Basil is a favorite crop in my garden- I love adding fresh leaves to salads and all kinds of Italian recipes. So when you see white spots on basil threatening your harvest, it can be worrisome.
The six main reasons for white spots on basil are these:
- Fungal disease
- Pest infestation
- Nutrient deficiency
- Salt buildup
- Normal color variation
In my experience, most causes of white spots are treatable, especially if you identify the problem quickly and take action. In this article, you’ll learn the details of each cause and what to do to get back to a healthy green color.
Let’s dive in!
6 Causes for White Spots on Basil Leaves
Treatment varies based on what exactly is behind these unsightly white spots, so it’s critical to recognize and deal with each of them appropriately.
If you don’t have time to read the post and just need a fast answer to what might be behind the white spots, here’s a quick breakdown:
How to Recognize
How to Treat
Powdery or fuzzy spots on leaves
Remove affected leaves, spray with homemade baking soda solution or commercial fungicide
White trails or scattered spots
Remove affected leaves, spray with garden-safe insect repellent
Large splotches after intense sun exposure
Remove affected leaves, provide shade
Entire leaves turining white/yellow
Apply organic fertilizer
Crystalline white deposits on leaf surface
Use a hard-water filter, avoid over-fertilizing
Normal Leaf Variegation
White margins are naturally-occurring on some varieties
Here are the causes and solutions broken down in more detail:
1. Fungal Diseases
Powdery mildew and downy mildew are the two most common fungal diseases. Both are spread by wind-borne spores, but their appearance is slightly different.
Powdery mildew is white and can affect both the top and bottom of the leaf. It resembles a sprinkling of fine white baby powder– hence the name. It’s a fairly common sight in the home garden, and I’ve had powdery mildew on my squash plants before:
Downy mildew presents first as yellowed leaves, similar to chlorosis or nutrient deficiency. After a while, those spots can start to turn white and a thin coating of black-brown spores develops on the underside of affected leaves. Eventually, the leaves will turn black and die.
Both these fungal diseases thrive in humid climates and perpetually damp areas. I suspect my powdery mildew problems spring up because I had my squash in a fence-corner area, which cut down on normal airflow.
How to Treat Basil Fungal Diseases
Cut away fungus-affected foliage and dispose of it in the garbage. Do not put it in the compost since this could cause the fungal spores to spread.
I’ve tried homemade spray for powdery mildew before with some success. The recipe I followed called for 1 gallon of water, 1 tablespoon of baking soda, 1 tablespoon of vegetable oil (I used grapeseed oil) and 1 teaspoon of dishwashing liquid. Mix it together and add to a spray bottle, then spray your plant down in the early morning hours before the sun can scorch the leaves.
This video from Cali Kim does a great job of showing how to apply a homemade spray and how to prune the diseased leaves:
Another treatment strategy is a non-toxic fungicide, which may produce more reliable results. Just be careful to follow the directions for how long to wait to harvest after application.
Make sure to water your basil carefully– excess moisture on the leaves could open the way for fungus to get a foothold. Always water at the soil level, not spraying the foliage or allowing water to splash the leaves.
Finally, don’t overcrowd your garden. Too many plants crammed in reduces airflow and makes a perfect environment for fungal infections to develop. Consider trimming plants back or even moving some plants out of an overcrowded area.
2. Pest Infestations
Leaf miners and whiteflies are common basil pests that leave white spots.
Leaf miners leave white serpentine trails as they eat through the inner membrane of the leaves.
Whiteflies are small gnat-like flies that prefer tomatoes but will also migrate to nearby basil plants– if you shake the leaves, a cloud of them will fly away.
How to Treat Basil Pest Infestations
For leaf miner problems, you won’t be able to get rid of the actual pest because it’s deeply embedded into the affected leaves. Instead, remove the affected leaves as soon as you notice them.
For whiteflies, use a strong stream of water to dislodge the pests. Repeat every day for a week for best results. For serious infestations, use Dr. Earth Final Stop Insect Killer or neem oil.
While basil typically likes several hours of sun exposure every day, excessive or harsh sunlight can scorch the leaves and leave behind white damaged spots.
Stacie Krljanovic, a Texas-based head groundskeeper and advisor at Patio Productions, shares why our plants can suffer from too much of a good thing. “The sun can cause the plant’s chlorophyll levels to drop, which results in the leaves turning white. This happens because the light affects the chlorophyll production process, causing it to stop functioning properly. It also makes it harder for photosynthesis to occur and thus prevents growth from happening at an optimal rate.”
In extreme cases of sun over-exposure, basil leaves can turn black.
How to Treat Basil Sunscald
Leaves with scalded spots won’t recover their green color, so pick them off and put them in the compost.
Next, give your basil some shade. If your plant is in a pot, move it to a shaded location, like under a covered porch or in an east-facing area that gets afternoon shade. If your basil plants are in the ground, put up a shade structure- shade cloth works wonderfully for this purpose.
In the future, planting tall, sun-loving plants next to your basil can provide natural shade (tomatoes are a perfect option).
4. Nutrient Deficiencies
The most common nutrient deficiency in basil is nitrogen, which gives the large leaves a deep green color. In hydroponics, magnesium deficiency is also common.
Both these can result in whitish spots and yellow leaves that gradually turn brown or black.
How to Treat Basil Nutrient Deficiencies
Use a balanced vegetable fertilizer at the beginning of the season, and liquid fertilizer and/or foliar spray every few weeks through growing and harvest.
For nutrient deficiencies in hydroponic gardens, use a multi-stage fertilizer system that includes micronutrients as well as macronutrients.
5. Salt Build-Up
In areas with hard water, the minerals in the water can build up as a white cast on basil leaves and on the soil surface.
Salt spots often show up as rings, like these:
Similarly, when there’s too much fertilizer in a planting bed, its components will build up on the soil as a hard white crust. Sometimes the plant itself will secrete excess fertilizer salts through the leaves. These salts on the leaves will be hard white dots that you can scrape off with a fingernail.
How to Treat Basil Salt Build-Up
We have extremely hard tap water where I live, so I use an inexpensive water filter pitcher for my indoor plants. For outdoor plants, a hard water filter for your hose is a great idea.
As for fertilizer buildup, this can be a serious problem over time. The best way to get rid of excess fertilizer is to flush the soil with several heavy waterings, and then refrain from fertilizing for several weeks. After your plants have had the chance to recover, fertilize lightly going forward.
6. The White Spots are Normal for the Variety
One basil variety has white markings naturally, and they’re nothing to worry about! Pesto Perpetuo naturally has white leaf borders, and sometimes also features white spots in the leaf center.
Since there’s no actual problem here, go ahead and enjoy your delicious fresh basil as much as you want!
Frequently Asked Questions about White Spots on Basil
I’ve had to deal with unhealthy-looking basil myself, and I know it’s not fun to find it in the garden. I hope this article has shown you that even if your basil starts sprouting white spots, you can still save it and enjoy a delicious harvest.
I’d love to hear from you! Do you have any other questions about white spots on herbs? Or maybe you have an effective strategy that you’ve developed in your own garden. We learn best from one another’s thoughts and experiences, so please share in the comments!