Weather Fleck

Do you see flecks of bleached or straw-colored areas on the leaves of your strawberries, summer squashes, or corn? Or flecks and bands of red, orange, yellow, or brown on conifer needles? Those discolorations are caused by ozone, and the damage is called weather fleck.

Tobacco leaf with weather fleck (R.J. Reynolds Tobacco Company, Bugwood.org) CC BY 3.0

Ozone injury to pine needles (USDA Forest Service) CC BY 3.0

Technically, it is weather fleck only when it affects tobacco plants, but for the sake of this discussion, we’ll use the term more broadly.


So what does ozone do?


Ozone

Ozone (O3) creates a barrier around the Earth, up in our stratosphere, protecting us from the sun’s harmful ultraviolet radiation. As a type of air pollution, it can also cause cardiovascular, central nervous system, respiratory, and reproductive problems for us. 


Excessive ozone can also reduce chlorophyll, carotenoid, and carbohydrate levels in plants while increasing ethylene gas levels. Ethylene is responsible for ripening, but excessive levels are considered air pollution. 


Ozone triggers plants to protect themselves, reducing fruit yield and quality. In 2022, a study found that East Asia lost $63 billion in crops due to ozone pollution in one year. Ozone forms as sunlight reacts with car and factory fumes, so there’s plenty of it. It is considered the most destructive air pollutant for plants in the United States. Nearly all plants in urban areas are affected to one degree or another. Ozone is a reactive form of oxygen that can cause many symptoms in broad-leaved plants.

Mature leaves of tobacco plant exhibiting weather fleck (R.J. Reynolds Tobacco Company, Bugwood.org) CC BY 3.0

Symptoms of weather fleck

Bleaching, bronzing, chlorosis, mottling, stippling, tissue death, and weather fleck are common symptoms of ozone damage. Weather fleck starts as small, dark green water-soaked areas. Within hours, those lesions turn brown, then tan or white. At higher ozone concentrations, this process can occur in just two or three hours.


You might mistake weather fleck for spider mite feeding, except that symptoms are usually seen on the upper surface of mature leaves first, followed by younger and older leaves. Spider mites almost always start on the underside of leaves, trying to stay hidden.

Spider mites on horseradish leaf

As ozone exposure continues, damage spreads to both sides of the leaves. Mature plants are more resistant to ozone than younger plants, but those damaged areas are dead tissue. Weather fleck doesn’t look very impressive, but those tiny damaged areas can merge and form bigger problems.


Bigger problems

Any time tissue is damaged, it becomes less able to protect itself. Plants exhibiting weather fleck are especially susceptible to Botrytis fungi. Botrytis is responsible for grey mold. These plants are also experiencing internal stresses that cause cell seepage, chemical imbalances, and other problems.

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Susceptible plants

The list of plants highly susceptible to ozone damage is rather long:

• ​​​alfalfa
• apricot
• avocado
• barley
• bean
• beet
• broccoli
• Brussels sprout
• carrot
• celery
• chicory
• Chinese cabbage
• citrus
• corn
• dill
• eggplant
• endive
• grape
• mint
• muskmelon
• onion
• oat
• parsley
• parsnip
• pea
• peach
• potato
• pumpkin
• radish
• rye
• scallion
• spinach
• squash
• strawberry
• sugar maple
• sweet potato
• Swiss chard
• tomato
• turnip
• wheat​

Ozone damage to sugar maple leaf (USDA Forest Service) CC BY 3.0

Severe ozone damage to potato leaf (Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org) CC BY 3.0

​Many garden ornamentals, such as dahlia, fuchsia, lilac, marigold, and salvia are also very sensitive to ozone levels.


​If you live in an area with lots of ozone, you may want to focus your gardening efforts on somewhat resistant cucumbers and peppers. What else can you do?


Aside from working from home and walking to the store instead of driving, you can protect your plants by not giving them more fertilizer than necessary. Plants are most susceptible to ozone damage during periods of rapid growth.


Did you know that ozone smells a little like chlorine? I didn’t, either.