Brown dry spot on tomato leaves - what it is and how to treat it

Diseases and pests

Alternaria can appear immediately after planting tomato seedlings in a permanent location; the fungus is dangerous in both open and closed beds. The disease develops and spreads rapidly, so it must be treated at the first sign of infection. The best defense is prevention, which involves several standard measures.

How does macrosporiosis manifest itself on tomatoes and why is it dangerous?

The causative agent is the moldy, spore-forming fungus Alternaria solani. The disease is known by various names: macrosporiosis, alternaria leaf spot, dry spot, zonal spot, or brown spot. The first symptoms can be seen on tomatoes still young after planting; in some cases, the disease appears later, after the plant already has fruit. If treatment is not started promptly, the fungus will gradually destroy the entire plant. Macrosporiosis causes a 60% yield loss.

The fungus actively reproduces and grows in a humid environment at temperatures of 25–27°C. The lower leaves are affected first, with round, white spots (6 mm to 2 cm in size) with distinct outlines appearing. After 3 days, the fungus begins to grow rapidly, gradually moving upward, infecting stems, leaves, clusters, and fruits. The spots merge, covering most of the leaf blade, dry out, and turn gray-brown, after which the affected leaves fall off.

Attention!

Spores are carried to healthy plants by wind, dew and raindrops, and insects. When fruit is affected, the fungus penetrates the seeds, which turn black and become unusable.

On the petioles and stems, the spots are clearly outlined, ring-shaped, and covered with a velvety, gray coating. They quickly grow and become deep. The plant's metabolism is disrupted, and the bushes stop developing. tomatoes are deformed, their flavor deteriorates. On the fruit, lesions are concentrated near the stalk; they are sunken and water-soaked, and the skin at the affected area is pale. Gradually, the spot enlarges, takes on a brownish-black color, and sometimes a light gray coating appears. Tomatoes fall off before they're ripe or, conversely, ripen prematurely.

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Causes of the disease:

frequent precipitation;
significant temperature changes (day and night);
mechanical damage to plants;
contaminated soil;
high humidity (night dew, excessive watering, short-term rain) in hot and dry weather;
epidemic or development of infection in neighboring areas.

Alternaria prevention

The fungus is activated in a specific microclimate and requires moisture to thrive. When planting tomato seedlings, follow the recommended pattern and avoid overcrowding. Each plant should be well-ventilated and receive sufficient sunlight at all stages of growth. Store-bought seeds from unknown manufacturers, and especially loose seeds, must be disinfected in a potassium permanganate solution before sowing. Potassium permanganate is rarely available commercially; other seed treatments can be used:

2–3% solution of pharmaceutical hydrogen peroxide, heated to 40°C (soak for 10 minutes);
biological preparations Trichodermin, Rizoplan, Fitosporin-M, Immunocytophyte (use according to instructions);
0.1 g of boric acid or 0.05 g of copper sulfate per 500 ml of water (keep the grains in the mixture for about 8 minutes);
1% acetic acid solution (soak grains for 120 minutes, rinse thoroughly with water).

Attention!

The seeds can be disinfected under an ultraviolet lamp; the grains should be kept there for no more than 3 minutes.

There are no tomato varieties that exhibit complete resistance to Alternaria. Some are less susceptible: Firebird, Aurora, Luch, Sanka, Hope, Lyana, Golden Bullet, Alex. To strengthen and boost tomato immunity, it's recommended to treat tomato plants with Immunocyt (half a tablet per liter of water) 3-4 times throughout the growing season. Before flowering, it's recommended to spray the plants with Trichodermin or Fitosporin. Other preventative measures:

removing side shoots and trimming bushes (cutting off lower leaves and leaves that thicken the crown) helps to avoid problems;
Weeds can be a source of infection; timely weeding will help prevent tomatoes from becoming infected with brown spot;
After each watering or rain, the soil around the bushes should be loosened; excess moisture will evaporate more quickly from the upper layers of soil. During rainy periods, it's best to remove mulch from the beds;
To protect tomatoes from fungus, keep the garden clean, remove trash and all plant debris. Affected parts of the plant should be promptly removed and burned.
When growing tomatoes, be careful with nitrogen-containing fertilizers; excess nitrogen often leads to the development of the disease. Potassium-based nutrient mixtures are effective in combating Alternaria blight;
in the fall, after removing all the tops, you should dig up the soil to the depth of a shovel;
Pest control sprays are essential, as they carry pathogenic spores. You can repel insects by planting spicy thara, marigolds, or calendula between the rows;
Tomatoes need to be watered only at the roots;
Tall bushes should be tied up.

If tomatoes are already infected with Alternaria, even after treating the plants, the soil remains contaminated. It's easier to disinfect the soil in a greenhouse; in open ground, it won't completely eliminate the infection, but the spore count will still be reduced. In the fall (a month before the first frost) and in the spring, after the snow melts, apply solutions of Baikal EM-5, Fitosporin, Baktofit, Planzir, or Alirin to the contaminated soil.

Biological products are often weak in effectiveness; in critical cases, they are best replaced with chemicals. In the fall, the soil should be treated with a 3% Bordeaux mixture. In early spring, during dry weather, apply a 4% copper oxychloride mixture or 2% Oxyhom to a depth of 12 cm. A small amount of Quadris, Homa, or Bravo can be added to the planting holes. It's important to remember that these products can destroy beneficial soil microflora. Sulfur checkers or formalin are recommended for disinfecting greenhouse walls.

How to cure brown spot on tomatoes

Biological products can be used, but they are more suitable for prevention. Chemical fungicides often provide a reliable, eradicating effect. A copper sulfate solution has shown excellent results: 20 g of the solution per bucket of water. To improve adhesion to the leaves, add about 200 g of grated laundry soap. Other effective fungicides include:

Abiga Peak;
Ridomil Gold MC;
Acrobat MI;
Poliram;
Ordan;
Infinity;
Flint;
Cuproxate.

Attention!

Any preparation should be diluted and used strictly according to the instructions, personal safety rules should be observed, and the time between spraying and harvesting should be monitored.

Treatments are allowed at intervals of 10-15 days (until signs of macrosporiosis disappear). It is recommended to spray tomatoes with fungicides no more than four times during the growing season. Most gardeners prefer Bordeaux mixture when treating tomatoes for brown spot, as it, unlike other treatments, is less hazardous to humans and the environment.

There are no effective folk remedies against Alternaria blight on tomatoes; trying various decoctions or infusions will only waste time and put your plants at serious risk. The key to combating any infection is following basic tomato cultivation practices. Rotate plantings of different crops, carry out preventative spraying (including pest control) on a timely basis, and keep your garden clean – this will ensure your tomatoes are free from any diseases.