Cercospora leaf spot of beet

Symptoms

• The spots are round, ash-coloured, 2–4 mm in diameter, often with a red-brown border.
• With severe disease development, more or less large areas of dead tissue form on the leaf, or the leaf dries out completely. Dead, curled leaf blades can remain on living petioles for a long time.

Signs of the pathogen

A characteristic sign is a silvery coating of the pathogen's conidiophores and conidia on the surface of the spots; it is more visible in wet weather or after dew. This coating is used to distinguish Cercospora leaf spot from zonate and bacterial leaf spot.

When to look

• The disease appears on well-developed leaves at the end of June or the beginning of July and is seen until the end of the beet growing season.
• Disease assessment starts when the first spots appear and continues every ten days until the end of the growing season on first-year beets and seed plants in permanent plots.

How to count

• The degree of disease development is determined by examining, along the field diagonal, 250 first-year beet plants and 125 seed plants at five evenly spaced points: at each point, 50 beets or 25 seed plants in succession in one row.
• During the period of strongest disease development (mostly in August), beet crops on farms are surveyed: the degree of disease development is determined along the field diagonal in 10 row sections of 50 plants each.
• The disease is assessed on a five-point scale. 0 points: a healthy plant with no spots on the leaves. 1 point: the spots are scattered, and affected leaves make up no more than 25 % of all leaves in the rosette.
• Assessment results are evaluated by three indicators: the percentage of affected plants, the mean infection score and the percentage of disease development.

No rate and no spray timing here: both come from the label and the register.

Agronomy

When the disease appears and the weather is warm and wet, soil loosening after rainfall, weed control and foliar feeding with phosphorus-potassium fertilisers are used.

Source of infection

The pathogen's infection reservoir persists in the soil.

Favourable conditions

• Development of the pathogen is favoured by heavy morning dews or light warm rains at a mean daily air temperature of +15..+20 °C.
• The disease is most widespread in August: its development is favoured by warm weather (mean temperature 20–22 °C by day and no lower than 15 °C at night) and high air humidity (no lower than 65–70 %).
• Disease development intensifies when long periods of wet weather give way to short dry spells, which contribute to suppressed beet growth, physiological ageing of the leaves and lower leaf resistance to the disease.

Where it is found

The most widespread and most harmful leaf spot in sugar beet crops. It occurs in the Forest-Steppe and Polissia oblasts.

Every fact rests on a quote from its source, and each quote was checked against the page it is printed on.

1. EPPO Global Database · European and Mediterranean Plant Protection Organization
2. Станкевич С. В., Положенець В. М., Немерицька Л. В., Журавська І. А., «Моніторинг хвороб сільськогосподарських культур», навчальний посібник, 301 с. · Інститут біоенергетичних культур і цукрових буряків НААН, Державний біотехнологічний університет; Житомир: Рута, 2022 · p. 180, 181, 182
3. Прогноз фітосанітарного стану агроценозів України та рекомендації щодо захисту рослин у 2025 році · Держпродспоживслужба, НААН, НАН · p. 104, 105
4. Photos: Jamie O'Neill, iNaturalist, CC BY 4.0