Cercospora Leaf Spot

Cercospora leaf spot is a polycyclic fungal disease caused by several Cercospora species. Cercospora beticola is the principal pathogen on sugar beet and table beet; C. arachidicola and C. personatum cause early and late leaf spot on peanut; C. carotae attacks carrot; and C. sojina causes frogeye leaf spot on soybean. The disease is among the most yield-limiting foliar problems in warm, humid growing regions worldwide, capable of cycling through multiple infection rounds in a single season.

The pathogen overwinters in infected crop debris and, on some hosts, in seed. Conidia spread by rain splash and wind and enter leaves through stomata during periods of wet foliage above 25 degrees C. Symptoms appear 7 to 14 days after infection. On beet, each lesion can produce millions of conidia that initiate new cycles throughout the season, making preventive timing essential.

Crops affected

Circular spots with pale centres and purple borders scattered across a green sugar beet leaf.

Identification plate for cercospora leaf spot: fungus, size shown to scale, with overwinters on debris & seed → splash- & wind-borne conidia → repeating cycles all season. — Identification plate - actual size, favours & spread, and disease cycle. View full size

Cercospora Leaf Spot at a glance

Heavy spotting yellows & collapses the leaf.

How to Identify Cercospora Leaf Spot

Sugar beet and table beet: circular spots 3 to 5 mm across, tan or grey centres with a reddish-brown to purple border. Heavily infected leaves yellow and collapse, exposing the crown to sunscald and secondary rots. Tiny dark specks (pseudostromata) may be visible in the lesion centres.
Peanut: early leaf spot (C. arachidicola) produces tan lesions with yellow halos, mainly on the upper leaf surface. Late leaf spot (C. personatum) produces darker, nearly black lesions primarily on the lower surface. Both cause premature defoliation under high pressure.
Carrot: oval to elongated water-soaked spots on older leaves, turning tan to brown with age. Severe infections defoliate the crop before harvest, reducing yield and storage quality.
Soybean (frogeye): circular spots with a grey-brown centre and a reddish-purple border, 1 to 5 mm across. Favoured by warm nights and overhead irrigation; QoI-resistant strains are widespread in major soybean-producing states.

Symptoms vary by host but share a common pattern: circular spots with a pale or tan centre and a distinct darker border. Key characteristics by crop:

Disease Cycle and Spread

Cercospora is polycyclic: a single infected plant can generate multiple infection cycles in one season. Primary inoculum comes from infected crop debris in the field and, in some host-pathogen combinations, from infected seed. Rain splash and overhead irrigation spread conidia over short distances; wind disperses them further. Infection requires 6 to 12 hours of wet foliage at 25 to 32 degrees C. Because the latent period before visible lesions is 7 to 14 days, symptoms on the plant always lag real infection pressure. Begin protective applications before conditions favor infection, not after lesions appear.

Vegalab Spore Control for Cercospora Leaf Spot

Spore Control contains 1% thymol, a naturally occurring phenol with membrane-disruptive activity against a broad range of fungal pathogens. Its mode of action is independent of DMI (FRAC 3) and QoI (FRAC 11) fungicides. Published research shows thymol activity against Cercospora beticola remains consistent across isolates regardless of DMI or QoI resistance profile, supporting its use in a resistance-management rotation alongside conventional programs.

Spore Control applications are preventive. The product protects clean, uninfected tissue at the time of application. Applications cannot cure established lesions or reverse tissue damage already visible. For best results, begin applications before or at the very first sign of disease, and maintain a regular program through the period of peak disease risk.

Use	Dilution	Product per 1,000 L water	Repeat interval
Preventive, early season or clean crop	1:1,000	1 L	Every 7 to 10 days
High disease pressure, active outbreak	1:500	2 L	Every 5 to 7 days

Resistance Management

QoI and DMI resistance in Cercospora beticola has spread widely across beet-producing regions of North America and Europe. Rotating Spore Control with conventional fungicide FRAC groups preserves the efficacy of the full program. Because thymol acts through a different mechanism, it contributes to rotation diversity without duplicating the mode of action of the products it alternates with.

Always check the current product label for the maximum registered rate before application. Rates shown are for guidance; label directions govern. Product claims, registrations, and availability vary by jurisdiction.

Important information

Product claims, registrations, and availability vary by jurisdiction. Always consult the current product label for approved uses, rates, and safety instructions before use. This page does not constitute a registered label or a guarantee of efficacy.

Recommended Vegalab solution: Spore Control

Spore Control · natural broad-spectrum fungicide (1% thymol) applied preventively before and during periods of disease risk, on a 7 to 10 day interval and tightened to 5 to 7 days under high pressure; rotate with conventional FRAC groups.

Role	Product	Use
Primary control	Spore Control

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Claims and product availability vary by jurisdiction. Always read and follow the product label.

Frequently asked questions

What crops does Cercospora leaf spot affect?

Cercospora species attack a wide range of crops. The most economically important hosts include sugar beet and table beet (C. beticola), peanut (C. arachidicola and C. personatum), carrot (C. carotae), and soybean (C. sojina, frogeye leaf spot). Warm, humid conditions and dense canopies favor rapid spread on all of these crops.

How do I know if my crop has Cercospora and not another leaf spot disease?

On beet, look for circular spots 3 to 5 mm across with a tan or grey centre and a distinct reddish-brown border. On peanut, early leaf spot (C. arachidicola) produces tan lesions with yellow halos mainly on the upper leaf surface; late leaf spot (C. personatum) produces darker lesions mainly on the lower surface. Laboratory confirmation is the most reliable identification method when symptoms are ambiguous.

Can Vegalab Spore Control cure an active Cercospora infection?

No. Spore Control is a contact protectant and applications are preventive in nature. The product protects clean, uninfected tissue at the time of application. Applications cannot reverse established lesions or clear existing infections. For best results, begin a preventive program before disease onset or at the very first sign of infection, and maintain applications through the period of peak risk. Under high disease pressure, applications at a higher rate can help limit the spread of new infections.

Is Cercospora beticola resistant to conventional fungicides?

QoI (FRAC 11) and DMI (FRAC 3) resistance in C. beticola is widespread in major beet-producing regions. Thymol, the active ingredient in Spore Control, has a different mode of action and published research shows consistent activity against both sensitive and resistant isolates. Rotating Spore Control with conventional fungicide FRAC groups is a sound resistance-management practice.