Late blight on tomatoes — symptoms and treatment

Severity: high

What is Late blight

Late blight of tomato is caused by Phytophthora infestans — not a true fungus but an oomycete, a water mould. The distinction is not academic: it explains why this disease travels at a speed no leaf spot can match, and why the products that stop Early blight do not stop it. Under favourable conditions it can devastate entire crops in days, taking leaves, stems and fruit together. This is the tomato disease that justifies dropping everything else.

Symptoms

Late blight does not make tidy spots. Lesions appear as large, water-soaked patches, greasy and grey-green as though the tissue had been scalded, and unlike Early blight they carry no concentric rings and show no preference for the oldest leaves — they can start anywhere on the plant. In humid weather, look at the underside of a lesion at its advancing edge: a white fuzzy growth appears as the pathogen sporulates. That is the giveaway. What follows is fast — the patches brown and leaves and stems collapse together.

• Early: large water-soaked, grey-green patches on leaves — patches, not defined spots.
• Progressing: white fuzzy sporulation on the leaf undersides in humid conditions.
• Advanced: rapid browning and collapse of leaves and stems; the crop can be lost in days.

If lesions are dry, ringed and confined to the bottom of the plant, you are looking at Early blight and you have time. If they are water-soaked, spreading and fuzzy underneath, you do not.

Causes and conditions

Phytophthora infestans sporulates on the underside of living lesions, and its spores travel on wind and rain — including from well beyond your own garden, which is why it can appear in a clean bed with no local source. It needs cool, wet, humid weather and prolonged leaf wetness; an extended wet period takes it from a lesion or two to a lost crop. Unlike fungi that overwinter on dead debris, it survives in living tissue — infected potato tubers, volunteer plants, cull piles — so those are the reservoirs to eliminate.

Treatment

Remove infected material — cultural

Timing: upon detection. Remove and destroy infected plants immediately — this is not a wait-and-see disease, and a single sporulating plant supplies the rest of the bed. Do not compost the material. Improve air circulation.

Metalaxyl + Mancozeb (Ridomil Gold) — chemical

Timing: before humid weather. Apply this preventive systemic fungicide before humid weather arrives — the aim is protection ahead of infection, not rescue after it. Critical during extended wet periods. Pre-harvest interval: 14 days.

Pesticide registrations vary by country — check local approval before use.

Prevention

• Grow blight-resistant varieties where you can — the most effective measure available to a home grower.
• Space plants generously and stake them so air moves and foliage dries fast.
• Water at the base, never over the leaves, and early enough that foliage dries before night.
• Destroy volunteer tomato and potato plants and never leave cull piles nearby — living tissue is where this pathogen waits.
• Inspect closely during cool, wet, humid spells.

Frequently asked questions

Is it contagious to other plants? Highly. Spores blow on wind and rain from plant to plant and garden to garden, and it takes potatoes as readily as tomatoes. An infected plant left standing endangers every tomato and potato nearby.

Can I save a plant that already has it? Realistically, no. Fungicide here is preventive — it protects tissue not yet infected. Once a plant shows spreading water-soaked lesions, the useful move is to remove and destroy it to protect what is still clean.

Can I eat the fruit? Discard any fruit with lesions. Sound, unblemished fruit from an affected planting can be eaten; if you have sprayed Metalaxyl + Mancozeb, respect the pre-harvest interval of 14 days.