Tomato late blight
This is one of the most damaging diseases of tomatoes worldwide
What is it?
Tomato blight is caused by the fungus-like organism Phytophthora infestans, and is one of the most damaging diseases of tomatoes worldwide. It can result in complete destruction of crops if weather conditions are favourable. Whilst outdoor plants are most commonly affected, devastating outbreaks can also occur in greenhouses. It can be very difficult to distinguish leaf and stem symptoms of blight from those of grey mould. Blight spreads quickly during wet or very humid conditions, destroying plants rapidly if left undetected. It can also affect potatoes, and was responsible for the Irish potato famine of the 1840's that led to the deaths of over one million people.
What does it look like?
Blight appears on leaves as pale green, water-soaked spots, which enlarge rapidly and turn dark brown. In high humidity, a white fungal growth is just visible on lower leaf surfaces, usually at the margins of the lesions. In dry weather, infected leaves quickly dry up and the white mould disappears. Infected areas on stems appear brown to black and entire vines may be killed in a short time when moist weather persists. Blight can also develop on green tomato fruit, causing brown lesions with a ‘marbled’ appearance. At first, the affected fruit remain firm. If conditions remain moist, abundant white mould growth will develop on the lesions and secondary soft-rot bacteria may follow, resulting in a slimy, wet rot of the entire fruit.
How is it spread?
Spores produced on infected tissues are dispersed by wind and rain splash, and in wet or humid conditions germinate to create new infections. Severe blight outbreaks tend to develop in mid to late summer, but can be found earlier if the weather is very wet. The disease often spreads to tomatoes from nearby affected potato crops.
How can I control it?
Plants can be protected by regular preventative fungicide sprays. A few tomato varieties with claimed resistance to blight have recently come onto the market, e.g. Ferline, Fantasio. Do not save seed from infected plants. Remove and destroy blight-affected plants, do not compost them. During very dry summers blight is unlikely to be a significant problem.
When do I test?
Blight symptoms often appear on tomatoes in mid to late summer, but the disease can develop earlier if spring and early summer are wet. Testing should be carried out as soon as symptoms are detected and monitoring for symptoms should continue until cropping is complete.
What do I test?
Where possible, select areas of leaf or stem which show symptoms of disease. For best results include material where healthy and diseased zones meet. Do not use completely dead plant material. As a general guide, use a sample of leaf material approximately 25mm x 25mm. For stem samples, use a piece approximately 5mm across.