When growing tomatoes, two of the most common threats to your plants are early and late blight. These terms sound vague, don’t they? After all, at its heart, a blight is merely something that spoils something.
Early blight spoils your tomatoes early in the season, while late blight spoils them later in the season. Simple!
Of course, it’s much more complex than it sounds. In tomato plants, early and late blight are fungal diseases that affect tomatoes in humid conditions. Both blights can be devastating to your tomato harvest, but early blight is much less destructive and easier to manage than late blight.
Let’s take a look at both these blights to protect your tomatoes from both of these deadly pathogens.
What are the Differences Between Early & Late Blight?
One blight comes early and the other comes late, of course, but what are the real differences between these two tomato diseases? Early and late tomato blights are actually caused by different fungi and they impact plants at different points in the growth cycle.
They cause similar symptoms, but since the fruit hasn’t fully formed when early blight attacks, you notice it on the leaves first.
Let’s dive into the specifics of each.
Tomato early blight is caused by the fungus can be caused by two, very similar fungi: Alternaria tomaphilia and A. solani. It’s one of the most common tomato diseases. In fact, it’s rare to have a tomato season go by without seeing a few signs of early blight.
Both A. tomaphilia and A. solani can affect other members of the nightshade family as well as tomatoes. That means your potatoes, peppers, and eggplants are at risk. If you’ve planted the nightshade vegetables in close proximity, early blight can spread quickly from plant to plant.
The Right Environment for Early Blight
Both blights affect tomatoes in humid weather. But early blight thrives in hot, humid temperatures. When the weather is 80F or above, humid, rainy, and damp, early blight comes out to play.
This is especially true early in the growing season. Young tomato plants are less hardy, and are using their energy to grow big and strong.
In most summer seasons, a patch of hot, humid weather rolls in. Sometimes it’s very hot and very humid, but other times it’s just moderately hot with a little bit of humidity. As long as the relative humidity is over 31%, early blight can start affecting your plants.
Signs of Early Blight
The first signs of early blight appear at the base of the plant. Take a close look at the lower stem of your tomatoes. In blighted tomato plants, you’ll see roundish, brown spots on the stem and lower leaves. Those brown spots will gradually expand into lesions. Eventually, the lesions will develop yellowish edges and darker interior rings.
If environmental conditions don’t change, or you don’t work to stop the spread of early blight, eventually the lesions will spread across the leaf – rotting it away completely. Or it may spread through the stem, killing the tomato plant.
When fruit is affected by early blight, the rot usually begins at the base of the stem. Then it spreads onto the fruit itself. The spots that form on the tomatoes themselves are often dark brown or black, and sunken, like pock-marks on the top of the fruit.
Early blight tends to be most destructive to the leaves of your tomato plants. A bad case of early blight can lead to leafless tomato plants and sun-scorched fruit.
While it’s not likely to kill your plants outright, early blight can turn a promising tomato season into a summer of disappointment. Even a mild case of early blight can reduce the quality and quantity of your tomato harvest as plants focus on repairing the damage instead of putting out fruit.
Late blight is the boogeyman of tomato growers everywhere. This fungus has a “take no prisoners” approach to nightshade plants. In fact, Late blight, caused by the oomycete (also known as a water mold) Phytophthora infestans, caused the infamous potato famine of the 1850s in Ireland.
When this blight attacks, it strikes hard and fast. In the right conditions, late blight can wipe out your entire crop of tomatoes in days. It’s an overwhelming disease that can be absolutely devastating when it occurs.
The Right Environment for Late Blight
Fortunately, late blight is much less common than its early counterpoint. Unlike early blight, late blight’s environment is very specific. Late blight thrives between 60-80°F, with the spores proliferating best at around 70°F.
Like early blight, late blight needs humidity to proliferate. But, while early blight can spread whenever humidity levels are between 31% and 91%, late blight needs total saturation. Late blight wants rainy, water-soaked leaves, damp air, and water droplets everywhere.
A dew-soaked plant on a humid morning works well, but drenching summer rains are ideal.
When the temperatures hover around 65°F for days, and clouds cover the sky, watch for the first signs of late blight in the tomato garden.
Signs of Tomato Late Blight
Remember, once you see the first signs of late blight, it’s already too late to save your tomatoes. This disease is fast and deadly. You’ll notice a few, water-soaked spots on the lower leaves at first. They’ll look pale green, almost like the diffused light of a water droplet. These spots start out at the edges of the leaves and quickly move inward.
Then, the spots turn velvety black, or purple. But before you can wonder what’s causing all these spots on your plants, the leaf will shrivel, turn brown, and die.
Stems will also sport lesions that weaken the stem. In fact, many people consider the brown stem lesions the primary sign of late blight.
These brown spots spread quickly and often develop a white, fungal growth as they spread. The lesions spread and quickly cause the whole thing to collapse as they cause the whole plant to rot away into a mushy mess of fungus and rotting plant material.
When late blight affects tomato fruit, the spots form beneath the skin. These reddish-brown spots quickly turn leathery, hard, and wrinkly. Then they soften as rot sets into the plant.
There are a few ways to reduce fungal and water mold diseases in general. If you keep your garden clear of debris and leave plenty of room for air to circulate between your plants, fungal diseases are less likely to spread. A clean garden is safer than a cluttered garden. Clean, open gardens don’t give last year’s fungal spores a place to hide.
It’s always a good idea to avoid overhead watering systems. Water your plants at the base to avoid over-saturating leaves and stems. In even moderately humid weather, wet leaves can quickly steam or wilt. They create the perfect environment for pathogen growth.
Crop rotation is another fantastic method for reducing the threat of blight. Don’t grow nightshade crops in the same bed year after year. Put a little space between our nightshades and rotate their locations every year to make blight less likely.
Early blight has a few useful treatment options. Because early blight consistently affects the leaves first, it’s easy to simply remove the affected leaves as they appear. Be sure to burn them or throw them in the trash, don’t try to compost blighted leaves.
You can use copper-based fungicides to reduce the spread of early blight. Consistent, weekly or bi-weekly treatments will keep the disease at bay. Use this fungicide to contain early blight while the symptoms are present.
Unfortunately, there is no successful treatment for late blight. Remember, this disease progresses so quickly, once it starts, you can’t stop it.
Because it’s not a fungal disease, you need to take a slightly different approach to try and control it long enough to harvest your fruits.
The second you notice late blight developing, or you even see the right conditions for it to take hold, treat your tomatoes with copper fungicides or a product containing the beneficial bacteria Bacillus subtilis.
A strong pretreatment can keep late blight from getting a stronghold on your tomatoes.
Blight Resistant Tomato Varieties
One of the best ways to reduce the devastating impact of blight on your garden is to plant blight-resistant cultivars. Because blights are the bane of tomato gardening, growers have been developing resistant plants for generations.
Of course, these resistant cultivars aren’t foolproof. In the ideal conditions for blight, it’s possible to have a breakthrough case of early or late blight. But resistant varieties are less likely to succumb to the fungi that cause early and late blights.
For a low-risk tomato garden, plant ‘Mountain Magic,’ ‘Jasper,’ and ‘Defiant PhR.’ These cultivars are resistant to both early and late blight. If you’re only concerned about early blight, try planting ‘Mountain Supreme’ or ‘Juliet.’ If you’re working to keep late blight out the garden, plant ‘Iron Lady’ or ‘Matt’s Wild Cherry.’
These cultivars are proven, hardy cultivars that can handle a lot of humidity without becoming hosts for the pathogens that cause early or late blight.
Certain areas only experience one type of blight or the other, so check with your local extension office so you can be sure which you need to be on the lookout for.