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Growing tomatoes is such an enjoyable endeavour with lots of culinary rewards. Yet, when we see the ominous signs of tomato blight, we might feel that all our efforts were for nought. However, with a little bit of preparation, knowledge, and forethought, we can be better equipped to keep this terrible infection at bay.
In this article, we’ll discuss how to treat and prevent tomato blight, so you can get back to building your garden and enjoying the fruits of your labour.
What is Tomato Blight?
Blight is a type of infection commonly affecting nightshade crops. Tomato blight is also known as “early blight or “late blight” depending upon the offending pathogen and accompanying weather conditions.
Early Blight
Early blight (Alernaria solani) often takes hold during very hot summer weather accompanied by intermittent wet spells. Typically, it is more of a concern in the USA. However, during exceptionally warm summers, early blight can affect UK growers as well.
Late Blight
Late blight, which comes later in the year, is an infection caused by a type of oomycete pathogen called Phytophthora infestans. It occurs during warm and moist weather conditions. As such, wet summers provide favourable conditions for this pathogen to thrive. In this article, we will mainly focus on late blight.
Once late blight appears on the plant, it quickly spreads through its tissues until it is widespread throughout the plant’s foliage and fruit. If it isn’t caught in time, it can wreak havoc on tomato crops and completely wipe out an entire season’s yield of fruit.
What Does Tomato Blight Look Like?
The main symptoms of late tomato blight are dark brown patches of rot and decay all over the plant. Under highly humid conditions, white, fungal-like growths may be visible on various parts of the stalk and on the underside of newly infected leaves. These leaves will eventually turn brown and shrivel. Fruits will then be exposed to the full force of the sun and suffer sun scalding. Deteriorating fruit may have dark brown spots at first. Then, as the disease progresses, the fruit will look bruised or turn brown as it begins to rot. Stems and branches will have black spots that spread and weaken the structure of the plant.
Blighted tomatoes should not be consumed and should be thrown out. The reason for this is because the damaged tissue can harbour other pathogens that may be harmful, or cause stomach upset for humans and animals.
When Does Tomato Blight Commonly Occur?
Tomato blight can happen at any stage of the plant’s development, but it often occurs when the plant is exposed to excessive moisture. Here in the UK, the risk of tomato blight can happen any time after June and lasts through late summer. When humidity is sustained over 90% for a few days and temperatures are over 10° to 18° C (50° – 64° F), blight can take hold.
During warm dry periods, blight can go dormant, but it will quickly reactivate with the onset of moist weather. With the cooler weather of autumn, the infection stops.
How Do I Prevent Tomato Blight?
It’s important to realise that there are no chemical pesticides or fungicides that cure blight once it has been contracted within the plant. Therefore, prevention is the best course of action.
Choose Resistant Tomato Varieties
Since there is no plant additive that prevents blight, one strategy may be to choose plants that are resistant to the infection.
Cultivars resistant to blight are available in our store. Our resistant cultivars include Honey Moon, Buffalosun, and Crimson Crush, our most resistant variety.
Also note, genetically modified (GMO) plants, developed to be completely immune, are not permitted within the UK.
Watering
Always water your tomato plants from the base of the plant. Try to never spray the leaves directly with water. In fact, it’s always a good idea to keep the leaves of your tomato plants as dry as possible. This is because ailments like tomato blight or powdery mildew can take hold when tomato plants are exposed to prolonged periods of humidity or rainy conditions. Also, it’s best to water your plants twice a day, once in the early morning and again at dusk.
Spacing
Always space your tomato seedlings adequately to allow for proper airflow between plants. In this way, plant foliage will have a chance to quickly dry off after rainfall. Space your particular tomato variety based upon the package instructions. Typically, 60cm (2 feet) is ideal for tomatoes, as healthy plants can become quite large.
Mulching, Trimming, and Plant Supports
Be sure to add mulch around the base of your tomato plants. This will help prevent any blight spores present within the soil from reaching the leaves of your plants. Similarly, remove low leaves and foliage around the base of your plant. Use lattice supports and/or tomato cages to encourage growth away from the ground.
Sun Exposure
Tomatoes like to live in sunny and gently breezy locations. With this in mind, plan to give your tomato plants at minimum 6 hours, or preferably 8 plus hours of full sunshine during the morning, followed by cooler dappled sun in the afternoon. Healthy plants are less susceptible to infection.
Planting Location
Avoid planting your tomato plants near or downwind of your potatoes or other nightshade crops, as UK winds move westerly for much of the year. If you are growing both potatoes and tomatoes, one crop can become infected and contaminate the other. In fact, most tomato blight infections usually come from small or large-scale potato farming in the area. Spores from infected crops are carried via wind currents and can land on or around your tomato plants. Blight spores can travel up to 30 miles!
Crop Rotation
It’s also a good idea to rotate your tomato crops every year. This way, if spores are present within the soil, crop rotation will enable enough time to go by for them to die out. Two to four years is a good timeframe in between plantings.
Avoid planting your tomatoes in areas that also grow plants from the Solanaceae family. This includes nightshade plants like peppers, eggplants, potatoes, and petunias.
Additionally, make sure your soil has adequate levels of nitrogen. Tomato plants with nitrogen deficiency are more likely to succumb to blight infection.
Use a Greenhouse
Using tomato tunnels or a greenhouse to grow your tomato crop can help reduce the risk of air-blown spores reaching your plants. However, the use of a greenhouse is not foolproof, but it may reduce the likelihood of exposure. If tomato blight is somehow able to take root within a greenhouse, it will spread rapidly, due to the high humidity.
Stay Informed
Check for forecasts that warn of tomato blight in your area. If there are confirmed cases of blight in your region, consider taking a proactive approach.
Here in the UK, there is a Blight Map application available via the Apple App Store. If you don’t own an Apple product, you can source data directly from the James Hutton Institute. For our friends over in the US, you can check this map via Plant Aid.org, for information about blight in your area.
Treating Tomato Blight
When treating tomato blight, you’ll want to move quickly. If it’s allowed to progress, your entire tomato crop can be decimated.
Use a Fungicide
While there is no cure for tomato blight once it has infected a tomato plant, the use of a fungicide can be used to kill spores present within the soil which may help contain or slow down the infection. However, blight spores are ever-changing and adaptive. Over time, they can build up a resistance to our best efforts.
Dispose of Contaminated Material
If you see the telltale signs of tomato blight, you’ll want to immediately remove any affected foliage to keep the tomato blight pathogen from spreading to the healthy leaves. If your tomatoes are in pots, remove the affected plants away from your other healthy plants.
When removing areas affected by tomato blight, be sure to burn contaminated plant matter. It should not go into the compost pile, where it can cause infection again the following year.
Tools
Disinfect all garden equipment that came into contact with the diseased tomato plants. This includes shovels, rakes, hand trowels, containers, and the like.
Soil
Any soil from contaminated plants should NOT be used for new tomato plants the following year, as this will only spread the infection to your healthy plants. The best solution may be to completely remove contaminated soil from your garden altogether.
However, if this is not feasible, you can instead use the soil with your other garden plants that are not susceptible to this infection. If you suspect that your tomatoes have blight, consider getting your soil and plants tested.
Conclusion
It’s amazing how gaining just a little bit of knowledge and making a few tiny changes can have big outcomes in the garden. Now that you’ve learnt about the ins and outs of tomato blight, you’ll be better equipped to prevent this devastating disease and protect your seedlings this year.
