Tomato blight can describe more than one destructive disease. Early blight, usually caused by Alternaria species, develops mainly on older foliage, while late blight is caused by the water mould Phytophthora infestans and can destroy a crop rapidly. Identifying which disease is present determines the most useful response.
Australian conditions shape the risk. Sydney’s humid winters, Melbourne’s damp and changeable spring, and Brisbane’s warm, rainy summers can each produce long periods of wet foliage. The same watering routine can therefore be harmless in one district and risky in another, especially on dense plants or in sheltered garden beds.
Late blight often starts as irregular, water-soaked patches that quickly turn dark brown or black. In humid weather, a fine grey-white growth may appear beneath the edge of a lesion. Stems can develop dark lesions, while fruit shows firm, greasy-looking brown patches that later decay and may develop white mould.
Early blight usually appears first on older, stressed leaves as small brown spots with concentric rings, creating a target-like pattern. Surrounding tissue turns yellow, and lesions may join until substantial foliage is lost. Dark, sunken patches can form on fruit near the stem, but the characteristic white growth associated with late blight is generally absent.
Rain, overhead irrigation and prolonged leaf wetness help both diseases spread, yet their development differs. Late blight can become apparent within days during mild, humid weather. Wind-driven spores may travel to nearby plants, while infected potatoes and volunteer tomatoes can provide additional sources of inoculum.
Early blight is more often a gradual problem. The fungus survives in infected debris and spreads by splashing water or contaminated tools. It commonly affects plants weakened by nutrient stress, crowding or heavy fruit load. Checking where symptoms began, the pattern within each spot and the weather before they appeared can help separate the two diseases.
No single symptom proves a diagnosis. Septoria leaf spot, bacterial speck, mite feeding, herbicide damage and nutrient problems can resemble blight, while a photograph alone may miss the underside of a leaf or an early stem lesion.
| Feature | Early blight | Late blight | Other common problems |
|---|---|---|---|
| Typical cause | Alternaria species | Phytophthora infestans | Septoria, bacterial speck, mites or non-infectious injury |
| Leaf pattern | Target-like rings, often on older leaves | Irregular wet patches turning dark brown | Numerous tiny spots, stippling or scorch |
| Humid-weather sign | Usually no white growth | Grey-white growth beneath lesion edges | Webbing, yellow halos or no pathogen sign |
| Fruit or stem | Sunken dark fruit lesions may occur | Greasy fruit patches and dark stem lesions | Damage may follow spray contact or feeding |
Ask a local nursery, state agriculture service or diagnostic clinic to examine a fresh sample, including affected leaves and any stem tissue. Look for similar symptoms on nearby plants, and avoid applying a fungicide until the cause is reasonably clear.
Act quickly when late blight is suspected, especially if spots are spreading after rain. Isolate affected containers, avoid handling wet plants and remove the entire plant if stems or fruit are involved. For limited early blight, pruning affected lower leaves may reduce inoculum, but heavily infected plants are rarely worth keeping.
Bag discarded plants and follow council green-waste rules; an ordinary backyard compost heap may not destroy the pathogen. Wash hands, clean pots and stakes, disinfect cutting tools between plants, and remove volunteer tomatoes and discarded potato plants. Do not carry suspect plants across state borders because interstate plant movement is subject to biosecurity controls. Discard blighted fruit rather than storing it beside sound produce. The storage lesson in short-term cigar storage is simple: airflow and moderate humidity matter when preserving stored goods, although neither measure can reverse an existing infection.
Fungicides protect healthy tissue or suppress early infections; they do not restore dead leaves or cure diseased fruit. In Australia, agricultural and veterinary chemicals are regulated through the APVMA under the Agricultural and Veterinary Chemicals Act 1991, with enforcement shared by state and territory authorities. Use only a product registered for tomatoes and the named disease, and follow its label for dosage, re-entry interval, withholding period and protective equipment. A registered copper product may reduce spread when applied at the correct preventive stage, but repeated use can accumulate in soil.
Neem products are useful for some insect pests but are not dependable fungicides for blight. Advice on neem oil for pests can help with aphids, whitefly or mites, yet it should not replace a product registered for tomato disease. Avoid spraying neem during high heat or when flowering plants are attracting bees, and observe the label.
Balanced feeding supports growth without creating the dense, succulent foliage that can be attractive to disease, while excessive nitrogen can worsen early blight. A guide to choosing a suitable fertiliser can help interpret fertiliser labels, although tomatoes need a complete feed suited to their growth stage. Improve soil with organic matter, avoid waterlogging and support plants without crowding them so foliage dries after rain.
Choose tomato cultivars sold locally as late-blight resistant, and rotate the bed to reduce early-blight carryover. Remove crop debris and volunteer hosts. Resistance can be incomplete and performance varies with the pathogen population, so it complements rather than replaces sanitation. In ordinary backyard and community plots, where seedlings and pots are often exchanged, clean containers and avoid accepting plants with unexplained leaf spots.
Water at soil level early in the day, mulch to prevent soil splash and space plants for airflow. Check sheltered corners after storms, because a roof or dense hedge can keep leaves wet longer than an open bed. Replace stakes and remove badly affected plants rather than hoping they will recover.
Keep a simple record of symptoms, weather and spray dates, then inspect plants at least twice a week during wet periods. The Bureau of Meteorology forecasts can help gardeners time preventive applications before prolonged rain, provided the product label permits the proposed use. Remove fallen fruit promptly and dispose of volunteer potato plants that could provide an inoculum source.
The issue is commercial as well as domestic. Major Australian growing regions such as the Riverland in South Australia, Carnarvon in Western Australia and Gippsland in Victoria supply fresh markets, while wholesale buyers in Melbourne and Sydney expect consistent, blemish-free tomatoes. A preventive routine protects home harvests and reduces crop losses before produce reaches the packing shed. Consistent sanitation, timely diagnosis and label-compliant care make a protected crop far more achievable.