What is Tomato Blight Caused by? 6 Ways to Prevent It


2 tomatoes with blight

Is tomato blight putting a damper on your harvesting spirit? Every gardener enjoys healthy harvesting tomatoes from their garden. In this article, I’ll take you through some of the causes of tomato blight and how you can avoid the unsightly fungal infection.

Tomato blight is caused by a fungus called Phytophthora infestans. It spreads via airborne spores. This particular type of fungus requires poor airflow and hot humid conditions to flourish. This fungus is responsible for late and early tomato blight.

Every type of fungus presents in a different way allowing gardeners to distinguish between them. Early blight appears all over the plant, causing fruit to fall, and late blight causes fruit to turn brown and wither. Once a gardener identifies the specific kind of spore affecting his plants, they can follow the six preventative measures I’ve put down in this article to safeguard future harvest.

How Can You Identify Blight?

Let us take a closer look at what tomato blight is and how it presents on the plant.

Blight is a common fungal infection that causes many gardeners to distress in different seasons. The disease is capable of infecting an entire farm of tomato trees resulting in untamable losses. Depending on the kind of fungus you have, other parts of the tomato plant can be affected.

While there is no cure for blight, there are ways to prevent it, and the first step is to know how to identify the disease in its different variations.

1. Early Blight

Early blight can be caused by either one of the fungi types, Alternaria Tomatophilia and Alternaria Solani. This blight resembles rings and presents first on the leaves and quickly progresses to the plant stem. The spots also show on the tomato, soon developing into large bruises. The affected fruits soon begin to fall because the blight appears close to harvesting time.

This blight is the most disappointing for a gardener because the infection happens so close to harvesting time. Luckily treatment for this kind of blight is simple and easy to put into effect.

This will not kill the whole plant, but it will automatically affect the yield. Even though you can shift to resistant cultivars, you aren’t always out of the hook. That’s why we have listed some of the steps you can take to reduce the damage.

  • Keep the tomato leaves dry. Remember, most fungi diseases are favored by wet surfaces. So, keeping the leaves dry helps to prevent the disease.
  • Keep off plants and weeds in the tomato family as they can also infect your tomatoes
  • Support your tomato plants with sticks
  • Always check and get rid of the infected plants

2. Late Blight

This type of fungus causes pale green blisters to form on the leaves and stems. Many gardeners term this fungus the most destructive of the three types. The blisters are quick to develop and turn into nasty purple-black lesions that will eventually encapsulate the stems. This fungus will often attack tomato plants in the rainy seasons that have cool weather at night.

This fungus does not spare fruits as they present brown rings that turn into crusty patches and become cesspools for rot. Any tomato gardener will tell you that there is little hope for a good harvest once this kind of fungus takes hold.

Here are some of the best tips that you can put in place to prevent late blight.

  • Always go for the right variety. Shop from a trusted dealer. You can also go for varieties that mature earlier before late blight kicks off.
  • Ensure that your tomato plants are well spaced. When you let every plant breath and receive light maximally, they will be able to fight and resist the disease.
  • Pay attention to the weather conditions. Remember, this disease is always favored by cool and wet weather. But dry weather tends to discourage it.
  • Avoid watering your tomatoes by a sprinkler or other overhead watering methods and keep the foliage dry.

If you would like to know more about blight the video below covers it in more detail along with other tomato diseases.

The Life Cycle of Blight Fungi

The blight spread through spores that can germinate in temperatures between forty-seven degrees and ninety degrees Fahrenheit. The spores also need free water and humidity greater than 90% to grow. When the spore lands on a leaf, the first signs of infection will show after at least five days. There are different ways a spore can move around the farm. Some of the transference methods include human contact and water.

By nature, blight fungi will infect the lower leaves of a plant before advancing towards the fruit. Only an eighth of an inch in diameter of spores need to contact the plant for an infection to take hold. The pathogen can also be transferred from plant to plant, sometimes crossing from farm weeds to food crops. Once the fungi have infected a plant, the only cure is to uproot the plant and dispose of it.

Six Ways You Can Prevent Tomato Blight

When you spot the fungus on the farm or garden, the first step is to uproot all affected plants and keep them as far away as possible from the healthy plants. The best way to dispose of the plants is to burn them or store them I sealed garbage disposal units. Because the fungus reproduces through air-borne spores, you might want to cut the plants down gently without agitating the plant.

Using mulch around the base of your plants will help keep the blight at bay. You can use grass or wood chips to this end. The mulch works by creating a barrier preventing the spores in the soil from getting to the plant. If the blight is too widely spread, you can use a fungicide to stop the spread.

Prevention during planting

There are a few measures you can put in place while planting to keep blight at bay.

1. Crop rotation

The concept of plant rotation is quite common in farming. Using a different piece of land to plant tomatoes each planting season will reduce the risk of getting your tomatoes infected. The potion of land you choose to grow your tomatoes next should not have any other member of the Solanaceae family plant planted there before. Plants in this family include eggplant, potatoes, or tobacco.

2. Choose the suitable tomato variety

There are tomato seeds in the market that will give you a crop resistant to tomato blight. The only way to know which sources to pick off the shelf is to read the fine print. If you pay close attention, you will notice the seeds that offer a blight-resistant crop; they might be a bit costly, but they are worth the extra coin.

3. Mulch Your Plants

tomato plant with green tomatoes and mulched with hay

After your plants have sprouted, you need to make sure that they are properly mulched. If you can, mulch your plants after planting the seed to give them a healthy start at life. Ensure that the plant base is well covered using whichever material you choose to use.

Suppose you are worried about your mulch turning white. It may be affected by Fungi. I wrote an article about what Fungi causes your mulch becoming white and how you will get rid of it. You can read it here.

4. Choose the Most Suitable Watering Mechanism

When watering your plants, ensure that you use a hose that reduces the amount of splashing that happens. Using overhead sprinklers for your tomatoes is highly discouraged. Water on the leaves will only increase the chance of the fungus catching on and decimating your plants. I would recommend using a soaker hose.

5. First Action

If you rarely inspect your garden, then you have little to no chance of catching the fungus at its earliest stages. I advise gardeners to examine their tomato plants as often as possible to increase the chances of spotting the first signs of damage, and acting fast after that will save the rest of your crops from getting blight.

6. Choose the Right Fungicide

Some gardeners might not be open to using fungicide on their crops because of the perceived health hazards some people believe they pose. Some fungicides have been approved for use, which means they do not have any adverse health effects on the consumer. I like to use a fungicide to control the fungus spread in good time, especially when all other measures have failed to yield desired results.

Additional tips

1. Keep an eye out for the weeds

Since some weeds are also susceptible to the pathogen, all gardeners should focus on de-weeding the tomato plants. Some weeds that can contract the fungi include black nightshade and the notorious nightshade.

2. Innovative mulching

When applying mulch, I advise gardeners to use plastic mulch as it provides an extra effective barrier compared to other materials used around the farm. If you hope to get your tomatoes blight-free, you should also avoid working on them when the weather is humid or when the plants are wet from dew, rainfall, or irrigation.

3. Stake your tomatoes plants

Always use drip irrigation instead of overhead irrigation for the tomatoes. Most gardeners have advised that staking does a great job drying out the plant base and reducing the level of contact between the leaves and spore-contaminated soil. If you are wondering what staking is, it is a technique that involves inserting a stick beside the plant to provide support for it as it grows. The tomato plant especially needs this to flourish as it has a week stem.

If you decide to use chemicals to combat blight, ask your supplier to alternate between the chemical families as often as possible. This prevents the pathogen in question from developing insensitivity to ingredients in the chemical mixtures. How the gardener goes about applying the chemical is also crucial.

4. Keep up with the tech

As always, farming technology keeps advancing every day, which is why gardeners should always stay on their toes regarding the new ways their colleagues are running successful farms. There is always a chance to learn from each other. Gardeners should apply the chemicals when the environmental conditions are agreeable to the fungi. In other words, the gardener should apply the fungicides in hot and humid conditions for the best outcome.

Conclusion on what causes tomato blight

Tomato blight can affect other plants, too, such as potatoes and eggplants. It is also important to note that two types of fungi are responsible for causing blight in any plant, namely, Alternaria emetophilia and Alternaria solani. Tomato blight can cause a gardener a bucket load of stress, but these six measures are just what every gardener needs to do. If you observe these six measures, you will have easier farming and an even better chance at a bountiful harvest.

Tomatoes are great to grow, especially if you follow the basic rules and growing methods. I wrote a detailed article on growing tomatoes at home to ensure perfect results every time. Read that here.

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Tony O'Neill

I am Tony O'Neill, A full-time firefighter, and professional gardener. I have spent most of my life gardening. From the age of 7 until the present day at 46. My goal is to use my love and knowledge of gardening to support you and to simplify the gardening process so you are more productive

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