Blight is one of the killers for anybody who wants to grow tomatoes. The problem is that many other problems (sunburn, windburn, magnesium deficiency) show similar symptoms to the early stages of blight so any grower has to make a decision whether to remove the infected plants to try and save the rest of the crop at a stage where they may actually be perfectly healthy plants.
Late Blight: Late Blight (or Blight) is caused by a wind and rain borne fungus (Phytophthora infestans). It is the same fungus as causes potato blight so if you are growing on an allotment, the chances are that your, or your neighbours, potatoes will infect your tomatoes.
The first signs of blight will be on the leaves. Brown marks will appear and they will spread rapidly to other leaves and plants. Once the plant is infected, blight will progress to the fruit and they will brown and rot. You will not get a crop. The only solution is to remove and burn the infected plants and hope that the blight hasn't spread.
The risk of blight increases when there has been 48 hours of wet and warm weather and after four days the spores will definitely be airborne. However, if your plants are not affected, a period of dry weather may help to slow down the problem. If you want to protect your plants, spray with a copper fungicide after 48 hours and repeat every 10 to 14 days.
Plants are less likely to get blight if they are grown in a greenhouse/polytunnel and you water them in a way which doesn’t get the leaves wet (either directly on the ground or - even better - use a drip watering system. It is also sensible to attend to your greenhouse tomatoes first as the blight spores can attach to your clothes and be transported into the greenhouse that way.
Blight stays in the ground from one year to the next on living material (old tomatoes, plants, leaves, etc.), so if you have any blight destroy all infected plants, fruit, leaves (absolutely everything). Burn them or put them in the dustbin, do not leave any part of the plant on the ground as it will probably cause re-infection next year.
Keep the plants well ventilated and try to avoid the atmosphere in the polytunnel/greenhouse getting damp. Do this by watering your plants in the morning (certainly not when the ground is hot during the day).
Blight can only be prevented, not cured. If you have blight in your crop, the best thing to do is to rip out affected plants and burn them. If you have caught it early, fruit can be used green but are unlikely to ripen (lots of chutney). However, you will probably have to use the fruit quickly as it could rot.
In the UK, the progression of blight across the country is reported by blightwatch.co.uk and is very useful to know whether there is blight in your area.
There are a number of commercial fungicides that can be used to try and prevent blight but we try to avoid using them if at all possible because they have to be used before blight appears and so may not be necessary.
Over the past few years, there have been a number of "Blight Resistant" Cultivars on the market. We've tried Crimson Crush and Mountain Magic with limited success. It should be noted that these cultivars are only described as "Resistant" not "Tolerant" and our experience has been:
- in years where there isn't much blight, they survive and grow as well as the rest of the crop;
- in years where there's a medium presence of blight, the plants survive and you will get a crop but (in general) the fruit is blight infected and much of it will rot if it hasn't ripened on the plant;
- in years where there's a heavy presence of blight, the plants survive but all of the fruit get blight.
Now, our experience may not be the same as everybody else's but we have decided not to grow blight resistant varieties but to focus on growing tomatoes in a way that minimises the chances of blight.
Early Blight: Early Blight is similar to Late Blight but is caused by a different fungus (Alternaria solani) and starts earlier in the season. Rather than killing the plant and rotting the fruit, Early Blight weakens the plant and significantly reduces the crop. It is less common in the UK, but relatively common in North America and therefore appears frequently on the web (which leaves everybody expecting to get early blight even if it doesn't happen). However, it can appear if the plants are wet and in cold springs.
Early blight is becoming more common in commercial potato crops in the UK as growers move from broad spectrum fungicides to more selective solutions and so may become more common in the UK.
A solution for early blight is to increase the ventilation around the plants by pruning off the leaves (which would be lost to blight in any case). Increased ventilation means reduced moisture and so will reduce the probability of early blight.