VIU Milner Gardens and Woodland

What causes tomato blight?

by Vancouver Island Master Gardeners Association

Question:  What causes tomato blight?

Answer:  Tomato blight is caused by different fungus infections that attack the tomato in varying ways and at different times.

Septoria Blight

Of the various tomato blights, Septoria Blight (or leaf spot as it is commonly known) is the least damaging of the blights. The scientific name for this fungus is Septoria lycopersici. The plant’s lower leaves have small dark circular spots with darker margins in the centre which are the fungal structures. (pycnidia). At this point rapid defoliation will begin if this infection is left untreated. This fungus favours warm nights (20-25 degrees C) with leaves that are wet from overhead irrigation, rain or dew.

Early Blight

This blight is caused by either Alternaria tomatophila or a closely related fungus Alternaria solani. Of the two, A. tomatophila is the most severe. This blight appears after the fruit has set. Rings that look similar to a target begin to appear on the lower leaves and stems of the plant. If it appears on the fruit, it will appear bruised, leathery and black, with raised concentric edges, and then the fruit will fall. Typically it appears near the stem. The fruit can be infected at any stage of maturity. The stems will turn brown and appear sunken and dry. Infections on older plant stems are oval to irregular with a dark brown concentric ring.

Late Blight

The fungus Phytophthora infestans is responsible for this very destructive blight on tomatoes. This fungus attacks both tomatoes and potatoes and is the fungus that was responsible for the great potato famine in the 1800’s.

Pale green/grey, watery looking spots begin to appear on the lower leaves and then morph into purplish black lesions and the stems turn black. The underside of the leaves will begin to show white fungal growth. This blight is particularly severe in inclement weather, coupled with cooler nights. Any affected fruits appear with brown, crusty patches and quickly rot.

Prevention is the same for each of the blights. 

Although there are fungicides available, and work with varying degrees of success; the easy solution is to not let blight take hold in the first place. Keep your tomato leaves as dry as possible.

As with all vegetables, crop rotation is a must ensuring that the tomatoes are not planted where other nightshade plants have been grown within the last two seasons. Remove any volunteer seedlings that pop up.

The easiest and most effective way to prevent blight is to space out your plants so air can freely circulate and help keep the plants dry. Stake your plants so tomatoes do not come in contact with each other,

Tomatoes prefer consistent watering practices and do not like to be dry. Water early in the morning and directly into the soil avoiding contact with any leaves, do not water from overhead and remove any leaves that may come in contact with the soil, either by water splashing or direct contact with the soil.

A mulch can be applied to provide a barrier between the soil and the tomato itself.

Fertilize regularly taking care not to over fertilize with potassium.

Remove any infected leaves immediately and dispose of…..do not compost.

After harvesting, remove all tomato debris and never compost infected plants.