Dark brown spots with rings like a target, starting on the oldest leaves.
That is early blight, one of the commonest leaf diseases of tomato and potato. It is usually less explosive than late blight. But in many warm regions it causes more steady yield loss, year after year.
Defoliation typically cuts yield by five to thirty per cent, and more when the disease starts early on a susceptible variety. On tomato, it also rots stems and fruit.
In this video: the cause, the symptoms at each stage, the look-alikes, the weather it loves, prevention, scouting and forecasting, fungicides chosen by FRAC group, and safety.
The advice is general and global.
Your local extension service and the product label always come first.
Early blight is caused by the fungus Alternaria solani. Some scientists use a separate name for the tomato pathogen,
and that debate is still open.
Potato and tomato are the main hosts, but eggplant, pepper, and nightshade weeds are infected too.
Between seasons, the fungus survives in crop debris, on the soil surface, in volunteer plants and weeds,
and on tomato seed. It produces large, dark spores, which wind and splashing rain carry to the lowest leaves of the next crop.
In tomato nurseries, the fungus can cause dark lesions at the soil line that girdle the stem, called collar rot, and damping-off of seedlings.
On growing plants, look at the oldest leaves first.
Small dark brown spots enlarge to about one centimetre, with concentric rings, like a target.
They are often surrounded by yellowing, and limited by the leaf veins.
The leaves then yellow and fall, and the lesions climb up the plant as the crop ages.
Later, dark sunken lesions with rings appear on tomato stems,
and at the stem end of the fruit. On potato tubers, the lesions are dark, slightly sunken and dry, with a leathery rot just below.
Do not confuse early blight with late blight.
Late blight makes water-soaked, fast-spreading lesions without rings, with white down on the underside in humid weather,
and it can start anywhere on the plant.
Septoria leaf spot of tomato makes many small, round spots with grey centres and tiny black dots, but no rings.
Brown spot, caused by a related Alternaria, makes smaller, often angular spots without clear rings,
and it is often mixed in with early blight.
And blossom-end rot, the dark patch at the bottom of a tomato, is not a disease at all. It is a watering problem, and spraying a fungicide on it fixes nothing.
The spores infect leaves when they are wet, and need only a few hours of leaf wetness.
Spots appear about two to three days later, faster in warm weather.
New spores are released as the leaves dry after dew, and the cycle repeats many times in one season.
Early blight develops fastest at about twenty-four to twenty-nine degrees Celsius, with dewy nights and dry days. That is warmer than late blight likes.
And stress matters as much as weather.
Old leaves, and plants short of nitrogen or water, or carrying a heavy fruit load, are much more susceptible.
That is why the disease climbs the plant as the crop ages.
Prevention starts before planting.
Rotate with crops outside the tomato and potato family for at least two to three years, and destroy volunteers and nightshade weeds. Use clean seed and healthy transplants, and check transplants for collar lesions before planting.
After harvest, bury or remove the crop residues.
Keep the plants unstressed, with enough nitrogen and water.
Stake or mulch tomatoes, so leaves and fruit stay off the soil, away from splashing spores.
Laisha's tomato guide explains why staking pays:
fruit held up off bare soil is far less exposed to soil-splash diseases such as early blight. Give the plants enough spacing for air to move, and avoid overhead watering late in the day, because foliage that stays wet overnight invites both blights.
In gardens and small plots, remove infected lower leaves.
Varieties differ: some tomato varieties carry moderate resistance, but none is immune.
And some Bacillus and Trichoderma products have reduced early blight in trials, mainly when disease pressure is low.
Inspect the lower leaves every week from flowering,
and twice a week in warm, dewy weather.
Where a forecasting model has been validated locally, use it to time the first spray. For tomato, the TOMCAST model adds up disease severity values from leaf wetness and temperature. In the Great Lakes region of North America,
growers spray when about fifteen to twenty units have built up since the last spray. For potato, a model in the Upper Midwest times the first spray at about three hundred physiological days from crop emergence.
These numbers need local calibration elsewhere.
Laisha names fungicides only by their FRAC group.
Protectant programmes start at the forecast threshold, or at the first symptoms.
The multi-site protectants are FRAC group M three, the dithiocarbamates and relatives, and M five, the chloronitriles. They act at many sites in the fungus, so the risk of resistance is very low. The single-site groups include group eleven, the quinone outside inhibitors, where a resistance mutation is widespread in some regions.
Group seven, the succinate dehydrogenase inhibitors, where resistance has been reported.
Group three, the demethylation inhibitors, which are systemic.
And group nine, the anilinopyrimidines, used in mixtures.
Reduced sensitivity to groups eleven and seven has been documented in North American and European populations.
So alternate the single-site groups, never apply the same one more than twice in a row, and mix them with a multi-site partner.
Use only products registered for your crop in your country, at the label dose, and respect the pre-harvest interval.
Now, safety.
Read the label before you buy, mix or spray.
Wear what the label asks for: chemical-resistant gloves, long sleeves and trousers, boots and eye protection.
Gloves matter most, because most farm poisoning happens through the skin,
and handling the concentrate while mixing is the riskiest step. Mix outdoors, measure carefully, and do not eat, drink or smoke while you work.
Calibrate your sprayer, spray when the wind is light, and leave a buffer zone near water. Wait out the pre-harvest interval before picking.
Store pesticides locked, in their original containers, away from food, feed, children and animals.
Triple-rinse empty containers, puncture them, and never reuse them. If someone is exposed, remove contaminated clothing, wash the skin with plenty of water, and get medical help at once, taking the label with you.
To recap.
Early blight makes target spots on the oldest leaves first, then climbs the plant.
Tell it apart from late blight, Septoria and blossom-end rot.
It loves warm days, dewy nights and stressed plants.
Rotate, use clean transplants, stake and mulch, feed and water evenly, and keep foliage dry overnight.
Scout weekly from flowering.
And if you spray, choose by FRAC group, partner single-site groups with a multi-site, and follow the label.
For every pest and disease, and full grow guides, visit laisha.ai. Thank you for watching.
