by Tamara Scully
If your late summer crops of beets, Swiss chard and spinach aren’t looking their best in the humid and overly wet weather much of the country has experienced recently, the culprit may be Cercospora beticola, a major pathogen of Chenopodium family crops. When you are growing a leafy crop such as Swiss chard, the C. beticola pathogen can cause the crop to be unmarketable, with unsightly leaf spots. In table or sugar beets, the juice, sugar content and root size can cause yield losses. Cercospora Leaf Spot (CLS) disease is considered the most important foliar disease of Chenopodium crops.
Another pathogen in the Cercospora family, C. carotae, causes issues in carrots, fennel, parsley, celeriac, celery and dill. On peppers, its Cercospora capsici causing concern. Cercospora canescans causes problems on legumes. In celery, early blight is caused by Cercospora apii, which appears on the leaves and stems. Cercospora citrullina effects cucurbits, and is a commonly found on watermelon.
With over 1,200 species, this family of fungi infects many vegetables, ornamentals and herbs. The Cercospora spp. fungi causes a variety of problems: root crops which fail to grow to size, particularly when infestation becomes severe; spotted foliage; defoliation; and unmarketable leafy greens. Severe disease can defoliate plants, and in some instances this is enough to cause total crop loss.
Hot, Wet, Wild
The Cercospora fungi cause leaf spots as a primary symptom, as do many diseases, including Phytophora, Septoria and a variety of others. While wet conditions favor the development of all of these – and many more – diseases, the temperature range which favors Cercospora spp. proliferation equates to those hot and humid summertime temperatures. Very humid weather, with temperatures between 75 and 95 degrees Fahrenheit, nighttime temperatures which remain above 60 degrees and prolonged periods of leaf moisture are keys to Cercospora disease development.
As summer winds down and early morning dew becomes heavy, even a hot season with average rainfall can see proliferation of Cercospora leaf spots. Irrigation that wets the leaves, particularly if they aren’t able to fully dry, is another factor in disease development, as this fungal pathogen requires extended periods of wetness in order to reproduce. Under optimal conditions, the pathogen can undergo multiple reproductive cycles in a growing season and pathogen population can grow exponentially.
Light brown or tan small leaf spots are indicative of the disease. They can be round, but sometimes appear angular. These spots normally will have reddish or purple margins. The center of lesions can develop a hole. The spots begin as separate entities, but in severe infestations can merge together. Older infected leaves can also become yellowed and eventually die.
Cercospora spp. produces sclerotia on the infected leaves. These specialized asexual reproductive bodies can often be seen under magnification as black dots in the center of the leaf lesions, and they are capable of long-term survival. During irrigation or rain events, sclerotia can splash onto plants, transmitting the pathogen. Prompt removal of infected leaves can reduce fungal pressures. By removing infected leaves in beets, for example, the root may be marketable in cases of early infestation and less severe cases of disease.
Blighted leaves can impact root growth, causing yield losses in root crops. If these crops are mechanically harvested, weakened leaves – due to the leaf spot infection – can impact harvest yield, as roots are left in the field if their leaves break during harvesting.
If left unchecked, Cercospora pathogens are often able to infect more than the leaves of plants. In beans, pods will show active lesions and discoloration. On peppers, the fruit itself is spared infection but the defoliation can cause yield loss. Peppers on defoliated plants are also exposed to sun scald, further reducing yields.
Seeds do become infected with Cercospora spp. pathogens and are a common cause of transmission. Planting disease-free seeds or transplants, practicing crop rotation for at least two years, properly spacing crops to promote leaf drying, removing all plant debris, burying of infected crop residues and protecting plant leaves from prolonged exposure to moisture will all help to reduce disease proliferation.
Fungicides are available for Cercospora spp., and some disease-resistant cultivars are available. If scouting finds evidence of infection at threshold levels, fungicide application is warranted. For organic producers, copper fungicides can be effectively utilized, when combined with Double Nickel biofungicide, to reduce incidence of C. beticola in Swiss chard, beets and spinach by about 30 – 40 percent, as shown in Cornell University studies conducted by Dr. Sarah Pethybridge.
While Cerpospora leaf spots affect many crops, C. beticola is a prevalent disease, causing significant crop loss in Chenopodium family vegetables – table and sugar beets, Swiss chard and spinach. The pathogen can readily overwinter in related common weed crops of lamb’s quarter, goosefoot and pigweed, serving as a source of inoculum.
For growers facing Cercospora leaf spot concerns, diligence when conditions are optimal (as they were for many this season) and precautions to curtail the future spread of the disease are warranted to prevent significant crop losses.