Fungal diseases are the most common cause of crop loss in home vegetable gardens. Unlike pest damage, which is usually visible on individual plants, fungal diseases can spread rapidly through a bed or even the whole garden, and by the time you notice the first signs, the infection is often already well-established. Early identification and a clear response plan make the difference between losing a few plants and losing an entire crop.
How Fungal Diseases Spread
Most fungal pathogens spread via spores - microscopic reproductive particles that travel on wind, water, insects, tools, and hands. Many overwinter in infected plant debris left in the garden or in the soil itself. Warm, humid conditions with poor air circulation are ideal for spore germination and infection; watering overhead (wetting the leaves) rather than at the base dramatically increases disease risk for many fungal pathogens.
This is why garden hygiene - removing and composting diseased plant material properly, or burning it if the compost pile can't get hot enough, cleaning tools between plants - is so important. You're breaking the infection cycle.
Powdery Mildew
Powdery mildew appears as a white or grey powdery coating on leaf surfaces, stems, and sometimes fruit. Unlike most fungal diseases, it thrives in warm, dry conditions - not wet ones - and is actually inhibited by free water on leaves. It's caused by a range of different fungal species that are host-specific: courgette mildew won't infect your roses, and cucumber mildew won't infect your squash (usually).
Treatment: At first sign, spray affected areas with a dilute sodium bicarbonate solution (1 tsp per litre of water with a few drops of soap to help it stick) or dilute milk (40% milk to 60% water) - both are surprisingly effective at slowing spread. Remove heavily infected leaves. Improve air circulation around plants by removing crowded growth. Potassium bicarbonate products (often branded as organic fungicides) are more effective than sodium bicarbonate for serious infections.
Early Blight (Alternaria)
Early blight affects tomatoes and potatoes most commonly, appearing as dark brown spots with concentric rings - a "bullseye" pattern - usually on older, lower leaves first. The lesions are surrounded by yellow halos. Unlike late blight, early blight rarely kills plants outright but weakens them significantly, reducing yield and making them more susceptible to other problems.
Treatment: Remove affected leaves immediately and dispose of them (not in the compost unless it runs hot). Apply copper-based fungicide spray preventively or at first sign of infection. Keep the base of plants clear of old leaves and mulch to prevent spores splashing up from the soil. Avoid overhead watering.
Late Blight (Phytophthora infestans)
Late blight is the most serious disease of tomatoes and potatoes - it was responsible for the Irish Potato Famine. It spreads with terrifying speed in cool, wet, humid conditions. On tomatoes, you'll see large, irregular, water-soaked patches on leaves that quickly turn brown and black; white fuzzy growth may be visible on the underside of leaves in humid conditions. Fruit develops firm brown lesions and rots rapidly.
Once late blight takes hold in warm, wet weather, it can destroy an entire tomato planting in a week.
Treatment: Act fast at first sign. Remove and bag all infected material - do not compost it. Apply copper fungicide spray immediately and repeat every 5 - 7 days. If a plant is more than 50% affected, remove it entirely to protect neighbouring plants. Blight-resistant tomato varieties (like 'Crimson Crush' or 'Lizzano') are worth considering if you garden in areas prone to late blight.
Damping Off
Damping off is the sudden collapse and death of seedlings, usually at the soil line. The affected seedling pinches in and keels over, looking healthy above the pinch point but completely rotted at the base. It's caused by several soil-borne fungi (Pythium, Rhizoctonia, Fusarium) and is almost always a result of overwatering, poor drainage, or insufficient air circulation around seedlings.
Prevention is the only real strategy - once a seedling damps off, it's gone. Use fresh, sterile seed-starting compost (not garden soil), water from below rather than overhead, provide good air circulation, and avoid letting seedling trays sit in standing water. A light dusting of horticultural grit on the surface of seed compost helps keep the immediate surface dry.
Downy Mildew
Confusingly named similarly to powdery mildew but caused by entirely different organisms, downy mildew thrives in cool, wet conditions rather than dry ones. It appears as yellow or pale patches on the upper leaf surface with a grey-purple "downy" fungal growth on the underside. Common on brassicas, lettuce, onions, and cucurbits.
Treatment: Remove infected leaves. Improve air circulation. Copper-based sprays have some activity. Avoid overhead irrigation. Choose resistant varieties where available - modern brassica varieties in particular have been bred for downy mildew resistance.
General Prevention Strategy
Regardless of which fungal disease you're dealing with, the same preventive practices reduce all of them:
- Water at the base of plants, not the leaves
- Water in the morning so any wet leaves dry quickly during the day
- Ensure adequate spacing between plants for air movement
- Remove diseased plant material promptly
- Rotate crops so the same family doesn't occupy the same ground for 3 - 4 years
- Choose resistant varieties, especially for notoriously disease-prone crops like tomatoes