Late blight on tomatoes (Phytophthora infestans) has been a serious threat for as long as home gardeners have grown them. In wet seasons - especially when cool nights follow rainy days - it can spread from a few leaf spots to total plant loss in a matter of weeks. Late blight spores travel through the air, sometimes many miles, and can arrive on seedlings sold at big-box stores. Once in your garden, the disease spreads soil-borne through splashing rainwater. In most US gardens it appears from late July through August, but in cool, wet years it can start as early as June.
TLDR: Late blight on tomatoes (Phytophthora infestans) is a fast-moving fungal disease that can destroy an entire plant in days. It spreads through the air and splashing water, and thrives in cool, wet weather. Spot it by the dark brown or purplish-black lesions on leaves and stems, often with white mold in humid conditions. Remove infected plants immediately - they won't recover. Protect remaining plants with a copper or chlorothalonil-based fungicide, and start a preventive spray program before the disease arrives.
Description of symptoms
When it shows up you see lesions on leaves and stalks; the plant loses its leaves; all its leaves. LB starts out on tomato leaves as pale green, water-soaked spots, often beginning at leaf tips or edges. The circular or irregular leaf lesions are often surrounded by a pale yellowish-green border that merges with healthy tissue. Lesions enlarge rapidly and turn dark brown to purplish-black.
Brown spots (lesions) on stems are one of the most visible early symptoms. They begin small and firm, then quickly enlarge. Under moist conditions, a white fungal growth develops and a soft rot collapses the stem of the tomato plant, which turns black.
The first appearance of lesions commonly occurs after periods of wet weather. Black lesions appear within 3-7 days of infection of leaves. On tomato fruit, late blight causes a firm, dark, greasy looking lesion from which the pathogen spore producing structures emerge under humid conditions.
Late blight can also develop on green tomato fruit, resulting in large, firm, brown, leathery-appearing lesions, often concentrated on the sides or upper fruit surfaces. If conditions remain moist, abundant white mold growth will develop on the lesions and secondary soft-rot bacteria may follow, resulting in a slimy, wet rot of the entire fruit.
All Spots On Tomato Leaves Are Not Always Late Blight
For instance, a very common and mild soil-borne wilt makes the bottom leaves of tomato plants grown in the same area year after year turn yellow; this is not late blight. Spots the size of a pencil eraser on tomato leaves aren't late blight either-the spots this nasty spider mite critter is hard to spot
If you're unsure whether you're looking at late blight or something else, UMN Extension's guide to identifying and managing late blight has a clear comparison for home gardeners.
Monitoring techniques and timing in season
This year in NE the LB hit in June while normal time is late July and August. The weather has to be wet, cool (60 to 80), and humid for LB to thrive and spread. Check your tomato plants twice each week; more often if there is wet weather. Look inside the foliage to the stems especially near the soil level. LB moves quickly so if you see a small lesion and its bigger the next day, that tomato goes into the trash.
If there are spots on just a few leaves, remove them and those around them and watch for a day or two. Remove all infected plants and place in the trash; not in the compost pile.
Best Fungicide Spray for Late Blight on Tomatoes
If you have found LB on one or more plants and have removed them, you may prevent the spread of the disease if you immediately spray the remaining plants with a fungicide. Your organic choices include Copper Sulfate or Actinovate. On the chemical side you have Daconil, a product that has 29.6% chlorothalonil in the concentrate. Ortho Disease Control and Bonide's Fung-Onil contain the same active ingredient. With both of these one can spray up to the day of harvest for tomatoes. Remember these products are only effective if used before the disease appears on the plant and should be reapplied every 5-7 days if wet weather persists.
If you choose the organic route we advise using alternately the Actinovate one week and the copper fungicide the next week; repeating this alternating approach until first frost.
For a full list of fungicide options including pre-harvest intervals and organic alternatives, see NC State Extension's fungicide guide for home gardeners.
How to Prevent Late Blight on Tomatoes
The best way to avoid problems of LB is to start your prevention program after the first frost and your plants are dead. The critical issue here is to have zero tomatoes left on the ground through the winter. LB spores can over winter on those innocent looking tomatoes. The spores do not overwinter on the roots or the stems and leaves.
You should then leave a 3 inch layer of organic mulch (straw or chopped leaves) over the tomato beds for the entire winter. If you had mulch under your tomatoes this year, remove that mulch and use it under shrubs or perennials that are not bothered by LB. The overwintering mulch is going to prevent any spores that do make it to late spring from splashing up on the plants from heavy rains.
If you don’t start your tomatoes from seed, avoid buying your tomato seedlings from the big boxes. Independent garden centers usually grow their own seedlings.
There are no tomato varieties resistant to LB. There will be some coming out in 2011,
Determinate varieties should grow on cages and indeterminate varieties should be on a trellis. You want your tomato plants to allow as much air move through the plant so it dries out faster after a rain. Mature plants should be at least a foot from the next plant. A soaker hose is the best way to water tomatoes so water does not unnecessarily get on the leaves.
Preventive Spray Program for Late Blight
The most reliable way to protect your tomatoes from late blight is to start spraying before the disease arrives - not after. Once you see symptoms on a plant, fungicides slow the spread but won't save it.
Start your preventive spray program 2–3 weeks after transplanting, or earlier if cool, wet weather arrives. Spray every 7–10 days throughout the season, and always reapply after heavy rain.
Organic options:
Alternate between two products each cycle - this reduces the chance of the pathogen building resistance:
- Actinovate (week one) - a biological fungicide that suppresses fungal pathogens on the plant surface
- Copper sulfate (week two) - a traditional copper-based organic fungicide
Compost tea and Myke can be added to support overall plant resilience, but work best as complements to the above, not replacements.
Conventional option:
Look for fungicides containing chlorothalonil - sold as Daconil, Ortho Disease Control, or Bonide Fung-Onil. Apply every 7 days when conditions favor late blight (cool nights, wet or foggy days). These can be applied right up to harvest day on tomatoes.
Consistency matters most. A gap of more than 10 days during a wet stretch is enough time for late blight to take hold.
Frequently Asked Questions About Late Blight on Tomatoes
Can tomatoes recover from late blight?
Rarely. Once a plant is infected, remove it and seal it in a bag for the trash - do not compost it. Your focus should shift immediately to protecting the unaffected plants around it with a fungicide spray.
What is the best fungicide for tomato late blight?
For organic gardeners, copper-based fungicides are the most effective option. For conventional control, look for products containing chlorothalonil - sold as Daconil, Ortho Disease Control, or Bonide Fung-Onil. Apply every 7–10 days when conditions favor the disease, and always reapply after heavy rain.
How does late blight spread in a garden?
Late blight spores travel through the air, sometimes many miles, and spread further through splashing water. Avoid watering overhead - use a soaker hose at the base of the plant instead, and water in the morning so leaves have time to dry before nightfall. Placement: Add this block directly above your conclusion section.
Late Blight on Tomatoes Is Manageable When You Catch It Early
Late blight moves fast, but so can you. The gardeners who lose the least are the ones who check their plants twice a week during wet weather, remove infected plants immediately, and start a preventive spray program before the disease arrives. Clean up the garden bed thoroughly after the first frost and you give yourself a much better chance next season.
