Cercospora Leaf Spot Detected in Nebraska

Published online: Jul 23, 2020 News Robert M. Harveson, Extension Plant Pathologist and Xin Qiao, Water and Irrigation Management Specialist, University of Nebraska Panhandle Research & Extension Center
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Source: University of Nebraska Extension

On July 13, symptoms characteristic of Cercospora leaf spot (CLS) were found on lower leaves of sugarbeets from research plots at the Panhandle Research & Extension Center at Scottsbluff, Neb.

This finding indicates that the infection likely occurred in early July, which historically has been early for western Nebraska. It also implies that many of the new leaves may already be infected but not exhibiting symptoms yet. Infection of the newer leaves is where the economic loss occurs with both root yields and sugar content. Curiously, this is almost the exact date when CLS was first observed in research plots in 2019.

CLS, a foliar disease of the beet, has long been a problem for sugarbeet production in more humid areas of the U.S. It has been a sporadic, but potentially severe problem in western Nebraska. The disease is caused by the fungal pathogen, Cercospora beticola, and is spread by spores moving in wind currents throughout and among fields.

The pathogen overwinters in infected residues and can serve as a source of inoculum the following season. Disease development is strongly dependent upon several very specific environmental conditions, including periods of high humidity or extended leaf wetness (more than 11 hours) with warm temperatures (higher than 60 to 65 degrees Fahrenheit at night and 80 to 90 degrees during the day). Disease initiation, spread and resulting damage to beet crops is greatly reduced without these exact conditions.

The fungal spores germinate, infect and grow within leaf tissues. Under optimal conditions, new lesions and spores will be formed within 10 to 14 days. This means that whenever you see the circular, ash-colored lesions (1/8-inch in diameter) surrounded by a dark border, infection has actually occurred approximately two weeks earlier.

Extension officials note that it is not time to panic or worry about treating with any spraying of fungicides, but it is time to begin scouting fields closely looking for symptoms. UNL Extension is also working this year on a new tool coordinated with the Cercospora Alert System that has been used in the past, and will share results of this work when available.