1. Introduction

Cercospora leaf spot is a fungal disease caused by Cercospora beticola. It is one of the most important, wide spread and destructive fungal diseases affecting sugar beets. It occurs wherever these crops are grown. The severity of Cercospora varies from year to year depending on weather conditions and varietal resistance. Cercospora can cause losses in susceptible varieties through reduced tonnage, reduced percent sucrose and increased impurities. Even fairly low levels of leaf spot may cause these effects.

2. Disease Symptoms

A symptom characteristic symptom of the disease is the development of circular spots on older leaves (see Image #1). These spots grow to 3-5 millimeters when mature. Individual spots converge as the disease progresses (see Image #2), and large areas of leaves become brown and necrotic.

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Image #1     Image #2

 Tiny black dots are often visible in the center of the mature lesions. Under humid conditions, the spots become gray and velvety with the production of conidia and conidiophores (for further explanation of terms, see next section, 'Fungal Morphology and Disease Cycle'.). The diseased leaves eventually collapse and fall to the ground, but remain attached to the crown. Heart leaves are normally lesion-free, but all other above ground parts are affected.

Elongated lesions occur on petioles, and circular lesions may occur on areas of the crown not covered by soil. The lesions vary in color from tan to light brown with dark brown or reddish-purple margins.

3. Fungal Morphology and Disease Cycle

As stated above, one of the primary symptoms of Cercospora Leaf Spot are the necrotic lesions that appear on the leaves. During high humidity, structures called stromata develop within the lesions, which are the sites of further innoculum production. The disease is spread by spores called conidia. The conidia are produced by specialized structures called conidiophores, which are located in the stromata.

The fungus survives on infected crop residue as spores (conidia) and as stromata. Under humid conditions, new conidiophores produced in the stromata generate more conidia. The conidia are spread to host leaves by rain splash and wind. The conidia germinate and penetrate host leaves through the stomata.

4. Conditions Favoring Disease Development

Cercospora leaf spot is favored by high temperature and long periods of high humidity or free moisture on the leaves. Optimum conditions for infection are day time temperatures of 27-32° C, night time temperatures above 16° C and relative humidity above 60% for at least 15-18 hours each day. Little, if any, infection occurs at temperatures below 15° C. Severe epidemics can be expected when the relative humidity stays above 96% for 10-12 hours each day for 3-5 days and the temperature remains above 10° C. The disease is spread from plant to plant by rain-splash, wind, irrigation water, insects and mites.

5. Disease Control

Leafspot management of Cercospora requires an integrated approach including:

• crop rotation,
• use of varieties that are less susceptible,
• scouting for disease,
• timely application of fungicide and
• more frequent applications when disease conditions are favorable.

Avoid planting next to last year's beets. This is especially important if last year's fields had high levels of Cercospora. If a field has been infected, a rotation scheme of 2-3 years with non-Cercospora hosts is recommenced.  To reduce inoculum potential for future sugar beet crops, remove infected beet tops. Also, deep ploughing hastens the breakdown of infected tops, leading to the death of the fungus.

In areas where the disease is prevalent, varieties with greater than average resistance to Cercospora should be planted. There are differences in Cercospora susceptibility among approved varieties. Cercospora may be somewhat easier to manage on varieties with higher than average tolerance to Cercospora. Conversely, varieties that are more susceptible than the average may need an extra spray in years that are highly favorable for Cercospora. Use of more tolerant varieties can be an important part of an integrated disease management plan.

Begin checking for Cercospora in late June or early July, making sure to check near last year's fields or shelterbelts. The first fungicide application should occur when conditions first favor disease or at disease onset. If the first application is late, control will be difficult all season, even if shorter than normal application intervals are used once applications start. When conditions favor disease, or disease is already prevalent, fungicide applications must be more frequent than when disease pressure is low.

Resistance and Tolerance to Fungicides. The terms "resistance" and "tolerance" are often used interchangeably. However, in the following discussion they are used with specific different meanings. Resistance is used to indicate that the Cercospora fungus is unaffected by a level of fungicide that previously prevented growth in the laboratory. Tolerance is used to indicate that the Cercospora fungus makes reduced growth in the laboratory on a level of fungicide that previously prevented growth in the laboratory. Resistant strains of Cercospora are not controlled by field applications of a fungicide. If tolerant strains are present, a reduced level of control will occur.

When conditions are extremely favorable for leaf spot development, supplemental fungicide sprays are often necessary. Applications of coppers, organotins or dithiocarbametes on a 10-14 day schedule are effective, provided they are applied very early in the disease cycle. Of the protectant fungicides, organotins (triphenyltin hydroxide) have given the best suppression.

Systemic benzimidazoles (i.e. benomyl, thiabendazole, thiophanate methyl) are also used in disease control. However, exclusive use of these chemicals has lead to the development or selection of resistant strains of Cercospora beticola.

If aerial application is made, make sure that areas around power lines and trees are side-dressed by the aerial applicator or by use of ground equipment. Improperly sprayed areas become focal points for Cercospora spread. 

Use higher label rates and shorter label application intervals when disease is severe or weather favors rapid disease buildup on susceptible varieties. Use 5-10 gals. water by airplane. Best results with ground equipment are obtained by using high pressure (at least 150 psi) and high volume. Label rates vary considerably for recommended gallonage with ground equipment. Research trials indicate that 20-40 gal. is adequate when high pressure is used. In the Rocky Mountain regions, fungicides may be needed as late as early or mid-September in many years; quitting the use of fungicides before this time may result in late-season damage that can reduce tonnage, sucrose and quality.

Special rates for fungicide application in your area can be found in the pesticide information section of the GrowerNET.