Cercospora leaf spot is the name applied to a variety of plant and tree diseases caused by fungi in the genus Cercospora. A diverse and speciose clade, scientists have formerly described more than 80 species, and there are undoubtedly just as many that await proper identification.
The fungi infect a similarly impressive list of plant species, including celeries, lettuces, peanuts, roses and bananas, and a handful infect trees and other woody plants. For example, sycamores, figs and mangos can be infected by these fungi, as can grapes.
Below, we’ll explain the basics of Cercospora leaf spot diseases, including details of the organism’s lifecycle, common symptoms infected plants and trees exhibit, and the basic strategy by which you’ll need to treat them.
Cercospora Leaf Spot: Fungal Details
Like most other fungal pathogens, Cercospora leaf spot fungi usually lie in wait among the vegetative debris found along the ground. They may also persist as small clumps of desiccation-proof cells, called pseudostromata. They can also colonize seeds, which means they spread relatively easily.
Cercospora spores are relatively hardy, but they will only infect plants under a relatively narrow range of environmental conditions. This primarily means warm and wet weather. Temperatures must generally be at least 60 degrees Fahrenheit and the humidity level must be in the 90% range. However, it is important to note that the “micro” humidity surrounding your plants will often be much higher after irrigation or rain that the humidity level discussed by the meteorologist on TV.
You can often predict the extent of Cercospora-caused problems by noting the summer weather. Infections are always most prominent (and difficult to treat) during particularly wet summers. Conversely, the disease is rarely much of a problem during extended droughts.
Cercospora rarely kills plants or trees outright. However, it is still a good idea to control it, as it can cause significant defoliation. This not only creates an eye-sore, it also exposes your plants and trees to secondary pathogens, which are more likely to kill the organisms it infects.
Symptoms of Cercospora Leaf Spot
At the outset, it is important to understand that Cercospora leaf spot can cause different symptoms – and these symptoms can vary in their severity – due to different growing or management conditions. For example, plants grown in a nursery setting may display different characteristic spots than those grown outdoors.
Nevertheless, the primary symptom of the disease is a series of dark-colored spots on the foliage. They are vaguely round in shape, though many become irregular over time. They’ll usually display a dark brownish-red outer circle, with a lighter center. In the very center of the spots, you’ll often be able to see black fungal fruiting bodies (essentially, the equivalent of mushrooms).
The exact appearance of the spots not only varies from one host plant species to the next, but also with cultivar, variety, and growth form. Typically, the oldest leaves become infected first, with the infection traveling up the plant or tree with time, thereby infecting newer growth. Leaves that have been infected for some time may become yellowish green in color.
The characteristic spots often appear in late spring, but the disease will often progress over the course of the summer. In fact, the environmental conditions that occur during the summer can exacerbate or mitigate the progression of the disease – the wetter the summer, the more likely the fungi are to thrive and compromise your plants’ health.
Treatment Strategies for Cercospora Leaf Spot
For the most part, treating Cercospora leaf spot means preventing the fungi from infecting your plants in the first place. There are a few treatments (which we’ll explain in a moment), but prevention is always the best strategy.
Begin by cleaning your gardening or tree-care tools after each use. This will help prevent shears or other tools from spreading spores from infected plants to healthy individuals. It is also imperative that you remove any old infected leaves from the area before installing new plants or trees in subsequent years, as these fungi survive quite well on the ground.
You’ll also want to use a soaker hose or drip-irrigation system as this will help limit the amount of splashing. This is crucial, as Cercospora spores often spread readily via water droplets. You’ll also want to ensure that you apply a thick layer of mulch around your plants and trees, as this will help form a barrier between your crops and the soil.
You should also inspect your plants on a daily basis and remove any infected leaves promptly. This can help limit the spread of the disease and give your plants sufficient time to produce new foliage to replace that which you remove. Just be sure to dispose of the infected leaves properly by either burying them or burning them. Do not simply allow them to fall to the ground below – this is a surefire way to lead to further problems.
It is also wise to select Cercospora-resistant cultivars when possible. Resistant varieties aren’t available for all Cercospora hosts, but growers are making strides in this regard constantly, so be sure to check the latest available literature when selecting your plants for an upcoming season.
Finally, as with dealing with any ubiquitous threat, proper crop-rotation can help keep these fungi at bay. If, for example, you’re battling Cercospora leaf spot on your sugar beets, you’ll want to try to plant a different crop on this land for at least 3 years before returning to sugar beets.
Fungicides for Cercospora Leaf Spot
There are a few fungicides that can help treat Cercospora leaf spot, but they’re not always recommended by horticultural professionals – at least in the context of homeowners dealing with the pathogen. Many annuals will survive the season despite developing leaf spot, and the prompt removal of infected leaves can help further protect your plants.
Additionally, there have been numerous instances in which these fungi have become resistant to popular fungicides. This is always cause for extreme concern, and means most horticulturists recommend that laypeople consult with a plant- or tree-care professional instead of haphazardly applying fungicides. It is also important to note that many of the most popular fungicides are extremely toxic to fish, and moderately toxic to birds, bees and earthworms. Many also represent a potential skin or eye irritant, so caution is advised.
Just be sure that if you decide to use a fungicide to treat your plants’ Cercospora infection, you must choose a product that is specifically labelled as being effective for the plant or tree species in question. You’ll also need to follow the preparation and application instructions explicitly.
Nevertheless, in the case of large horticultural collections or professional agricultural situations, fungicides may be necessary. Some of the most commonly used fungicides include those that contain chlorothalonil or thiophanate-methyl. Here are our top picks for each type of fungicides:
- SYSTEMIC FORMULA: The systemic formula cures and prevents listed diseases for roses, flowers, shrubs, or even houseplants
- DISEASE CONTROL: Fungicide controls Black Spot, Powdery Mildew, Crabapple Scab, Boxwood Blight, and more
- RAINPROOF PROTECTION: Systemic rainproof protection lasts up to 2 weeks
- CONTROLS A VARIETY OF DISEASES - Infuse prevents and stops black spot, dollar spot, rust, brown patch, powdery mildew, leaf spot,...
- TOTAL COVERAGE OF LAWN - Provides a broad spectrum of disease control for roses, flowers, lawns, trees, shrubs, and multiple other...
- WON'T WASH AWAY - Fungicide is absorbed into the plant ensuring it can't be washed off by rain or sprinklers after the spray has...
Check the label’s instructions, but these products are usually designed to be applied directly to the leaves (particularly those at the bottom of the plant). Typically, you’ll want to do so before any spots are noted or as soon as is possible after noting symptoms.
Just understand that these are not systemic products. They will not be absorbed and redistributed by the plant; they work “on contact.” They’re typically most effective on treating uninfected leaves preventatively, though they will not help treat leaves that haven’t yet burst from their buds. This means you’ll need to reapply the fungicide every two weeks or so (again, check the label instructions).
Cercospora leaf spot can certainly cause headaches for farmers and homeowners, but with a bit of forethought and careful plant management, you can often limit the damage these fungi cause. Just remember to select resistant cultivars whenever possible, inspect your plants and removed infected leaves promptly, and always dispose of infected vegetation properly.
If you do these things, you’ll likely find it possible to keep these frustrating fungi in check.