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What Causes White Spots on Tomato Leaves?

powdery mildew on tomato leaves

Tomatoes are grown in gardens and greenhouses all over the world. Somehow homegrown tomatoes always taste better than storebought. Since you’re reading this article, you are probably wondering: what are these white spots on my tomato leaves? Should I be worried? What should I do?

There are several reasons, why the leaves of tomato plants turn white or have white spots. In this article, you will find out what causes these white spots and what actions to take to grow healthy, delicious tomatoes with great yield.

#1 Reason – Powdery Mildew

Powdery mildew is a fungal disease that can affect many plants, including tomatoes. It is easily identifiable and usually is not a very serious disease as it can be treated with various methods. If left untreated, powdery mildew can reduce the taste of tomatoes and can reduce crop yield.

Powdery mildew thrives in moderate temperatures and high-humidity environments. Growing tomatoes in glasshouses are very common, but for powdery mildew, it is an ideal place to thrive.

How to Identify?

Infected plants have distinct white powdery spots on leaves and stems.

white spots on tomato leaves
Powdery mildew on tomato leaves
Severe powdery mildew infestation on tomato plant
Severe powdery mildew infestation on tomato plant

What Causes Powdery Mildew Infection?

Powdery mildew usually affects tomato plants, when they are overwatered or overfertilized, the humidity level is very high combined with poor air circulation. Sometimes too little sunlight can also be a factor.

How to Get Rid of Powdery Mildew?

There are three main ways to get rid of powdery mildew, removing infected parts of the plant, chemical control and non-conventional chemical control.

#1 Pruning

Pruning is an effective way to stop the growth of a fungal disease and also promote new healthy growth. Do not prune more than one third of the plants leaves or stems, even with a serious case of powdery mildew, it will be the end of your plant.

Use sharp, disinfected tools to remove the most infected leaves, and avoid touching healthier parts of the plant with your hands while doing this.

#2 Non-conventional Chemical Control

Neem oil, milk spray, natural sulfur and metal salts are used to treat powdery mildew. We suggest neem oil and milk, because they are easily found in stores and there aren’t any additional problems when using these non-conventional chemicals.

Apply neem oil directly to the powdery white spots on tomato plants. Tomato plants should heal in 6-7 days. Neem oil will also get rid of many annoying pests such as aphids, beetles and mealybugs.

If you do not have neem oil, you can try to make a diluted milk spray. Make a mixture of milk and water (1:10) and spray it on powdery mildew. It will not heal as fast as neem oil, but as you keep spraying diluted on tomato plants every week, you’ll get rid of the powdery mildew infection.

#3 Conventional Chemical Control

It is possible to control powdery mildew with fungicides that contain triadimefon and propiconazole. Apply them to your tomato plants when you first see white spots appear on the leaves or stems. Although I would consider using non-conventional chemicals first.

#2 Reason – Late Blight

Late blight is another fungal infection, which is much more severe than powdery mildew. It usually comes around late in the season, hence the name late blight. It can kill all of the tomato plants in a matter of weeks.

How to Identify?

White spots on tomato leaves can be Late blight. Additionally to white spots, late blight will turn leaves and stems brown and the fruit will start rotting.

brown stem caused by late blight
Brown stem caused by late blight
Small white spots and browning of the leaves caused by late blight
Small white spots and browning of the leaves caused by late blight

What Causes Late Blight?

Late blight is caused by phytophthora infestans, which is a fungus-like microorganism. Keeping tomatoes in dry conditions and providing a lot of sunlight will prevent this incredibly severe tomato disease, unfortunately, this is easier said than done during fall, when it rains a lot.

How to Get Rid of Late Blight?

Late blight can be cured with the heavy use of chemicals, which aren’t suitable for home gardens. If you do not want to deal with this disease, some cultivars of tomato are exceptionally late blight resistant such as Mountain Magic, Jasper, and Matt’s Wild Cherry.

Once your tomatoes do suffer late blight, it is advised not to plant tomatoes in that spot for 3 or 4 years. Dispose of the disease-ridden plants into garbage, not compost. When you do notice late blight very early, you might be able to save your plants by quickly pruning away leaves that are infected and providing dry conditions for the tomato plants.

#Reason 3 – Stinkbugs

Stinkbugs are known as common pests of tomato plants, they adore ripe tomato fruits. Stinkbugs also leave small white spots on the plants. If you do not know whether there are stinkbugs on tomato plants or not, search for

stinkbug on a tomato
Stinkbug on a tomato Source: Thomas

How to Identify?

Stinkbugs also leave small white spots on the plants. If you do not know whether there are stinkbugs on tomato plants or not, search for punctures on ripe tomatoes. If there are holes in ripe fruit, you are definitely dealing with stinkbugs.

How to Get Rid of Stinkbugs?

Luckily even milder insecticides will get rid of stinkbugs, it’s best to prune away hurt leaves to encourage fresh healthy growth. Apply neem oil on the leaves to get rid of the annoying stinky pests.

#Reason 4 – Lack of Nutrients

Lack of essential nutrients can cause white spots on tomato leaves, although this is fairly uncommon, compared to other causes. To be sure, feed tomato plants with quality fertilizer, which has magnesium and calcium in it. If the white spots disappear, white spots were due to lack of nutrients.

#Reason 5 – Sunburn

Intense summer sun in hot regions can cause white spots on tomato leaves, especially when the plant is exposed to it suddenly. Tomato plants need to acclimatize to sunlight, especially when grown indoors first and then brought outdoors.

How to Identify?

White spots or blotches will appear on the leaves. The leaves, which have a bit of shade will appear completly unaffected.

How to Get Rid of Sunburn?

Keep the soil of tomato plants moist, while providing the tomato plants some shade. There isn’t a good way to get rid of sunburn or sun scalding, the easiest way to deal with this problem is prevention. Let your plants acclimatize to the sun step by step and provide them with some shade during the hotter summer months.

Tips to Growing a Healthy Tomato Plant

Here’s a list of tips to keep tomato plants growing healthily without major problems:

  1. Provide enough space for growth, plant tomato plants 1.5 to 2 feet (45 to 60 cm) apart. This will allow the air to flow between the plants
  2. Increase air circulation, important for glasshouses. Open both doors of the glasshouse or make small holes or openings to the glasshouse, which you can open when humidity levels get above 70 %rh.
  3. Provide an appropriate amount of sunlight, at least 6 to 8 hours of sunlight per day. Provide some shade during periods of very intense sun.
  4. Prune tomato plants regularly, it will induce new, healthy growth and prevent diseases.

Final Thoughts

White spots on tomato leaves can be caused by diseases, pests, too much sunlight, or lack of nutrients. Keep your tomato plants happy by giving them the appropriate amount of sunlight, do not let the humidity level rise too high, and check regularly for stinkbugs!

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