Robert M. Harveson, Extension Plant Pathologist
Xin Qiao, Water and Irrigation Management Specialist
University of Nebraska, Panhandle Research and Extension Center, Scottsbluff
On July 13, 2020 we found symptoms characteristic of Cercospora leaf spot (CLS) on lower leaves of sugarbeets from research plots at the Panhandle Research and Extension Center at Scottsbluff.
This finding indicates that the infection likely occurred back in early July, which historically has been early for us in 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 our plots in 2019.
CLS, a foliar disease of the beet, has long been a problem for sugarbeet production in more humid areas of the United States. But it has been a sporadic, but potentially severe problem for us 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 60-65 degrees Fahrenheit at night and 80-90 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 10-14 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.
We are making this brief report to simply inform everyone about its presence in Scotts Bluff County. 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. We are 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.