Did you seemingly overnight find white spots all over your tomato plants? Don’t worry. You are not alone.
Here in this article, I will tell teach you how to fight back and how you can prevent, or at least minimize, problems in the future.
Quick answer: A fungal disease called powdery mildew is often the culprit when you see white spots forming on tomato leaves. Powdery mildew thrives in humid growing conditions with low levels of sunlight and poor air circulation. There are many things you can do to prevent powdery mildew infections. You can treat powdery mildew with Neem oil spray solutions and commercial fungicide sprays marked as safe for use with edible plants.
What does “white spots on tomatoes’ leaves” actually mean?
Healthy tomato plants have vibrant leaves with a distinctive smell. And you should always pay attention when your tomato plant’s leaves start to develop spots or discolorations. These are warning signs that something is wrong.
But there is no need to panic. It is all about doing a few critical things right.
Powdery mildew and white spots on tomato leaves
I never use commercial sprays to treat my herbs, vegetables, peppers, and tomato plants.
This may also be one reason why I have battled against many different pests, bugs, and unwanted insects over the years. Notable adversaries include slugs, fungus gnats, whiteflies, leaf miners, aphids, and the fungal disease powdery mildew.
How to prevent powdery mildew
Powdery mildew will spread from spores that blow in from elsewhere. And sometimes there is nothing you can do if the conditions are damp and humid.
Still, for me, prevention and early detection are keys in the fight against pests and plant diseases. And it all starts with trying to create an ideal growing environment for your tomatoes.
I know that powdery mildew thrives in a dense, shady, and humid growing environment.
I choose a growing spot with good access to sunlight and give the tomato plants space to allow for good air circulation. And if the tomato plants grow too dense, I prune suckers to maintain good airflow.
It may strike you as too simplistic. But I can tell you from experience that good planning goes a long way to preventing powdery mildew and other pests and diseases.
How do I treat powdery mildew?
Still, even with all this thought and planning, some of my beefsteak tomatoes did suffer from powdery mildew this growing season. I will tell you what went wrong later on in this article.
I treat powdery mildew by pruning infected tomato leaves where possible. If pruning is not an option, I use a Neem oil and water solution to spray infected leaves.
Spraying tomato plants is a tedious but necessary process and you should be prepared to treat your plants at least twice with 3-5 days in between treatments.
Read more about how to mix and apply Neem oil to tomato plants in the article How to use Neem oil on tomatoes (and other plants)
Can I compost plants infected by powdery mildew?
Removing or pruning leaves infected with powdery mildew can be very effective, especially when you catch the disease early.
It is safe to compost the infected leaves as the as powdery mildew will only survive on a living host. Also, if you have a hot compost, the heat generated from the composting process will kill most pathogens known to affect tomato plants. 
How I got powdery mildew on some of my beefsteak tomato plants this year
I planted 9 beefsteak tomato plants outdoors in one of my garden beds.
As it turned out, I moved the plants outdoors too soon as we were hit by strong winds and temperatures that may have reached a minimum below 10 degrees Celsius (50 degrees Fahrenheit).
As a result, the plants showed signs of stress, and the leaves turned white, brown, and crispy.
The plants did survive but I did not prune as I usually do. I welcomed all new green leaves as a sign of life and left the plants to recuperate.
As I am sure you realize by now, the plants grew too dense and were infected by powdery mildew in late summer.
Conclusion: Not even the best laid out intentions can protect you against pests and fungal diseases such as powdery mildew.
Other reasons tomato plants get white spots on leaves
Direct sunlight can cause sun scalding
Tomato plants thrive in full sun but need protection against direct sunlight during the hottest times of the day.
Direct sunlight can cause sun scalding where leaves go white and burn with telltale brown edges and leaves crumbling to the touch.
Excessive wind cause leaves to go white
Strong wind causes leaves to turn white and wither. I experienced this firsthand this year when I moved tomato plants outdoors too early.
Tomato plants need nutrients to grow. Feed your plants regularly during the growing season to avoid leaves losing their strength and vigor.
Be careful to feed your tomato plants nitrogen-rich nutrients late in the growing season. Nitrogen will stimulate the growth of new leaves when you want the plant to focus its energy on producing fruits.
Fresh new leaves will also lead to dense plants, increasing the risk of fungal diseases like powdery mildew.
Will a powdery mildew infection kill my tomato plant?
Powdery mildew is a nuisance and will overtake your plant if left untreated. But it will not kill your plant; the fungi will feed on your tomato plants’ cells and give you fewer and smaller tomatoes to harvest.
However, as long as you take action to treat your tomato plants, you will find that your tomato plants will bounce back from powdery mildew with little to no adverse effects.