Tomato Late Blight vs. Anthracnose: Identification and Organic Control
Late blight and anthracnose are the two most destructive fungal diseases affecting tomatoes during humid, rainy summer months. They look different, behave differently, and require different management approaches — yet both are frequently misdiagnosed, leading to ineffective treatment and crop loss. This guide provides a definitive identification guide for both diseases, explains the environmental conditions that trigger each, and outlines organic control methods that work without synthetic fungicides.
Table of Contents
- Quick Identification Guide
- Understanding Late Blight (Phytophthora infestans)
- Understanding Anthracnose (Colletotrichum spp.)
- Organic Control Methods
- Prevention: Creating Disease-Resistant Conditions
- Quick Reference Summary
- Frequently Asked Questions
- References
Quick Identification Guide
Primary location
Late Blight
Leaves, stems, green fruit
Anthracnose
Ripe fruit (sunken spots)
Leaf symptoms
Late Blight
Water-soaked, then brown/black patches
Anthracnose
Minimal leaf symptoms
Fruit symptoms
Late Blight
Brown, greasy-looking firm lesions
Anthracnose
Circular, sunken, dark lesions
Diagnostic sign
Late Blight
White mold on leaf underside (humid)
Anthracnose
Salmon/pink spore masses in lesions
Spreads fastest in
Late Blight
Cool, wet conditions (18–22°C)
Anthracnose
Warm, wet conditions (24–28°C)
Progression
Late Blight
Very rapid — can kill plant in 1–2 weeks
Anthracnose
Slower — primarily affects ripe fruit
Understanding Late Blight (Phytophthora infestans)
Late blight, caused by the water mold Phytophthora infestans, is the same pathogen responsible for the Irish Potato Famine of 1845–1852. It spreads through airborne spores that germinate on wet leaf surfaces and can devastate entire plantings within 7–10 days under ideal conditions [University of Minnesota Extension, 2023].
How Late Blight Spreads
Spores travel on wind and rain splash. A single infected plant can release millions of spores that travel hundreds of meters. Cool nights (below 18°C) followed by warm humid days create the perfect cycle: spores germinate overnight on wet leaves, infect tissue, and produce new spores by morning.
Recognizing Late Blight
On leaves: Begin with pale green or water-soaked patches, typically at leaf tips or edges. Within 24–48 hours, these turn brown-black. In humid conditions, white, downy mold appears on the underside of the lesion — this is the diagnostic feature that confirms late blight.
On stems: Brown, water-soaked sections that can wrap around the stem and kill the entire branch above. Affected stems feel soft when squeezed.
On fruit: Firm, brown lesions with a greasy appearance on green or immature fruit. The fruit does not soften in the lesion area (unlike anthracnose).
Understanding Anthracnose (Colletotrichum spp.)
Anthracnose is caused by Colletotrichum coccodes and related species. Unlike late blight, it primarily targets ripe or ripening fruit. Infections often begin during flowering or fruit set but only become visible as fruit matures [Cornell University Plant Disease Diagnostic Clinic].
Recognizing Anthracnose
On fruit: Small, circular, water-soaked spots that expand into sunken, dark depressions (1–3 cm diameter). As lesions mature, salmon-pink or orange-colored spore masses appear in concentric rings within the lesion — this is the diagnostic feature of anthracnose.
On leaves and stems: Generally minimal. Some yellowing of lower leaves may occur due to soil splash.
Why Anthracnose Is Often Missed Until Harvest
Anthracnose infections are latent — the fungus enters the fruit during the green stage but produces no visible symptoms until the fruit begins ripening. A fruit that appears fine may develop anthracnose lesions within 2–3 days of being brought inside.
Organic Control Methods
Copper-Based Fungicides (Approved for Organic Use)
Copper fungicides (copper hydroxide, copper sulfate/Bordeaux mixture) are among the oldest and most effective organic treatments for both diseases. Copper ions disrupt fungal cell membranes and inhibit spore germination. Apply preventively during rainy periods every 7–10 days. According to the Rodale Institute, regular preventive copper applications significantly reduce late blight progression [Rodale Institute].
Application notes: Apply in the morning on dry days. Reapply after rain. Follow label rates carefully — copper accumulates in soil with repeated use.
Baking Soda Spray
A solution of 1 teaspoon baking soda + 1 drop dish soap per liter of water changes leaf surface pH, making conditions less favorable for fungal spore germination. More effective as a preventive than a curative measure.
Neem Oil
Cold-pressed neem oil disrupts fungal reproduction. Effective against both diseases when applied preventively. Mix 2–3 ml neem oil with 0.5 ml dish soap per liter of water. Apply weekly during high-risk periods.
Physical Controls
- Remove and bag (do not compost) all infected plant material immediately
- Avoid overhead watering — use drip irrigation to keep foliage dry
- Stake and train plants to improve air circulation
- Apply straw or compost mulch to prevent soil-splash infections
Prevention: Creating Disease-Resistant Conditions
- Choose resistant varieties — Many modern tomato varieties carry late blight resistance (look for "LB" resistance codes)
- Rotate crops — Never plant tomatoes in the same location in consecutive years
- Improve drainage and air circulation — Well-aerated, fast-draining gardens dry faster after rain
- Mulch heavily — A 7–10 cm compost mulch layer prevents soil splash that carries spores from soil to lower leaves
Quick Reference Summary
Pathogen
Late Blight
Phytophthora infestans
Anthracnose
Colletotrichum spp.
Primary target
Late Blight
Leaves, stems, green fruit
Anthracnose
Ripe/ripening fruit
Diagnostic sign
Late Blight
White mold on leaf underside
Anthracnose
Salmon spore masses in fruit lesions
Best organic control
Late Blight
Copper fungicide (preventive)
Anthracnose
Copper + neem oil; improve air circulation
Remove infected material?
Late Blight
Yes — immediately
Anthracnose
Yes — discard affected fruit
Frequently Asked Questions
Q: Can I eat tomatoes affected by anthracnose if I cut out the lesion? A: The surrounding firm, unaffected fruit is generally safe to eat if consumed immediately after cutting away the lesion. Anthracnose does not produce toxins that spread into unaffected fruit tissue. However, affected fruit deteriorates rapidly — use immediately.
Q: Is late blight contagious to pepper plants? A: Phytophthora infestans primarily affects tomatoes and potatoes (all Solanaceae family). Peppers are less commonly affected but can be infected. Maintain strict crop separation and remove infected plants promptly.
Q: Can I put diseased tomato plants in my compost? A: For late blight: Do not compost. P. infestans spores can survive in compost that does not reach thermophilic temperatures. Bag and dispose in trash or burn. For anthracnose: Home compost piles that regularly reach 55°C+ can safely process infected material, but when in doubt, dispose rather than compost diseased plant material.
Q: Why do my tomatoes keep getting blight every year? A: Late blight spores overwinter in infected potato tubers left in the soil and arrive annually on wind from regional sources — you cannot prevent spore arrival. Your only defense is preventive spraying during high-risk weather windows and growing resistant varieties.
References
University of Minnesota Extension. (2023). Tomato Late Blight. https://extension.umn.edu/plant-diseases/late-blight
Cornell University Plant Disease Diagnostic Clinic. Anthracnose of Tomato. https://plantclinic.cornell.edu/
Rodale Institute. Organic Disease Management in the Home Garden. https://rodaleinstitute.org/
Royal Horticultural Society. Tomato Diseases. https://www.rhs.org.uk/disease/tomato-diseases
USDA National Organic Program. Allowed and Prohibited Substances for Organic Production. https://www.ams.usda.gov/
