John J. Gallian, Associate Professor of Plant Pathology and Extension Sugarbeet Specialist, University of Idaho, Twin Falls Research and Extension Center.

Cercospora leaf spot, caused by the fungus Cercospora beticola, is a serious disease of sugarbeets in the midwestern U.S. It can often be found in the Idaho, eastern Oregon and eastern Washington sugarbeet production areas, but does not normally cause economic losses due to the dry climate of this region. Occasionally, however, in some areas, primarily under sprinkler irrigation, it can become severe enough to warrant fungicide treatment. In 1997, for example, several sprinkler irrigated fields near Marsing, Idaho, that were in low-lying areas near the Snake River and had frequent rain showers developed as much as 15-20% infection (Figure 1), which is far higher than the 3% economic threshold for this disease. Cercospora causes reduced tonnage, reduced sucrose and increased impurities, and losses of up to 30% in recoverable sucrose under moderate disease conditions are fairly common. The pathogen only infects the leaves, but roots of affected plants do not store as well as roots from healthy plants.

Initial symptoms appear as small, circular spots, 1/8 to 3/16 inch in diameter, with tan to gray centers and dark brown to reddish purple brown borders, beginning on the older leaves and progressing to younger leaves (Figure 2). In humid weather as spores of the fungus are actively being produced, spots may have a gray to steel blue fuzzy appearance. As the disease progresses, leafspots coalesce and kill large portions of the leaf (Figure 3). Severely diseased leaves wither and die, resulting in defoliation.

Cercospora leaf spot may be confused with bacterial leaf spot or Ramularia leaf spot. Bacterial leaf spot may develop during periods of rainy, cool weather, and usually appears about two weeks earlier than Cercospora. Spots are larger than those of Cercospora, have dark gray centers, dark brown to black borders and do not have tiny black dots in the centers as Cercospora does when examined with a hand lens. Ramularia leaf spots are larger and more irregular than Cercospora and have light brown centers, which become white to light gray during moist weather as spores are produced. Neither bacterial leaf spot nor Ramularia leaf spot have been observed to cause economic loss in the sugarbeet production areas of the Pacific Northwest.

C. beticola survives in infected plant residue in the soil, which is the primary inoculum source. Other inoculum sources, which are considered to be minor, include infected seed and weed hosts such as redroot pigweed, lambsquarter, mallow and bindweed. During periods of humid weather, spores are produced and spread by wind, rain and irrigation water, and insects. Spore production occurs most readily at 68 - 79° F temperatures and 90 - 100% relative humidity. Infection occurs during periods of frequent rainfall, warm temperatures (77 - 95° F), high relative humidity (90 -100%) and night temperatures above 60° F. The optimum for spore germination and infection is during these warm temperatures when there is free moisture on the leaf surface for a minimum of 8.5 hours. These conditions are the most favorable, but infections can develop more slowly at a wider range of conditions. Disease will not develop at temperatures below 50° F and rarely when relative humidity is below 70%. It must be remembered, however, that temperature and humidity within the plant canopy can differ considerably from the conditions outside the canopy, and these differences can be even greater after row closure and under sprinkler irrigation. Isolated field situations where air drainage and fog can periodically accumulate and/or may be influenced by a body of water such as a lake or river can sustain high humidity and favor Cercospora leaf spot development. Growers should be aware of their particular situation and use judgment based on the information presented here.

An integrated approach including cultural practices, resistant varieties and fungicide applications have proven to afford the most effective control in growing areas where the disease is a perennial problem. In the Pacific Northwest, growers must rely on cultural management and proper timing of fungicide applications because Cercospora leaf spot resistant varieties are not adapted and will likely not be developed for the area.

Cultural Practices - A three year crop rotation is recommended because the pathogen overwinters on infected sugarbeet leaves and can remain viable for 2-3 years. Fungal spores are carried with the wind, and sugarbeets should not be planted within 100 yards of a field that was infected the previous year. Fall tillage, preferably plowing, to bury the infected plant material is useful in reducing inoculum survival and dispersal.

Chemical Control – Early detection and monitoring the disease progress is essential to prevent economic loss. If leafspots cover at least 3% of the foliage by harvest (equivalent to about 80 spots/leaf), economic loss has already occurred. When favorable conditions exist and are expected to continue, and Cercospora leaf spot is present and increasing, it is recommended that the field be sprayed when disease is at an average 0.5% infection (about an average 12 spots/leaf), before the 3% economic threshold is reached. The same guidelines should be used for repeat application. This disease has the ability to increase very rapidly and spray treatment should not be delayed once these conditions have been met. Fungicide application will not eradicate the existing disease but can reduce or prevent further infections. The fungicide must be applied and allowed to dry before rain begins or sprinkler irrigation is started to minimize washing the applied chemical from the leaf surfaces.

Triphenyltin hydroxide (Super Tinâ ), mancozeb, maneb, benomyl (Benlateâ ) and thiophanate methyl (Topsin-Mâ ) are registered for control. It is highly recommended that the systemic benzimidazole fungicides, Benlate or Topsin M, not be used alone because of the risk of building up resistant strains of the fungus that persist in the environment for many years. This recommendation is not to be taken lightly. In the Red River Valley, for example, resistant strains of the pathogen developed to the benzimidazoles after only three years of use and are no longer effective. Resistant strains can build up with Super Tin, although more slowly than with the benzimidazoles.

Fungicide Resistance Management - Fungicide rotation and tank mixing are the primary methods of preventing the development of resistant strains of the pathogen. For example, if Super Tin is applied, change to mancozeb or maneb for the following application. In most cases in the Pacific Northwest, a tank mix containing a systemic fungicide should not be necessary for Cercospora leaf spot control if recommended cultural practices are being followed and a protectant fungicide is applied with the recommended timing. If disease is severe and a systemic fungicide is warranted, a combination of mancozeb + a benzimidizole would be the preferred tank mix containing a systemic fungicide. Super Tin + Topsin M has been one of the best performing combinations in the Red River Valley, but resistance to both Super Tin and the benzimidizoles has been detected and they are no longer making this recommendation. No matter which tank mix is used, a systemic fungicide should should not be applied more than once during the season. Following these resistance management procedures along with good cultural practices, and applying fungicides only when needed can reduce the possibility of developing resistance and prolong the useful life of these important tools.

Acknowledgements: The input from Art Lamey, North Dakota State University, Fargo, and Carol Windels, University of Minnesota, Crookston, in the preparation of this publication is gratefully acknowledged.