Treating blight starts with identifying which type you’re dealing with, then acting fast with the right spray, pruning strategy, and environmental changes. Blight spreads quickly in cool, moist conditions, and once it takes hold, your goal shifts from curing infected tissue (which can’t be saved) to protecting healthy growth and slowing the spread. Here’s how to handle it at every stage.
Identify Which Blight You Have
Early blight and late blight look different, spread differently, and respond to different treatments. Getting this right matters before you reach for any spray.
Early blight is caused by a fungus called Alternaria solani. It shows up as small, dark, papery spots on lower leaves first. Those spots grow into brown-black circles with a distinctive bullseye pattern of concentric rings. You’ll often see a yellowish-green halo around each spot. It typically hits older foliage and works its way up the plant. Early blight affects tomatoes, potatoes, and other plants in the nightshade family.
Late blight is caused by a different organism, Phytophthora infestans, and it’s far more aggressive. Leaf spots start as pale to dark green, irregularly shaped blotches that aren’t contained by leaf veins. They grow rapidly into large purplish-black areas. The telltale sign: in cool, damp weather, you’ll see white, fluffy fungal growth on the undersides of leaves. Late blight can destroy an entire planting in days under the right conditions. It hits tomatoes, potatoes, and occasionally eggplant.
Remove Infected Plant Material Immediately
No spray will reverse tissue that’s already dead or dying from blight. Your first step is always physical removal. Cut off every leaf, stem, or fruit showing symptoms using clean pruners. Cut several inches below the visible damage to catch what you can’t yet see. Disinfect your pruners between cuts with rubbing alcohol or a diluted bleach solution so you don’t carry spores from one plant to the next.
How you dispose of that material matters. Blight spores survive on dead plant debris and can spread through wind and rain to reinfect your garden. Bag the removed material and tie the bags closed before moving them. Burning is the most effective disposal method where local regulations allow it. If you compost diseased material, use it only in the same location (not elsewhere in your garden), and know that a backyard compost pile rarely reaches temperatures high enough to kill blight spores reliably. For late blight especially, bagging and sending it out with household trash is safer than composting.
Apply Fungicide to Protect Remaining Growth
Fungicides work best as protectants, meaning they prevent new infections on healthy tissue rather than curing what’s already infected. Research consistently shows that protectant fungicides are most effective when applied before spores land on leaves, ideally within a day of expected exposure. That’s why timing matters more than the product you choose.
Copper-Based Sprays
Copper fungicide is the most widely available option for home gardeners and is approved for organic use. For late blight, mix about 2 fluid ounces of liquid copper concentrate per gallon of water. Apply it to all leaf surfaces, including the undersides, using a pump sprayer. Reapply every 7 to 10 days, and always reapply after rain.
A few important precautions with copper: time your application so plants get at least 12 hours of dry weather afterward. In cool, damp conditions below 60°F, copper can cause leaf burn (called phytotoxicity). Some plant varieties are more sensitive than others, so if you’re unsure, spray a few leaves first and wait 48 hours to check for damage. Don’t mix copper products with lime. Copper is also toxic to fish and aquatic life, so avoid spraying near ponds, streams, or drainage areas.
Conventional Fungicides
Products containing chlorothalonil or mancozeb are common conventional options for blight control. Research on late blight has found that a combination of chlorothalonil with another active ingredient (propamocarb hydrochloride) had the strongest suppressive effect among tested fungicides. Single-ingredient protectants like mancozeb work well but over a narrower timing window, so you need to be more precise about applying them before infection rather than after.
If blight has already established and you’re trying to limit damage, curative fungicides (those that work after infection) exist but are generally more effective within one day of infection. After that, their effectiveness drops significantly. This is why regular preventive spraying on a 7 to 10 day schedule during blight season is more effective than reacting after you see symptoms.
Baking Soda Spray
A simple kitchen remedy that many gardeners use: mix 3 tablespoons of baking soda into 1 gallon of water and spray it on foliage once per week. The idea is that baking soda raises the leaf surface pH from a neutral 7.0 to about 8.0, creating conditions where blight spores can’t colonize. If the fungus is still spreading after three weekly applications, increase to 3.5 tablespoons per gallon and continue weekly. This approach is inexpensive and worth trying for mild early blight, though it’s less proven against aggressive late blight outbreaks than copper or conventional fungicides.
Biological Controls
Products containing the beneficial bacterium Bacillus subtilis offer another option. In greenhouse trials on a related blight pathogen, B. subtilis reduced disease severity by up to 52% compared to untreated plants. It works through direct antagonism, essentially inhibiting the blight organism’s growth. One notable advantage: in trials, it was effective even against strains of blight that had developed resistance to conventional fungicides. Biological controls work best as part of a broader strategy rather than a standalone treatment.
Control the Conditions Blight Loves
Blight thrives in cool, wet weather. Temperatures between roughly 55°F and 73°F (13°C to 23°C) with high humidity above 75% and prolonged leaf wetness create ideal conditions for rapid spread. You can’t control the weather, but you can reduce how much moisture sits on your plants.
Water at the base of plants, never overhead. Use drip irrigation or a soaker hose. Water in the morning so any splash on foliage dries quickly. Increase airflow by spacing plants generously, pruning lower branches that touch the ground, and staking or caging plants to keep foliage off the soil. Mulch around the base of plants to prevent soil (which may harbor spores) from splashing up onto lower leaves during rain.
During extended cool, rainy stretches, blight can move extremely fast. These are the weeks when preventive fungicide applications matter most, even if you don’t see symptoms yet.
Prevent Blight From Coming Back Next Season
The early blight fungus survives winter in infected plant debris in or on the soil, persisting for at least one year and possibly several. Late blight doesn’t survive as long in most climates without a living host, but it can overwinter in infected potato tubers left in the ground.
At the end of the growing season, remove all plant debris from affected beds completely. Don’t leave old tomato or potato plants to decompose in place. Rotate your crops so that nightshade-family plants (tomatoes, potatoes, peppers, eggplant) don’t grow in the same spot for at least two to three years. This starves soilborne spores of the hosts they need.
When selecting varieties for next season, look for cultivars bred with blight resistance. Disease resistance is usually noted on plant tags or in seed catalog descriptions. For tomatoes, varieties like Garden Treasure carry resistance to late blight. Many seed companies now label resistance with codes like “LB” for late blight or “EB” for early blight. Resistant varieties aren’t immune, but they tolerate infection far better, giving you more time to respond and reducing the need for constant spraying.
Starting with certified disease-free seed potatoes and healthy transplants is equally important. Blight often enters a garden on infected starter plants rather than blowing in from miles away, so sourcing clean stock from a reputable nursery is one of the simplest things you can do to avoid the problem entirely.