White Spots On Tomato Leaves? (Possible Causes and How To Treat It)

Ryan Womeldorf
by Ryan Womeldorf

Growing vegetables in a small garden can be beneficial. In addition to adding color to any garden, it can mean fresh vegetables to be used in the kitchen. It should not be surprising that one of the most popular garden vegetables are tomatoes.

From time to time, you may notice white spots on the leaves of your tomatoes. More often than not, the white spots are the result of powdery mildew. Though it could also be due to late blight and sun scalding, powdery mildew is the most likely suspect.

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How to Spot Powdery Mildew and Why it is Bad

There are a couple of ways in which to spot powdery mildew on your tomato plants. The first type of powdery mildew is known as Oidium lycopersicum and they are white spots that appear on the upper portion of the leaves. You may also notice that powdery mildew on the stems as well.

Why powdery mildew is bad. If you think that the powdery mildew spots are just a minor aesthetic inconvenience, think again. In the most severe of cases, powdery mildew can potentially lead to withering of foliage and even massive leaf loss.

Powdery mildew tends to be most common when buying greenhouse-raised seedlings. If you grow one on your own. The humid, warm conditions can potentially cause that fungal disease to grow, develop, and then spread.

What Causes Powdery Mildew

Powdery mildew is actually a fungal disease. Moreover, it is caused by a few different types of fungi. In general, fungi do well in darker, moist, stagnant areas. When humidity starts to climb, those areas become a harvesting ground for mildew.

If your tomato plants aren’t getting the proper amount of air circulation or sunlight, that is a telltale sign. In those conditions, your tomato plants provide the ideal space for that fungus to not only grow but reproduce. Thus, the white spots that you see on the leaves and even the stem.

Younger plants. Generally speaking, powdery mildew tends to be most prevalent among younger plant leaves. The main reason that young leaves are the prime target is due to potential over-fertilization. When your tomato plants get too many nutrients and too much fertilizer, it accelerates growth.

With a greater growth rate, there are a lot more new leaves for the powdery mildew to potentially form on. Sure, fertilization can lead to a higher yield. Over fertilizing, though, can potentially lead to a buildup of powdery mildew.

How to Treat Leaves with Powdery Mildew

If you do spot powdery mildew on your tomato leaves, do not fear. There are a few remedies available that are quick, simple, and can restore your tomato plant back to its previous glory. Spotting the issue early is the key; when the mildew takes hold, it can be more difficult to fully remove.

Try Pruning

The very first thing that you should do in the event of spotting powdery mildew is to break out your pruning shears. Pruning helps prevent the spread of powdery mildew into the other areas of the plant (like the stem). Furthermore, it actually improves the circulation throughout the plant, further preventing powdery mildew. That’s a win-win.

1.) Cut carefully. When pruning, it is important that you exercise caution. Don’t just start lopping off leaves because you can definitely over prune. Try to cut away the leaves that are clearly impacted. Light pruning may be enough to get the job done.

2.) Always clean your shears. Whenever you are done pruning away powdery mildew, make sure to sanitize the blades. If you do not sanitize, there is a chance that you can spread the fungi to other areas of the plant or other plants entirely. A quick cleaning can save you a lot of trouble in the end.

3.) For heavily impacted areas. If you notice that your tomato leaves are nearly entirely covered with powdery mildew, put the focus on those that are the most severe. Remember, over-pruning is bad for the plant. Not only can it damage your tomato plant, but it could potentially kill it. In severe cases, it helps to start a fungicide treatment as well.

Other Methods of Treating Powdery Mildew

Of course, pruning is just one method for treating and eliminating powdery mildew. It tends to be the quickest, easiest, and cheapest method, but sometimes drastic times call for drastic measures (of sorts).

1.) Neem oil. Neem oil is actually an organic fungicide that can be used in the treatment of powdery mildew. This all-natural oil also acts as an organic insecticide. It can repel many different insect types like mealybugs, aphids, as well as whiteflies. Those are all harmful to tomato plants, so you can also reap the benefit of preventative pesticide treatment. All you need to do is spray it on the leaves about once a week until you notice the symptoms start to diminish.

2.) Milk Spray. Neem oil may not be readily available. In fact, it is entirely possible that you may not be able to find it at a local store, facilitating the need to order online. So, in a pinch, milk spray is another effective method for treating tomato leaves.

Simply mix 3 parts water with 2 parts milk to create the concoction. Mix it into a spray bottle and spray over the infected leaves thoroughly. Keep doing this on a weekly basis until you see that the powdery mildew has begun to dissipate.

How to Prevent Powdery Mildew

Now that you know what to look for and how to treat powdery mildew, why not prevent it from showing up in the first place? When caught early, powdery mildew is pretty easy to treat. Not having to deal with it is even better.

1.) Pruning and spacing. One of the leading causes of powdery mildew is poor air circulation. Making sure that you space out your tomato plants is essential for the prevention of powdery mildew. Make sure that your plants are spaced between 18 and 24 inches from one another. Overcrowding keeps air from properly circulating around the plants, giving powdery mildew the ideal conditions needed o thrive.

Prune back the branches and leaves whenever necessary. It can be a minor pruning, but it will go a long way towards keeping your tomato leaves happy and healthy. Besides, it is easier to do a little pruning here and there than having to perform a mass repair.

2.) Lighting. When your tomato plant fails to get sufficient light, it is ideal for powdery mildew. Remember that they thrive in moist, dark areas. So, make sure that your tomato plants get the proper amount of sunlight.

The average tomato plant needs anywhere from 6 to 8 hours of full exposure to sunlight to thrive. They are known as sun-loving plants for a reason. If they are not in sunlit areas, make sure to move them to a more exposed space.

3.) Go with a resistant variety. If you have had experience with powdery mildew, there is good news. There are resistant varieties of tomatoes that can stand up better than others. If your garden has a history of mildew issues, try one of these resistant varieties.

Sun Scalding Could Also be an Issue

While the most likely suspect is powdery mildew, it is not the only reason why your tomato leaves may be white. Sometimes tomato plants can develop white spots due to a condition known as sun scalding.

Sunlight is good, but too much direct sunlight can be hazardous. The leaves that get exposed to too much direct heat and sunlight can develop those white spots. These are more common in plants that are used to being in more shaded areas.

Withering and dying. If left exposed, the white spots may be the tip of the iceberg. Leaving them in the sun can cause withering of the leaves and they may even fall off entirely. Furthermore, it can inhibit the growth of your tomatoes. Ensure that your tomato plants get plenty of sunlight but not too much.

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Late Blight is the Last Reason for White Spots

Finally, another possible reason for the white spots that you see on your tomato leaves is known as late blight. Late blight is serious. It is a severe disease that can potentially devastate a crop of tomato plants.

Deadly impact. Like powdery mildew, late blight thrives in cool, dark, damp conditions. And like powdery mildew, it can cause the branches and leaves of the tomato plant to eventually shrivel up and perish.

If that weren’t enough, the tomatoes themselves will develop large, noticeable brown spots as well as the typical white fungi spots on the leaves.

Pruning. Should you notice the symptoms of late blight, prune the infected areas away immediately. Make sure that you do not compost the infected plant debris since late blight can come back through the soil.

Copper fungicide. After you have finished pruning, use a copper fungicide on a weekly basis. The fungicide will help battle the late blight, returning your tomato plants back to normal provided it hasn’t been caught too late.

Ryan Womeldorf
Ryan Womeldorf

Ryan Womeldorf has more than a decade of experience writing. He loves to blog about construction, plumbing, and other home topics. Ryan also loves hockey and a lifelong Buffalo sports fan.

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