Cercospora Leaf Spot of Swiss Chard, Beets, and Spinach
Cercospora leaf spot, caused by the fungus Cercospora beticola, occurs wherever table beets, swiss chard, sugar beet, and spinach are grown and is one of the most important diseases affecting the Chenopodium group. It can result in significant losses, particularly in late summer when conditions are favorable (high temperatures, high humidity, long leaf wetness periods at night). Leafy greens become unmarketable, and beet roots fail to grow to full size when disease is severe.
Symptoms occur as numerous, initially small circular leaf spots (see photo). Spots have a pale brown to off-white center with a red margin. Lesions expand in size, coalesce, turn gray as the fungus sporulates, and can result in extensive loss of foliage. Leaves at the center of the plant are often less severely affected. The pathogen produces sclerotia or stromata which can be seen with a hand lens as small, black dots in the center of lesions. Lesions may also occur on petioles, flower bracts, seed pods, and seeds. Leaf symptoms are similar to those caused by Beet Phoma (Phoma betae), except that the phoma will have more obvious tiny fruiting bodies in the lesions and can also affect the roots.
Source and survival
C. beticola survives between crop cycles in residues from infected crops (as sclerotia), in weed hosts, and on seed. It can survive in the soil for up to two years. High levels of disease can result from just a few infected plants since each lesion produces numerous conidia. Several cycles of infection and conidium production may occur with favorable environmental conditions. Spores can penetrate the leaf directly through open stomates. The pathogen is favored by high relative humidity and temperatures between 75-85˚ F and is spread by rain splash, wind, irrigation water, insects, workers, and equipment. Leaf wetness during the night, even with dry conditions during the day, encourages disease. Successive plantings made close together can allow disease to move from one planting into the next.
- Bury infected crop residues and destroy volunteer plants and weed hosts.
- Start with certified, disease-free seed, or treat seed with hot water or fungicides.
- Rotate to non-host crops (outside of the Chenopodium family) for 2-3 years.
- If disease is present, do a once-over cut rather than cutting chard or spinach for regrowth.
- Avoid planting succession crops close together.
- Avoid overhead irrigation if it will result in prolonged leaf wetness periods (e.g., through the night); irrigate mid-day when leaves will dry fully or use drip irrigation.
For optimum results use protectant fungicides as a preventive treatment, prior to infection and symptom development. Pathogen populations resistant to sterol demethylation-inhibiting (DMIs, FRAC Group 3) fungicides have been reported, so although these products are labeled, fungicides with other modes of action should be used. These include azoxystrobin (Quadris) (Group 11); basic copper sulfate (Basic Copper 50W HB and other copper products) (Group M1); pyraclostrobin (Cabrio) (Group 11); trifloxystrobin (Flint) (Group 11). Do not alternate Group 11 strobilurin fungicides with each other (Cabrio, Quadris and Flint). Products that simply kill spores on contact will not prevent the continuing production of spores nor protect leaves from new infections.
Crops that are affected by this disease:
-By Bess Dicklow, Rob Wick, and Ruth Hazzard, UMass Plant, Soil, and Insect Science Dept.