or this year to do more tgan spray an organic copper treatment which didn’t seem to work.
    Many thanks.