How to Prevent Late Blight on Tomatoes: A Grower's Guide

Late Blight: Not Like Other Tomato Diseases

Late blight strikes tomatoes and potatoes, quickly ruining entire crops and providing infection sources for other gardens and farms via wind-dispersed spores. Unlike other tomato diseases that only affect leaves or cause limited fruit damage, late blight kills plants outright and is highly contagious. Understanding its unique threat is critical.

Late blight on a tomato plant.

Be Prepared

• Monitor USA Blight website (real-time tracking) regularly during growing season.
• When late blight is detected in your region, consider weekly preventive spraying.
• Cornell pathologist Meg McGrath recommends Actinovate (beneficial bacteria Streptomyces lydicus) as preventive, adding copper-based product (e.g., Copper Fungicide) when blight is present.

Copper spray

Actinovate

Key Prevention Strategies

Choose Resistant Varieties

No tomato is completely immune, but breeders are developing resistant varieties. Look for them when selecting seeds/plants. Start your own from seed or buy from trusted local source. Plant some early-maturing varieties—if blight strikes late, you may still get a harvest.

Prevent Overwintering

The fungus (Phytophthora infestans) needs living tissue to survive winter. It can't overwinter on cages/supports. But infected potatoes can carry it. Destroy volunteer potato plants. Buy certified disease-free seed potatoes.

Give Plants Space

Avoid planting tomatoes/potatoes where you had them last year. Space generously per variety recommendations. Maximize airflow and light. Use trellises/supports to keep vines off ground.

Fuzzy growth on leaf underside indicates late blight fungus producing spores.

Late blight stem lesions

Early blight: symptoms on older leaves near base; note "target" concentric rings.

Septoria leaf spot: appears on oldest leaves after fruit ripens.

Drought stress: lacks fuzzy spore growth and stem lesions.

Avoid Overhead Watering

Use soaker hoses or drip irrigation to keep foliage dry. Water early so foliage dries before nightfall.

Be Alert

Learn weather conditions that foster spread: cool, wet weather accelerates; dry weather slows it. Check USA Blight site regularly. Stay in touch with local gardeners. When blight is detected in your region, start preventive spraying.

Spray Preventively, If Necessary

Key word: preventively. Once infected, it's too late. Actinovate as preventive; add copper when blight present. Read labels, follow directions. Effective protection often requires weekly spraying all season. Monitor closely even when spraying.

Know It When You See It

Distinguish late blight from early blight, septoria leaf spot, and drought stress (images above). If treated plants show continuing infection or unsprayed plants succumb, act fast: pull plants, seal tightly in trash bag or secure under black plastic (sun's heat kills spores). Do not compost blight-infected plants. Left unattended, disease spreads to neighbors and farms. Garden responsibly!

Related Resources

• USA Blight: report occurrences, submit samples, view maps, sign up for alerts.
• Late blight images from Cornell University.
• Cooperative Extension services: find local master gardeners.