Tomatoes are amongst the most popular plants you will find in home gardens globally. They are easy to grow and need minimal maintenance. But, unfortunately, some diseases love tomato plants as much as we do!
Three pathogenic diseases fall under the general heading tomato blight. They are early blight, late blight, and septoria leaf spot. Many people don’t realize it, but late blight is the same infamous disease that caused the devastating Irish potato family in the 1840s!
Types of Blight
Tomato blight is a fungal disease that is carried by wind and water, as well as insects and animals. If it isn’t treated, it can systematically kill your tomato plants as it affects the tissue of the stems, leaves, and fruits.
When it rains, the fungal spores reproduce and the condition worsens. There is no cure for blight on tomato plants or in the soil, but there are ways you can control it.
There are three types of tomato blight, but their names don’t give us much of a clue as to what they are. For instance, early blight and late blight can occur at any stage in the growing season.
Treatment for all types of tomato blight is the same, but the signs you will see that identify them are different.
Early Blight (Alternaria solani)
You can identify early blight by small, dark brown lesions with target-like concentric rings that form on the leaves and stems of affected plants.
It usually starts after the first fruits have formed.
Although it doesn’t affect the fruit, the leaves of infected plants die and drop off. Because the tomato fruit is then exposed to direct sunlight, it often gets sunscald, which damages and often kills the fruit.
Late Blight (Phytophthora infestans)
Late blight affects the leaves, which get light brown edges. The lesions that form have an irregular shape and they often develop a lighter ring or halo around them.
If you notice white, fuzzy patches on their underside, the disease is already far gone.
Late blight can attack at any time during the growing season and often affects the younger leaves at the top of the plant. It spreads quickly in damp, humid conditions, and if it isn’t treated, it can spread to the fruit.
Septoria Leaf Spot (Septoria lycopersici)
You can identify plants affected by septoria leaf spots by tiny, dark circular spots that form on their leaves. Yellow rings often form around the spots.
Septoria leaf spot tends to start on the plant’s lowest leaves. As the lesions spread, the leaves start falling off.
This form of tomato blight doesn’t usually affect the fruit directly or indirectly.
What is the cause of tomato blight?
All tomato blight needs to spread are the right weather conditions.
Tomato blight is caused by fungal pathogens that thrive in cool, wet weather. According to a paper published by North Carolina State Extension (NCSE), the spores that cause late tomato blight thrive in night-time temperatures of 50-60°F (10-15.5°C) and day-time temperatures in the 80s (late 20s).
In cloudy or wet weather, the clouds protect the spores from the sun’s UV rays. The rain enables the spores to infect the leaves.
This means that heavy dew and foggy conditions often promote late blight infections. But the spores are also commonly spread by wind and irrigation.
If blight is already rampant in an area with 5-10 miles (8-16 km), it can be blown into your field or veggie patch very easily. Even if your tomato plants have never been affected by blight, a strong wind might blow the airborne spores into your garden at any time.
What are the first signs of tomato blight?
The first signs of tomato blight will vary, depending on the type of blight your tomatoes are suffering from.
The specialists at the University of Massachusetts Amherst Center for Agriculture, Food, and the Environment encourage tomato growers to watch out for signs of blight. They advise that its early detection and management are the key to preventing blight epidemics.
First Signs of Early Blight
Look out for small brown to black lesions on older leaves. Often the area around the initial lesion turns yellow.
When the lesions get bigger, they often develop concentric rings. This is why people often compare their appearance to a target.
First Signs of Late Blight
The most common symptoms you will pick up are dark green or brown lesions on the leaves of tomatoes, and brown lesions on the stems. They usually appear to be sunken.
A white fuzzy, fungal growth under the leaves is also common. Sometimes you will also find firm, brown spots on the fruit.
First Signs of Septoria Leaf Spot
The first signs of septoria leaf spot are round tan or grey lesions with a dark brown outline that appears on the lower leaves. Often the lesions get bigger quite quickly and the infected leaves turn yellow and then brown.
If you magnify the lesions with a lens, you might spot black spots in the center. These are peculiar to septoria leaf spot and help to distinguish it from early and late blight.
Signs of Tomato Rot
Sometimes, the blossom end of tomatoes becomes sunken and decayed. It might look like tomato blight to you, but it isn’t.
Blossom end tomato rot is caused by a lack of calcium in the tomato. Remember, blight usually attacks the leaves!
You will spot tomato rot because your tomatoes will quite literally be rotting at the blossom end!
How do you get rid of tomato blight?
If you catch tomato blight quickly, you can treat it and prevent it from spreading.
Remove all the affected leaves and stems and burn or get rid of them. Don’t put them onto your compost heap or the disease will spread.
Mulch around the base of all your tomatoes with wood chips, straw, or any other natural mulch. This will prevent the fungal spores from splashing up onto your tomato plants in wet weather.
To prevent blight from spreading, it’s a good idea to spray with an organic fungicide that isn’t toxic.
A homemade recipe recommended by Dr. Martin Draper, a plant pathologist, contains:
- A teaspoon of vegetable oil
- A little mild soap in a gallon of water
Mix these ingredients and spray your plants regularly.
Clean up your garden in the fall to prevent the spores from overwintering on any plants you leave in the garden.
How to prevent tomato blight?
Of course, prevention is always better than cure. So, how can you prevent tomato blight from attacking your tomato crop?
Here are a few golden rules to help you.
Plant blight-resistant varieties. It isn’t rocket science to realize that these will be much less likely to get blight than a susceptible tomato variety.
Always rotate crops in the nightshade (Solanaceae) family. Common varieties include tomato, potato, eggplants, and chili peppers.
Once you have grown any of these, plant other vegetables for the next 2-3 years. This is to prevent the build-up of pests and diseases that attack them, including blight.
Stake your tomato plants or support them in cages to prevent the foliage from touching the ground. Staking or caging them also improves air circulation around the plants, which helps them to dry more quickly after wet weather.
Make sure your plants are healthy. Stressed tomato plants are more likely to be attacked by blight.
Weed your beds regularly. Various types, including horsenettle and nightshade, are hosts of blight, particularly septoria leaf spot.
Water your plants regularly, but don’t water overhead, and don’t over-water. Water early in the day and don’t water in wet weather.
Don’t fertilize outdoor tomatoes until they are well established and in full blossom. And don’t mulch until the soil has warmed up.
Use a fungicidal spray, preferably one that is organic. It won’t cure blight, but it will protect new plants from infection.
Can you eat tomatoes with blight?
Blight normally affects tomato leaves rather than the fruit. If it has affected the fruit, some plant pathologists say it’s safe to eat the parts that haven’t been affected, though they may remain firm or become unduly mushy.
But some food safety specialists warn that they could harbor organisms that might cause food-borne illnesses. They also say the infected fruit is likely to taste bitter.
So, the answer to this question is that it’s best not to eat tomatoes with blight.
Growing tomatoes is really easy. But if your tomatoes become diseased, it’s not much fun anymore.
These garden tips tell you how to avoid tomato blight in your veggie garden, and how to identify the different types if it does occur. We have also outlined steps that you can take to prevent tomato blight and how to get rid of it.
Chances are your tomatoes won’t get blight. But if you live in a wet, windy area, there’s a possibility they might.
We’ve provided all the tips to help you cope if it happens. Good luck!