How to prevent them from destroying your crop

If you purchase an item through links on this page, we may earn a commission. Our editorial content is not influenced by commissions. Read full disclosure.

When growing tomatoes, two of the most common threats to your plants are mildew and mildew. These terms seem vague, don’t they? After all, basically, a plague is simply something that spoils something.

Alternaria damages your tomatoes early in the season, while late blight damages them later in the season. Simple!

Of course, it is much more complex than it seems. In tomato plants, mildew and mildew are fungal diseases that affect tomatoes in humid conditions. Both blights can be devastating to your tomato crop, but late blight is far less destructive and easier to manage than late blight.

Let’s take a look at these two plagues to protect your tomatoes from these two deadly pathogens.


What are the differences between downy mildew and downy mildew?

One comes early and the other comes late, of course, but what are the real differences between these two tomato diseases? Tomato early and late blight are actually caused by different fungi and affect plants at different times in the growth cycle.

They cause similar symptoms, but since the fruit is not fully formed when Alternaria attacks, you notice it on the leaves first.

Let’s dive into the specifics of each.

Alternaria blight

Early blight of tomato is caused by the fungus can be caused by two very similar fungi: alternarian tomaphilia and A.solani. It is one of the most common tomato diseases. In fact, it’s rare that a tomato season goes by without seeing some signs of Alternaria.

Both A. tomaphilia and A.solani may affect other members of the nightshade family as well as tomatoes. This means that your potatoes, peppers and eggplants are at risk. If you have planted the nightshade vegetables nearby, Alternaria leaf blight can spread quickly from plant to plant.

The right environment for Alternaria

Both plagues affect tomatoes in wet weather. But early blight thrives in warm, humid temperatures. When the weather is 80 F or higher, humid, rainy and humid, mildew comes out to play.

This is especially true early in the growing season. Young tomato plants are less hardy and use their energy to grow tall and strong.

In most summer seasons, a zone of hot and humid weather arrives. Sometimes it’s very hot and very humid, but other times it’s just moderately warm with some humidity. As long as the relative humidity is above 31%, Alternaria can begin to affect your plants.

Alternaria signs

The first signs of alternaria appear at the base of the plant. Take a close look at the bottom stem of your tomatoes. In burnt tomato plants, you will see rounded brown spots on the stem and lower leaves. These brown spots will gradually develop into lesions. Eventually, the lesions will develop yellowish edges and darker inner rings.

If environmental conditions don’t change or you don’t work to stop the spread of Alternaria, the lesions will eventually spread onto the leaf and rot it completely. Or it can spread through the stem, killing the tomato plant.

When fruits are affected by early blight, the rot usually begins at the base of the stem. Then it spreads on the fruit itself. The spots that form on the tomatoes themselves are often dark brown or black and sunken, like pustule-like marks on the top of the fruit.

Early blight tends to be more destructive to the leaves of your tomato plants. A bad case of early blight can result in leafless tomato plants and sunburned fruit.

Although unlikely to kill your plants, Alternaria 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 crop, as the plants focus on repairing the damage instead of producing fruit.


Late blight is the boogeyman of tomato growers around the world. This fungus has a “take no prisoners” approach to nightshade plants. In fact, downy mildew, caused by oomycete (also known as water mold) Phytophthora infestans, caused the infamous potato famine of the 1850s in Ireland.

When this plague attacks, it strikes hard and fast. Under the right conditions, late blight can wipe out your entire tomato crop in a matter of days. It is an overwhelming disease that can be absolutely devastating when it occurs.

The right environment for late blight

Fortunately, mildew is much less common than its early counterpoint. Unlike alternaria, the environment of downy mildew is very specific. Downy mildew thrives between 60 and 80°F, with spores proliferating best at around 70°F.

Like alternaria, downy mildew needs moisture to proliferate. But, while mildew can spread when humidity levels are between 31% and 91%, mildew needs full saturation. Mildew wants rainy, water-soaked leaves, moist air, and water droplets everywhere.

A dew-soaked plant on a wet morning works well, but heavy summer rains are ideal.

When 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 that once you see the first signs of late blight, it’s already too late to store your tomatoes. This disease is fast and deadly. You will first notice some water-soaked spots on the lower leaves. They will look pale green, almost like the diffused light from a drop of water. These spots start 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 those spots on your plants, the leaf will shrivel, brown and die.

The stems will also bear lesions which will weaken the stem. In fact, many people consider brown stem lesions to be the primary sign of late blight.

These brown spots spread quickly and often develop white fungal growth as they spread. The lesions spread and quickly cause the entire plant to collapse as they cause the entire plant to rot into a mushy mess of fungi and decaying plant material.

When late blight affects tomato fruit, spots form under the skin. These reddish-brown spots quickly become leathery, hard, and wrinkled. Then they soften as the rot takes hold in the plant.

Prevent burns

There are several ways to reduce fungal diseases and water mold in general. If you keep your garden free of debris and allow enough space 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 sprinkler systems. Water your plants at the base to avoid over saturating the leaves and stems. In even moderately humid weather, wet leaves can quickly evaporate or wilt. They create the perfect environment for the growth of pathogens.

Crop rotation is another fantastic method to reduce the threat of blight. Do not grow nightshade crops in the same bed year after year. Put some space between our nightshades and rotate their locations every year to reduce the risk of burning.

Treat the burn

Alternaria has a few useful treatment options. Since alternaria always affects the leaves first, it is 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 the withered leaves.

You can use copper-based fungicides to reduce the spread of early blight. Regular, weekly or bi-weekly treatments will keep the disease at bay. Use this fungicide to contain early blight while symptoms are present.

Unfortunately, there is no effective treatment for mildew. Remember that this disease progresses so quickly that once it has started, you cannot stop it.

Because it’s not a fungal disease, you need to take a slightly different approach to trying to control it long enough to reap your rewards.

As soon as you notice late blight developing, or even see the right conditions for it to take hold, treat your tomatoes with copper-based fungicides or a product containing the beneficial bacteria. Bacillus subtilis.

A strong pre-treatment can prevent late blight from taking hold on your tomatoes.

Varieties of blight resistant tomatoes

One of the best ways to reduce the devastating impact of blight on your garden is to plant blight resistant cultivars. Because blight is the bane of tomato gardening, growers have been developing resistant plants for generations.

Of course, these resistant cultivars are not infallible. Under the ideal conditions for late blight, it is possible to have a hard-hitting case of mildew or mildew. But resistant varieties are less likely to succumb to the fungi that cause alternaria and late blight.

For a low-risk tomato garden, plant ‘Mountain Magic’, ‘Jasper’ and ‘Defiant PhR.’ These cultivars are resistant to both mildew and downy mildew. If you are only concerned about Alternaria, try planting ‘Mountain Supreme’ or ‘Juliet’. If you are working to keep mildew out of the garden, plant ‘Iron Lady’ or ‘Matt’s Wild Cherry’.

These cultivars are proven, hardy cultivars that can withstand a lot of moisture without becoming hosts for pathogens that cause alternaria or late blight.

Some areas only experience one type of burn or the other, so check with your local extension office to be sure what to look for.

Was this article helpful?

Yes No ×

We appreciate your helpful feedback!

Your response will be used to improve our content. The more feedback you give us, the better our pages will be.

Follow us on social networks:

Facebook pinterest

Leave a Comment