The white spots that appear on rosemary are typically caused by powdery mildew. Small, dust-like spots appear on the rosemary and spread until the entire plant is covered in a powdery gray or white fungus.
Use an organic fungicide, such as neem oil, to rid your rosemary of the white spots caused by powdery mildew. You can also use a solution of baking soda or compost tea. Fill a spray bottle with your anti-fungal solution and generously spray the rosemary.
I will explain it in a way that is easy to understand by using my work and hobby experience of touching plants.
-  Rosemary Leaves With White Powder (Spots) May Have Powdery Mildew
-  Pest Infestation Causing White Spots
-  Sunburn
-  Nutrient Deficiency
-  Cold Water
 Rosemary Leaves With White Powder (Spots) May Have Powdery Mildew
If you find what looks like white powder on your rosemary all of a sudden, you might have powdery mildew.
Powdery mildew is a disease that spreads quickly once it appears. Therefore, it is critical to treat powdery mildew as soon as possible.
In the earliest stages of infection, the fungus causes tiny white circular spots on the broadest leaf foliage, much like a light chalky dusting.
The white powder is millions of microscopic fungal spores carried from plant to plant on the wind.
The fungus starts as a white spot but quickly spreads throughout the plant and, in the worst cases, covers the whole plant.
Powdery mildew is caused not by a single pathogen but by many; these pathogens can differ from one plant to the next.
The death of rosemary infected with powdery mildew will not occur suddenly.
But if the disease isn’t treated, photosynthesis will stop, the plant’s defenses will get weaker, and the plant’s growth will slowly get worse.
What Conditions Cause Powdery Mildew?
Let’s go over what makes powdery mildew a real possibility.
- Temperature: Temperatures between 15 and 30 degrees Celsius (59 and 86 degrees Fahrenheit) are optimal for fungal growth.
- Humidity: The fungus tends to multiply more rapidly in high humidity conditions.
- Lack of Light–When rosemary plants are grown indoors in low light or shade, they are more likely to get powdery mildew. The problem usually goes away when you move the plant to a sunnier or brighter spot.
- Stuffy conditions – Fungus spores can spread quickly in places that don’t get enough air or are too crowded.
Powdery mildew typically occurs between April and October, when temperatures are suitable for fungal growth.
Additionally, the disease necessitates additional care during the rainy season and immediately following precipitation when humidity levels are at their highest.
How Does Powdery Mildew Spread?
The pathogen of powdery mildew is a type of fungus called a filamentous fungus, a member of the fungi family.
The pathogen of powdery mildew multiplies by releasing spores. Spores are carried by the wind but can also be transmitted by water.
In other words, water applied to a leaf with powdery mildew can spread the disease to other leaves. Therefore, avoid washing away powdery mildew with water just because you find it.
How To Treat Rosemary With Powdery Mildew
– Immediate Steps
In a rat race, the more pathogens there are, the more spores will spread.
To stop the disease from spreading further, it is crucial to first cut down on the number of pathogens!
Remove powdery mildew-covered rosemary leaves to cut down on the number of pathogens.
You can use scissors or your bare hands to remove the leaves and branches, but be sure to disinfect both frequently with alcohol to stop the spread of the disease.
Take your rosemary out of a humid place like the kitchen or bathroom. It likes to be in an airier, drier place with a lot of ventilation.
– Spray Your Rosemary with a Fungicide to Treat Powdery Mildew
An organic fungicide is a great way to eliminate the fungus on rosemary plants. I prefer copper-based fungicide spray (Amazon link) for moderate to severe powdery mildew infections.
- Follow the manufacturer’s instructions when putting the fungicide into a spray bottle.
- Spray the affected plants thoroughly, coating the undersides and tops of the leaves. Apply until your rosemary is glistening with the neem oil solution.
- Reapply the neem oil solution every 1-2 weeks until the powdery mildew symptoms disappear. Do not spray your rosemary if it is water-stressed or if the temperature rises above 90°F (32°C).
– Use Baking Soda
Another homemade remedy for powdery mildew on rosemary plants is baking soda. It can also help your rosemary resist fungal pathogens.
It alters the pH of the leaf surfaces, making them hostile to the fungi that cause powdery mildew.
- Mix 1 gallon of distilled water with 1.5 tablespoons of baking soda.
- Add three tablespoons of mild soapy detergent or light horticultural oil. Pour the solution into a spray bottle after thoroughly mixing it.
- Spray the undersides and tops of your rosemary plant’s foliage, branches, and other affected parts.
- Apply it once a week until the symptoms disappear.
– Use Compost Tea Solution
A compost tea solution has anti-fungal properties that can come in handy in preventing and controlling powdery mildew pathogens. You can purchase a ready-to-use compost tea (Amazon link).
Or, preferably, you can prepare your own:
- Mix water and aged compost (preferably with some manure) at a 5 or 6 to 1-ratio.
- Allow 1-2 weeks for the solution to sit before straining it.
- Gradually add water until the solution resembles brewed tea in color.
- Spray your plant with the compost tea solution every two weeks.
- Continue spraying throughout the growing season for the best results.
How to Prevent Powdery Mildew in Rosemary Plants
Prevention is the most effective way to protect your rosemary from powdery mildew. One of the best ways to do so is by providing your rosemary with excellent care & growing conditions.
- Improve air circulation – Stagnant air gives powdery mildew fungus the ideal conditions for development. Consider using a small fan to increase ventilation around your indoor rosemary plants.
- Prune overgrown rosemary plants to boost airflow between the foliage.
- Check your rosemary regularly for signs of fungal problems and act immediately. Apply baking soda or compost tea solution regularly during the growing season.
- Rosemary plants thrive in warm spots with more than 6 hours of sunlight daily. If you’re growing yours indoors, place the container
- on a windowsill with temperatures between 60-70°F (15-21°C) and bright, indirect sunlight.
- Don’t overcrowd your plants. Ensure a 2- to 3-ft. space between them to facilitate airflow.
- Avoid excessive use of nitrogen-based fertilizer.
 Pest Infestation Causing White Spots
Pest infestations can also cause white spots. However, they are usually caused by leaf damage or chlorosis, which occurs when the foliage cannot produce chlorophyll efficiently (green pigment).
Spider mites, aphids, mealybugs, and whiteflies are the most common pests.
- Spider mites appear as white spots on the undersides of leaves at first. You’ll notice some white cottony webbing on the branches and foliage of your rosemary plant in the advanced stages.
- Mealybugs are small, flat white bugs. These bugs will be visible as they try to burrow into your rosemary’s stem, branches, or leaves.
- Whiteflies are winged insects that have a waxy, white powdery coating. They congregate on the undersides of the leaves, forming a mass of waxy whitish-silver debris and sticky honeydew.
- Aphids are minor sap-sucking bugs with soft bodies. They can be white, black, green, or even red. If you notice a string of small white dots or spots on the tips of the rosemary leaves, they are most likely aphids.
How to Get Rid of Rosemary Pests
- When you notice pests, spray your rosemary with insecticidal soap or neem oil.
- Remove the problems and their eggs using a cotton swab soaked in rubbing alcohol.
Rosemary is a plant that thrives in the sun. Too much direct sunlight, on the other hand, can scald the foliage. Sunburn causes the leaves to appear bleached, whitened, or to develop white spots.
You will most likely notice white spots on the sun-facing leaf surfaces if you have sunburn. The affected leaves will turn yellow or white before turning brown, dying, and dropping off.
How to Fix
Sunburn is not harmful but can make your plant appear unhealthy and unsightly.
Move your potted rosemary to a location where it will receive 6+ hours of bright, indirect light.
 Nutrient Deficiency
When your rosemary is deficient in essential nutrients such as nitrogen, potassium, or phosphorus, it may develop white spots. Spots most commonly appear on leaf tips, margins, and new growth.
Affected leaves are small and yellow or pale green. The affected leaves will turn brown and die if the nutrient deficiency is not treated.
How to Fix
- Use a nitrogen-rich liquid fertilizer to feed your rosemary. I recommend halving the strength listed on the label.
- Compost can also be added to the growing medium.
 Cold Water
Watering your rosemary with cold water may result in white spots as well. This is because the cold shock causes the palisade cells on the leaves to break down, leaving behind white callus-like spots.
Using tap city water can also cause mineral deposits to form on your plant, resulting in white spots.
How to Fix
For your rosemary, use room temperature or lukewarm water. I prefer distilled rainwater or filtered water.