Cercospora leaf spot is caused by the fungus Cercospora beticola.
It occurs wherever table beets, swiss chard, sugar beet, and spinach are grown and is one of the most important diseases affecting the Chenopodium group which causes significant losses, particularly in late summer when conditions are favorable (high temperatures, high humidity, long leaf wetness periods at night).
The leafy greens become unmarketable, and beet roots fail to grow to full size when disease is severe.
This disease is highly influenced by weather conditions and plantings that mature in cool weather often escape severe infection.
DISEASE CYCLE
- beticola survives between crop cycles in residues from infected crops as sclerotia, in weed hosts, and on seeds. 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/spores.
Spores germinate when humidity is high or free moisture is available under high temperatures and can penetrate the leaf directly through open stomata.
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.
Several cycles of infection and conidium production may occur with favorable environmental conditions
SIGNS & SYMPTOMS
Infection and lesion formation initially occur on older leaves before progressing to newer one, thus leaves at the center of the plant are often less severely affected.
Symptoms occur as numerous, initially small, circular to angular, brown to dark green spots with a reddish brown margin form on older leaves.
Severely affected leaves wither and die from coalescing lesions. A diagnostic feature is the presence of tiny black dots (pseudostromata) that form in leaf substomatal cavities within the grayish-tan lesions. The pseudostromata produce conidiophores borne in clusters that serve as conidia-bearing structures. Pseudostromata are visible with a hand lens, and after exposure of leaves to high humidity, entire lesions appear fuzzy due to the presence of numerous conidia.
Lesions may also occur on other parts like the petioles.
MANAGEMENT
Chemical method
For optimum results, protectant fungicides are highly recommended for preventive treatment, prior to infection and symptom development.
Although many fungicides are available for managing the disease, fungicide resistance management must be considered and monitored carefully, since C. beticola populations have been identified in several production areas that are newly resistant and/or tolerant to major classes of fungicides.
The following fungicides are recommended for use against Cercospora leaf spot of spinach;
- CHARIOT 500SC 20ml/20l
- BRADLEY 500SC 10ml/20l
- RANSOM 600WP 15g/20l
- DOMAIN 250EC 10ml/20l
- DUCASSE 250EC 20ml/20l
- EXECMPO CURVE 250SC 15ml/20l
- GREENCOP 500WP 50g/20l
- MEGAPRODE LOCK 525WP 30g/20l
- MILLIONAIRE 690WDG 45g/20l
- ABSOLUTE 375SC 10ml/20l
- CADILLAC 800WP 50g/20l (for prevention purposes only)
Non-chemical methods
- Bury the infected crop residues and destroy volunteer plants and weed hosts.
- Start with certified, disease-free seed, or treat seed before planting.
- Rotate to non-host crops (outside of the Chenopodium family) for 2-3 years.
- 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.
- Reduce periods of leaf wetness by decreasing planting densities and orientating rows parallel to the prevailing wind direction.
Tips!
- Always mix the fungicide with INTEGRA 3ml/20l, which is a sticker, wetter, spreader and penetrant that improves efficacy of the chemical.
- Always do repeat sprays for effective control. A single spray is never adequate.
- Timely management of the disease is very important.
- A proper nutrition is essential for improved crop immunity.
