U of I researchers target mounting fungal threat to Idaho potato industry

University of Idaho Extension soil sample testing reveals that three-quarters of Idaho potato fields are now infected with some level of the pathogen Spongospora, which can cause the fungal disease powdery scab.

Spongospora is also a host that transmits the potato mop-top virus (PMTV), though not all of the infested soils in Idaho have yet reached a threshold to where the diseases will occur.

The two diseases are increasingly culpable for fresh potato packing facilities and international trade partners rejecting U.S. potatoes, as well as processors docking prices paid to growers.

Phillip Wharton, an associate professor in plant pathology, and James Woodhall, an Extension specialist and associate professor in plant pathology, hope to find answers for potato growers regarding these increasingly problematic diseases through a planned research project to be funded by a two-year, $130,000 Specialty Crop Block Grant (SCBG). The U.S. Department of Agriculture finances the SCBG program, and the Idaho State Department of Agriculture administers it.

Wharton and Woodhall will evaluate cultural and chemical control measures to develop new management strategies for reducing crop infections, such as irrigation management, variety resistance and testing the efficacy of fungicides and other products that have been reported to be effective against powdery scab in other regions.

“Powdery scab is one of the potato diseases that people really haven’t been able to get any kind of a handle on in terms of control,” Wharton said, adding planting clean, certified seed is among the best options for growers.

If a field has a history of powdery scab, the grower can get the soil tested by Woodhall’s lab to ascertain levels.

The Idaho Potato Commission (IPC) and the Northwest Potato Research Consortium placed helping growers combat powdery scab and PMTV among their top research priorities for 2025. The new U of I research should be especially important for producers of Idaho potatoes shipped to Mexico, which is now the largest market for U.S. potatoes. Mexico had long restricted shipments of fresh U.S. potatoes to within 16 miles of the U.S. border prior to opening access to the entire country in 2022.

The grant application included letters of support from IPC and J.R. Simplot Co.

“When we ship our potatoes to Mexico, powdery scab is one of the things that they look for and sometimes find,” said Travis Blacker, vice president of research and industry relations with the IPC. “We’re looking at ways to control that better, so we don’t have these finds when we ship potatoes to Mexico.”

The researchers suspect milder winters and wetter springs may play a role in the spread of Spongospora in Idaho soils, noting a study from Australia showed a correlation between early spring moisture and infections later in the season.

Recent surveys by Woodhall have shown that the prevalence of Spongospora has hugely increased in the past decade.

“In 2013, we did a large-scale survey of Idaho fields in collaboration with Simplot. At that time, 20 to 30% of fields had powdery scab,” Wharton said.

Currently, as much as 75% of Idaho potato field soils contain some level of Spongospora, and a third of them harbor PMTV.

“More recently people have been requesting a lot of tests for powdery scab,” Woodhall said. “We’re testing hundreds of samples per year for the presence of this organism.”

Powdery scab symptoms can include discoloration, gall-like growths on roots, ruptured pustules on the tuber surface and internal lesions in tubers. The disease impacts potato marketability rather than yield. PMTV can cause rust-brown arcs to form within tuber flesh, tuber necrosis, reduced yields and raised rings on the tuber surface. Certain potato varieties may be asymptomatic for both diseases.

The researchers will conduct research trials at the U of I Aberdeen Research and Extension Center to assess the efficacy of combining less susceptible potato varieties with various fungicide combinations in enhancing disease control. A review of products demonstrating efficacy against powdery scab in other potato-growing regions is currently underway, and the selected products will be utilized in the field trials in 2026 and 2027. Additionally, irrigation management trials will be conducted at the U of I Parma Research and Extension Center, where specific fields exhibit high disease pressure. They hope to identify the window of time when managing soil moisture is most critical for disease control.

Past SCBG-funded research from Woodhall’s lab led to the development of new soil tests for Spongospora and PMTV. The researchers will use these tests to assess levels of the diseases in the research trials.