Disease management requires growers to look at a number of factors - those that influence disease spread, increase inoculum and lead to infection.

Cercospora

Description

Cercospora leafspot, caused by the fungus Cercospora beticola, is the most serious disease of sugarbeets in eastern North Dakota and Minnesota. This disease can cause reduced tonnage and sucrose and increased impurities. Losses of 30 percent in recoverable sucrose are fairly common under moderate disease conditions. Roots of affected plants do not store as well in the pile as roots of healthy plants.

Many of the currently grown high-yielding sugarbeet varieties are susceptible or moderately susceptible to Cercospora. The epidemic of the 1980 and 1981 growing seasons was favored by optimum weather conditions and the very susceptible varieties grown at that time. The epidemic of the 1995 growing season was again favored by optimum weather conditions, frequent rainfall that washed fungicide off the leaves, and the widespread (southern Minnesota and the southern Red River Valley) presence of strains of Cercospora with tolerance to the tin fungicides.

Symptoms

Click image for larger view Image 1

Cercospora infection of the sugarbeet leaf produces circular spots about 1/8 inch (occasionally 3/16 inch) in diameter with ash gray centers and dark brown to reddish purple brown borders (image #1). During warm, rainy, humid weather, the spots may coalesce and kill entire leaves, particularly on susceptible varieties. In humid weather, these coalescing spots may be covered with areas of steel blue to light bluish-purple fuzz.

These are masses of spores of the Cercospora fungus. Severely diseased leaves wither and die, resulting in severe defoliation. The disease begins on the older leaves and progresses to the younger leaves. Diseased leaves usually remain attached to the crown of the plant.

Bacterial leafspot can be confused with Cercospora leafspot. The symptoms of bacterial leafspot frequently develop during cool rainy weather but may occur intermixed with Cercospora leafspot. Bacterial leafspot often appears on leaves a week or two earlier than Cercospora leafspsot but can be present throughout the season. Bacterial leafspot produces irregular-shaped to circular spots that are 3/16 to 1/4 inch in diameter. They have dark gray centers (Cercospora has light gray centers) with very dark to almost black borders. In areas where bacterial leafspots coalesce, portions of the leaf tend to tear, producing a ragged leaf.

Bacterial leafspot is sometimes confused with Cercospora leafspot.

To distinguish Cercospora leafspot from bacterial leaf-spot, examine the spots with a hand lens. The gray centers of Cercospora leafspots usually will have tiny black dots in them, or if there is profuse spore production, the centers will have a fuzzy blue-gray appearance. There are no black dots or blue-gray fuzz (fungal spore mass) in the centers of bacterial leafspots.

Ramularia leaf spot may also be confused with Cercospora leaf spot. Ramularia leaf spots develop light brown centers with dark brown or reddish brown borders. The spots are slightly larger than Cercospora, about 3/16-1/4 inch in diameter. Ramularia leaf spots have a more irregular to angular appearance. In moist weather, masses of spores form tiny white dots in the centers of the spots or may even form a silver gray to white fuzzy surface -- this contrasts sharply with Cercospora. Ramularia is favored by cooler weather than Cercospora.

Alternaria leaf spot may be confused with Cercospora leaf spot. Generally Alternaria attacks only yellowed leaf tissue. Spots may be 1/8 to 3/8 inch in diameter or slightly larger. Spots are roughly circular, dark brown to black, often with a target pattern of concentric rings and a dark brown to black fuzzy growth of spores. Alternaria leaf spot usually first develops near the leaf margins, then the spots spread and coalesce and occupy much of the leaf area between the main veins.

Ramularia and Alternaria rarely cause enough damage to warrant control by fungicides. Bacterial leafspot is rarely of economic concern and is not controlled by fungicides.

Development

Cercospora leafspot develops rapidly in warm, humid and rainy weather. The Cercospora spores are produced most readily at temperatures of 68-79 degrees Fahrenheit and relative humidities (RH) of 90-100 percent. Spores do not form at temperatures less than 50 F. Optimal spore germination and infection occur when the temperature is 75-77 F and the RH is 100 percent for at least 8 ½ hours. In general, day temperatures of 80-90 F and night temperatures above 60 F favor disease development. Under favorable conditions, leafspot symptoms may occur in as few as five days after infection, with more spores produced in another five days.

Spread

The most common source of the Cercospora fungus is infected beet debris in the field. The fungus and spores survive over winter on this debris. The Cercospora spores are spread by wind, water (irrigation and rain), and insects. The fungus also can be carried on the seed, although this is usually of minor importance. The fungus may infect some common weeds such as redroot pigweed, lambsquarters, mallow, and bindweed, but there is little evidence that these weeds are important in the disease cycle.

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.