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What do artichokes, barley, beets, patience dock, rhubarb, and strawberries have in common? Besides being delicious, they are all susceptible to a fungal disease called Ramularia leaf spot (RLS). Cotton, daffodils, jonquils, and narcissus are also vulnerable, along with several other wild plants, such as cowslip.  Severe Ramularia infection of strawberry (Jerzy Opioła) CC BY-SA 4.0 Ramularia leaf spot symptoms Like most other leaf spot diseases, this one is characterized by, you guessed it, leaf spots. Reddish-brown spots occur on both upper and lower leaf surfaces. Those lesions often have a yellow halo but can be angular, circular, or rectangular, making this disease difficult to identify. Some even call Ramularia the master of disguise. To get a clear identification, growers refer to the five R's of Ramularia infection:
Brown ulcers may also occur on flower bracts. Eventually, those bracts will curl up and dry out. Growers believe this disease is spreading because it is difficult to identify with certainty. But you can always assume that spots warrant a closer look.  Ramularia on patience dock (Tristram Brelstaff) CC BY-SA 3.0 In the case of Ramularia leaf spot, entire leaves will eventually shrivel up and die, taking millions of white fungal spores with them. Then those spores are splashed onto neighboring plants through rain and overhead watering. Ramularia leaf spot lifecycle Ramularia fungi have a rather complicated life cycle. There are several varieties with slightly different symptoms, but let's keep it simple, shall we? Most importantly, this disease spreads through infected seeds and plant debris and on the wind. Once spores come into contact with a plant, lesions form on lower leaves, spreading to upper leaves, then flowers. Those flowers produce infected seeds that continue the cycle.  Ramularia lesions (James Lindsey) CC BY-SA 3.0 Ramularia leaf spot management
Infected commercial crops are sprayed with fungicides up to eight times a year with limited success. Of course, you must wear protective clothing and limit the time spent around these chemicals to stay safe, so you might want to think twice before applying them to your groceries. Since stressed plants are more likely to become infected, keeping your plants healthy with these tips can prevent many problems.
Figuring out what leaf spots are telling you can be tricky, but it's worth the effort to find out. Do you see spots 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 ozone damage, and it is called weather fleck.
Technically, it is weather fleck only when it affects tobacco plants, but 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 but threatening us with several health problems. It is a reactive form of oxygen that forms as sunlight reacts with car and factory fumes, so it’s abundant. Ozone triggers plants to protect themselves, reducing fruit yield and quality. As a type of air pollution, excessive ozone can 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. Nearly all plants in urban areas are affected to one degree or another. It is considered the most destructive air pollutant for plants in the United States. In 2022, a study found that East Asia lost $63 billion in crops due to ozone pollution in only one year.  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 weather fleck 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 gray mold. These plants are also experiencing internal stresses that cause cell seepage, chemical imbalances, and other problems. Susceptible plants Many garden ornamentals, such as dahlia, fuchsia, lilac, marigold, and salvia are very sensitive to ozone levels. The list of edible plants highly susceptible to ozone damage is rather long.
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. A well-lit garden path can be a pleasant experience on a summer evening, but what do those lights do to your plants? Let’s find out.  Landscape lighting (Asaph441) CC BY-SA 4.0 Natural cycles Light at night is not natural. Nothing, us included, evolved to deal with artificial light at night. And we are all light-sensitive to one degree or another. Chickens produce more eggs in summer when their chicks have the best odds of survival. So chicken farmers put their hens under lights to get as many eggs as possible, as quickly as they can. This arrangement works well for the farmers. But it is hard on the hens. Left to live a natural life, the average hen can live 10 to 20 years. Commercial hens generally live only 3 to 4 years. All of those lights and productive demands are hard on people and plants, as well.
Sources of night light There are many sources of artificial light at night. Street lights, porch lights, decorative path lighting, and security lighting are the most common sources of light at night. Depending on where you live, headlights, factory lights, and strip mall lighting may also be sources of artificial light for your plants. And indoor plants are subject to our overhead lights, reading lamps, and computer and television screens. It’s a wonder anything gets any sleep these days! Natural cycles The natural cycle of light and dark triggers circadian rhythms within us that allow us to sleep, repair ourselves, and recover from the demands of our days. Plants have similar needs. While plants use sunlight to perform photosynthesis during the day, they need darkness to produce an important compound called phytochrome. What is phytochrome? Phytochrome regulates several functions within plants, including abscission, dormancy, photoperiodism, and seed germination. Without healthy seed germination, our gardens might be rather dismal places. And photoperiodism allows plants to anticipate the seasons, triggering them to bloom, grow, and rest at the proper time of year. That is where ‘short day’ and ‘long day’ varieties come in. They use photoperiodism to tell them if the correct balance of daylight and darkness hours is right for them to bloom and grow safely.  Plants need darkness to stay healthy (Pixnio) Public Domain Plants are much more sensitive to the number of daylight hours than you might expect. For some plants, as little as one-minute exposure to a 25-watt bulb is enough to halt blooming. Strawberries are very light-sensitive. If they don’t receive enough darkness, you might not receive any berries.
The number of hours of daylight can also cause bolting, which can shorten the useful life of your lettuces and spinach. If bolting is a problem in your garden, you may want to turn off some of those lights. Abscission and dormancy As the days get shorter and the nights get longer, plants produce hormones used to protect themselves through the colder winter months. Those phytohormones tell plants to pull nutrients out of leaves and let them fall. If any part of that natural cycle is interrupted or interfered with, plants can be unprepared for winter weather, making them more susceptible to pests and diseases. Night lights and insects We all know that moths are often attracted to porch lights, but nighttime lighting attracts several other insects to our landscapes. Research from the University of Exeter has shown that night lights “trigger complex effects on natural food webs [and that they] may have more permanent, widespread impacts on wildlife and ecosystems.” Managing light pollution Now we know: too much light at night can adversely affect our plants. So it’s up to us to reduce light pollution (and air pollution) to improve the health of our plants and the food they produce for us. Try some of these tips to help reduce light pollution in your home and landscape:
With a good night’s rest, you and your plants will both feel better. |
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