Tomato blight is caused by the same fungus, Phytophthora infestans, that causes late blight of potatoes (see factsheet DC17 - late potato blight). The disease is common on outdoor grown tomatoes, especially late in the season. A greenhouse or polytunnel provides some protection from the wind blown blight spores, but indoor crops can also be affected.
Symptoms of tomato blight
Symptoms of tomato blight
Foliage and stems: Dark brown/ blackish round patches appear, often surrounded by a pale yellow halo, that quickly spread to rot the whole leaf. The underside of the infected leaf develops a downy white coating of spores in moist conditions, particularly at night. Dark streaks and spots may develop on infected stems. The early stages of blight are easily missed, and not all plants are affected at once. If weather remains warm and damp, the disease will rapidly spread; in a severe attack the foliage may be reduced to a rotting mass within a few days.
Fruit: Dark markings develop on green fruit. Mature fruits quickly develop a dryish brown rot, which may only appear a few days after picking. A whitish-grey mould may accompany this. The fungus is not poisonous to humans, it is fine to eat any ripe tomatoes not visibly affected by the fungal blight. Fruits showing symptoms are not pleasant to eat and will not ripen or store as the fungus rapidly spreads through the fruit even after harvesting.
The fungus survives the winter on infected potato tubers, or in tomato
seeds. Plants growing from these produce fungal spores which are spread
on the wind. The initial infection in a crop may come from a local source,
such as a neighbouring potato crop or a garden rubbish dump, or the spores
may be blown in from many miles away.
Spores can only develop and infect if they land on wet foliage or fruit. Spread is very rapid throughout the crop when temperatures are above 10°C and humidity is over 75% for two days or more.
Prevention and control
- Variety choice: Tomato varieties vary in their susceptibility
to blight. Growing earlier maturing and smaller fruiting varieties may
mean that crops can be harvested before blight strikes.
There are a few varieties on the market that are said to be resistant to blight - Ferline and Legend for example. They are worth trying, but may only delay the infection rather than ensuring a healthy crop. Check current seed catalogues before buying your seed for other resistant varieties that may have been recently developed. Some so-called resistant varieties can resist some strains of the fungus and not others, so their performance may vary depending on the strains of blight around.
- Healthy start: Do not save seed from infected plants. Buy good quality seed from a reputable supplier. Remove and destroy any volunteer potatoes which may potentially harbour the pathogen.
- Watering: Keep the plant leaves dry .Water at the base of plants and early in the morning to reduce long periods of moisture on foliage, which encourages blight. A mulch will reduce the amount of watering needed and control weeds. Try to avoid brushing past tomato plants when they are wet, increasing the likelihood of spreading any infection.
- Timing: Tomato blight infection occurs during a period of warm, moist weather in mid to late summer. Wind carried spores spread the disease very quickly and can soon infect a whole crop if the conditions remain warm and moist. Growing earlier maturing and smaller fruiting varieties does mean that you can harvest fruit before blight strikes.
- Increase airflow: Space plants widely and stake to reduce humidity levels within the crop. Greenhouses and polytunnels should not only be well ventilated but also leak proof. Plants growing in leaky greenhouses and polytunnels can easily succumb to blight.
- Monitor: Check plants on a regular basis for signs of blight. Remove infected leaves as soon as you notice them. See below for composting infected material.
- Fungicide spray: Bordeaux mixture is a copper based spray traditionally used by organic growers to control blight. It can protect healthy foliage, but cannot cure the disease once established. So, to be effective, it has to applied regularly to healthy plants as a preventative measure which is not considered good organic practice. For this reason, Garden Organic does not recommend the use of copper sprays as a way of combating blight.
- Blighted fruit: Blight symptoms may show after a few days, so eat fruit as soon as possible. If you have any green tomatoes on a plant affected by blight, pick off and turn into chutney as soon as possible. Trying to ripen the fruit will usually just end up in a pile of blighted, unusable tomatoes.
- Composting: Leaves and stems of plants affected by blight can
be added to your compost heap; the fungus will not survive in dead plant
material. Do not compost blighted fruit, as the fungal spores can survive
in seeds to grow and reproduce next spring, carrying blight onto your
new crops; either bury deeply (60cm/ 2ft) or put in a dustbin. Alternatively,
take all affected plants to the local recycling centre. These centres
compost on a very large scale, thus generating high temperatures which
will kill the disease.