Wet, Humid Conditions Favor Cercospora Leaf Spot in the Panhandle - UNL CropWatch, July 13, 2011
July 14, 2011
Unusually wet and humid conditions in the Panhandle favor higher levels of Cercospora leaf spot than the area has seen in recent years. For the 13th year, the UNL Panhandle Research and Development Center will be implementing the Cercospora Alert Forecasting System for area sugarbeet growers.
Three common leaf spot diseases of sugarbeet:: (from left to right) Cercospora leaf spot, Alternaria leaf spot, and Phoma leaf spot.
Cercospora leaf spot (CLS) is caused by the fungal pathogen, Cercospora beticola, and has played a defining role in sugar beet cultivation throughout the central and eastern production areas of the United States.
This disease became a major limiting factor in early Nebraska production and was a primary reason for the shift of sugar beet production from eastern Nebraska to the Panhandle 100 years ago. The disease was of minor importance in western Nebraska until the mid 1980s, when it began to cause significant problems. Today it is the most serious foliar disease of sugar beets in the Central High Plains.
Disease development strongly depends on specific environmental conditions characterized by periods of high humidity or extended leaf wetness (more than 11 hours) and warm temperatures (higher than 60°F at night and 80-90°F during day). Without these conditions, disease spread and damage to beet crops is greatly reduced or inhibited. In our arid climate. These conditions are most common in mid July through late August in irrigated fields.
The Cercospora Alert Forecasting System is designed to estimate the potential for disease to develop based on the relative humidity and temperature measured within fields. The plant pathology program at the Panhandle REC continues to spearhead this regionwide service in collaboration with the agricultural staff of the Western Sugar Co-op, and several crop consulting services.
With this system, growers receive daily updates as to the potential for disease development. To subscribe to this service, contact UNL Research Technician Kathy Nielsen at email@example.com. The daily updates also can be viewed on the Panhandle REC's website at panhandle.unl.edu.
We have not had any widespread, serious yield-limiting outbreaks throughout the Western Sugar Grower area since the mid 1990s; however, this year's conditions may result in higher disease levels than we have recently experienced. Please be aware that we have begun reporting leaf spot alerts over the last week, and pay attention to the results from the site nearest your fields.
Because of the high degree of variability between and within fields, the alert system requires that growers use the predictive information as a guide to local environmental conditions and the potential for disease development in their fields. Therefore, it is still necessary to scout fields for the appearance of cercospora leaf spot.
Growers can also monitor nighttime high temperatures. This disease is most apt to develop with warm nights with periods of heavy dew. We have adequate leaf moisture at night, but often our temperatures drop to the 50s at night, helping us escape from the serious infections that plague Michigan or the Red River Valley. We have sufficient temperatures during the day, but unless irrigations are occurring, the moisture on leaves is not normally high enough to initiate severe infections.
Identifying Cercospora Leaf Spot Symptoms
It is important to learn to recognize the symptoms of cercospora leaf spot and how to distinguish them from those caused by other common foliar diseases (Figure 1). Individual leaf spots are 3-5 mm (1/4 inch) in diameter at maturity and are nearly circular to oval in shape. Lesion centers are tan to light brown with dark brown to reddish purple borders. Generally, cercospora leaf spot is the only foliar disease in this region that would seriously impact yields and usually is the only one that would require a fungicide application.
Incorrectly identifying this disease could lead to an unnecessary fungicide treatment, affecting a grower's costs and profitability.
Extension Plant Pathologist, Panhandle REC, Scottsbluff