Tomato Blight

Tomato blight (also known as potato blight) is a fungal disease which is caused by wet, damp conditions and mainly affects tomato plants growing outdoors.

Greenhouses that are poorly aerated can also be a cause of blight because of high humidity.

There are two types of blight:

Early Blight, (below) shows itself as dark round spots surrounded by concentric rings that fade to yellow. Wet leaves combined with cold temperatures in the Spring will leave a plant susceptible to early blight.

Late Blight (see pic below) is the most common and devastating type.
It starts as brown patches on the edges of leaves that eventually spread to the entire plant - stems and fruit too.

The combination of wet leaves and high humidity over a prolonged period is almost curtain to cause blight. As a fungal infection, it can easily spread from plant to plant and on fingers too, so it’s best not to touch plant leaves.

It is common to have a touch of fungal infection on one or two lower leaves when growing outdoors - just remove them and keep the base of plants free from decaying leaves. Good air circulation at soil level helps keep plants healthy.

However, if blight has affected a number of leaves it is time to spray with Dithane 945 or apply Bordeaux Mixture. This may stop the disease spreading, and more importantly infecting other plants, but there is no cure for blight, only ways to stop it getting worse.

In this pic you can see that blight has affected the stems and the fruit.

How to prevent tomato blight

If growing outside provide some kind of shelter for your plants if possible. Wet leaves are ok for a few hours, but over-night in damp conditions followed by another wet day will lower the plants defenses.

Ferline and Legend are supposed to be blight tolerant - that is they have a greater defense against blight than other varieties. The season (2008) was very wet and every variety that I grew, including Ferline, contracted blight.

Suggestions to follow:

Keep tomato leaves dry - provide some shelter from the rain if possible.
Water in the morning to that plants are not stood in excess water overnight.
Remove some of the lower leaves (by pulling off - not cutting) if they show signs of infection.
Don’t touch leaves of plants and spread the disease on fingers.
If all else fails the last resort to save your plants is spraying with a protective fungicide. This is available from garden centres in the UK.

I sometimes hear people say things like: “a drop of rain will do them good” and “when you water, water the leaves too” my advice is to keep those leaves dry. The only time I will allow my leaves to get wet is when I foliar feed which I do on a dry day.