Getting the diagnosis wrong costs you time, fruit, or an entire planting. If your tomato leaves and stems show rapidly spreading dark, greasy lesions and fruit rot, pull and destroy those plants now; if only a few lower leaves have small concentric rings, try to save the plants with pruning, improved airflow, and a treatment plan. The word blight covers several diseases, so identifying which one matters for what to do next.
I write from central Ohio, USDA zone 6b, where midsummer heat and humid nights make both early and late blight possible. I will show you simple visual checks, same day actions you can do this afternoon, a season treatment plan, and exact prevention steps for next year tied to your last and first frost dates. I will also name varieties worth planting to avoid the problem.
Blight on tomato leaves, stems, and fruit, what to look for
Leaf spots that form concentric rings, browning that starts on older leaves
Early blight, caused by Alternaria, usually shows up on older leaves first. Look for round to irregular brown spots with concentric rings, like a target. Spots start 1 4 inches across and spread outward, turning the whole leaf brown and crispy.
Check lower third of the plant first, especially leaves within 6 inches of the soil. If only older leaves are affected, you can often prune them away and save the plant, provided stems are clean and fruit is not heavily infected.
Water soaked lesions, dark greasy stems, and gray fuzzy growth under humid nights
Late blight, caused by Phytophthora infestans, looks different. Lesions are dark, water soaked, and may ooze. Stems can become dark and greasy. Under humid nights you may see gray to white fuzzy sporulation on the undersides of leaves or on lesions.
Late blight spreads very fast in cool wet weather, 50 to 65 degrees Fahrenheit nights with high humidity. If you see this fuzzy growth or rapid spread, remove plants immediately and follow disposal steps in the next section.
Fruit lesions, sunken dark spots, and white mold like growth inside cracks
Fruit shows the final answer. Early blight gives sunken brown spots, often near the stem or where fruit touched foliage. Late blight produces irregular, greasy dark patches that can quickly engulf the fruit and show white mold in cracks or around stem scars.
Cut open suspect fruit. If you see white fuzzy growth inside cracks or at the stem end, treat the plant as late blight and remove it. If the fruit only has a few sunken spots you can trim them and eat the rest when the plant remains otherwise healthy.
Most likely causes of blight in midsummer, ranked by frequency
Early blight, why Alternaria loves stressed, older foliage in warm humid weather
Early blight is the most common blight in midsummer here. Alternaria favors warm days, 75 to 85 degrees Fahrenheit, with high humidity. Plants stressed by heat, irregular watering, or nitrogen deficiency are more vulnerable.
Typical pattern: lower leaves infected first, then move upward as spores splash from soil or mulched debris. I have lost yield when I let the lower 8 to 12 inches of foliage stay wet for days after watering.
Late blight, why Phytophthora spreads fast after cool wet nights and overhead watering
Late blight is less common but more destructive. It likes cooler nights, 50 to 65 degrees Fahrenheit, and continuous leaf wetness for 6 to 12 hours. Overhead watering at night and long periods of fog or rain are a perfect storm.
Late blight can wipe out a bed in a week in the right conditions. If you garden in a cooler, coastal, or high humidity area, treat any gray fuzz or rapid collapse as an emergency and remove plants.
| Feature | Early blight | Late blight |
|---|---|---|
| Typical temperature | 75 to 85 F | 50 to 65 F |
| Leaf signs | Concentric rings on older leaves | Water soaked lesions, gray fuzzy sporulation |
| Spread speed | Slow to moderate over weeks | Fast, days to a week |
Same-day actions to stop spread, what to do right now
Work in this order, start with what you can do in an hour. Wear gloves and wash hands after. Do not compost affected material.
- Remove affected leaves, then bag them. Cut 2 to 4 inches past visible lesions on the stem. For heavy late blight symptoms, pull the whole plant and remove the root ball.
- Bag and seal the removed plant material in heavy trash bags. Do not add to your compost pile. I once composted infected vines and reintroduced spores to the garden, so trust me, disposal matters.
- Sanitize tools between plants. Wipe pruners with a solution of 1 part household bleach to 9 parts water for 30 seconds, then rinse. Or use 70 percent isopropyl alcohol on a rag.
- Stop overhead watering. Switch to drip or soaker hoses and water at the base with 1 to 1.5 inches of water per week, adjusting for rainfall. For timing, water in the morning during summer. See more on timing at this guide to when to water vegetable plants in summer: when to water vegetable plants in summer.
- Increase air flow, prune suckers to open the canopy, and space plants at least 18 to 24 inches for determinate and 24 to 36 inches for indeterminate varieties. Tie vines up to 4 to 6 feet high if using cages or stakes.
Treatment plan for this season
Decide the next steps based on your diagnosis. If early blight and only lower leaves are affected, try to save plants. If late blight or more than 30 percent of the canopy is infected, remove plants now.
- After pruning, apply a protective spray if you choose to treat. I prefer a layered approach, cultural then chemical. For a noncommercial homemade option, mix 1 tablespoon baking soda, 1 teaspoon light horticultural oil, and 1 teaspoon plain dish soap per gallon of water. Spray the whole plant every 7 to 10 days and after heavy rain until conditions improve. Effectiveness varies, so treat as a stopgap.
- For stronger protection, use a labeled copper-based fungicide or a contact fungicide like chlorothalonil, following label rates and reapplication intervals, usually 7 to 14 days and after heavy rain. Contact your local extension for product recommendations and exact dosages for your region.
- Monitor every 3 days for new lesions. Remove and bag any new infected tissue immediately. Harvest ripe fruit promptly to reduce load on the plant and limit spread.
- Record the date you first saw symptoms and actions you took. If you remove plants, plant another crop only after rotating out of Solanaceae for at least one season, or wait 12 months in the same bed.
Prevention steps for next year, exact actions and timing tied to frost dates
Prevention starts now and continues in fall and next spring. Tie your schedule to your last and first frost dates, not the calendar. In central Ohio, my last frost is usually around May 10 and first frost around October 15. Adjust earlier in warmer zones and later in colder ones.
- Plant resistant varieties. For late blight resistance, try Mountain Magic or Defiant PhR. For general disease resistance, Iron Lady and Celebrity are good choices. I avoid heirlooms like Brandywine in beds that had blight because they are often more susceptible.
- Rotate crops. Do not plant tomatoes or other nightshades in the same bed for at least one season. If your last frost is May 10, plan rotation now and mark beds for next spring.
- Improve soil and drainage. Raise beds at least 8 to 12 inches high if your soil puddles. I lost a bed once because I skipped raised rows, so drainage comes first.
- Use 2 to 3 inches of organic mulch like straw or shredded leaves once soil has warmed to 55 degrees Fahrenheit after your last frost. Mulch prevents spores in soil from splashing onto lower leaves.
- Space and stake properly. Indeterminate tomatoes need 24 to 36 inches between plants, determinate 18 to 24 inches. Keep leaves 6 to 12 inches above the soil line where practical by burying stems 2 to 3 inches deeper at planting or pruning low growth in season.
- Plan to plant after your last frost once soil temperature is above 55 F for transplanting tomatoes. If you are in a colder zone, wait until soil is reliably warm; in a warmer zone, move earlier but keep an eye on cool, wet spring nights that favor late blight.
Common failure mode and how to avoid it
The most common mistake I see is composting infected plants. If you compost blighted vines, spores survive and will reinfect the next year. Do not compost. Bag and throw away or burn only where allowed.
Another mistake is overhead watering late in the day. Wet foliage at night creates a perfect environment. Switch to drip irrigation, water in the morning, and watch nightly lows. If nights fall into the 50 to 65 F range and humidity stays high, be extra vigilant.
For help diagnosing ambiguous cases, take clear photos of the top and underside of leaves, stems, and fruit and contact your county extension. They can confirm early versus late blight and recommend local fungicides and exact label rates.
FAQ heading
Can I eat tomatoes from a plant with early blight?
Yes, if only a few fruits have small sunken spots, cut the spots away and use the rest. If fruit shows widespread soft, greasy rot or white fuzzy growth, discard it. When in doubt, throw it out. Always wash hands after handling infected material and sanitize knives.
Will copper spray cure blight once it starts?
Copper sprays are protective, not curative. They reduce new infections if applied before heavy disease pressure. For active, spreading infections, copper can slow spread but rarely cures the plant. Follow reapplication intervals on the label and consult your extension for product selection.
How long can blight spores survive in soil or stakes?
Alternaria and Phytophthora spores can survive months to years on plant debris and in soil under the right conditions. Clean equipment, remove debris, rotate crops, and replace or disinfect stakes and cages. Do not reuse infected mulch or compost without high heat treatment.
If I pull infected plants, when can I replant tomatoes in that bed?
Wait at least one season before replanting tomatoes or other nightshades in the same bed. If you must plant sooner, use containers and fresh, sterile potting mix. Mark the bed now and plan a rotation crop like beans or corn for the season after blight.
Where can I get a definite diagnosis and local spray recommendations?
Your county extension office can confirm whether it is early or late blight from photos or samples, and they will give product recommendations and precise label rates for your area. In Ohio, contact the state extension for lab tests and guidance before applying fungicides.
If you are growing tomatoes in containers next season, there are small tweaks to spacing and watering. Read my guide on how to grow tomatoes in containers for exact pot sizes, soil mixes, and watering volumes: how to grow tomatoes in containers. For bed layout basics, see raised bed gardening for beginners: raised bed gardening for beginners. Finally, if you are new to outdoor veggies, skim common mistakes new gardeners make so you skip easy errors: common mistakes new gardeners make.
