In this Sept. 27, 2017, file photo potatoes await harvesting at Green Thumb Farms in Fryeburg. Credit: Robert F. Bukaty | AP

PRESQUE ISLE, Maine — The rise in rejected seed potatoes from last season’s crop can be blamed on new strains of an old virus, a Maine Potato Board plant pathologist said on Feb. 18.

The Maine Potato Board announced recently that 60 percent more seed potato lots tested in the 2019-2020 Maine Department of Agriculture, Conservation and Forestry’s annual post-harvest test were rejected than in 2018-2019. A total of 7.3 percent of all seed potato lots were rejected.

It was the highest rejection rate for a Maine post-harvest test since 2012-2013. But the results were emblematic of a more significant problem experienced by farmers across the country: new strains of potato virus Y that are harder to spot during production than previous strains, Maine Potato Board plant pathologist Andrew Plant said.

More than 5 percent of the samples tested must be positive for either potato virus Y or potato leafroll virus in order for a lot to be rejected.

Credit: David Marino Jr.

Plant, who tests crops from around Maine for viruses in the post-harvest test, said the primary cause of these rejections was recombinant strains of potato virus Y. He singled out potato virus Y strains N-Wilga and NTN, which he said have become more common within the potato industry in the past 10 years.

Potato virus Y is a pathogenic virus whose presence Plant said is one of the biggest issues facing the potato industry. The virus can make potatoes at risk to contain rot — dark spots on the potatoes — decrease yield and potentially cause the complete death of crops.

These new strains have displaced the previous cause of rejections — potato virus Y strain O. That strain had a higher likelihood of killing the potato plants it infected, but it was also far easier for growers to spot than more recent strains.

Potatoes infected with the previous strain could be easily pulled out by farmers to prevent the spread of disease because they were visually obvious — a change in the color of leaves. In contrast, potatoes infected with new strains are virtually indistinguishable from healthy potatoes.

Plant said that the work being done at his lab, and other post-harvest tests across the United States, play the most important role in being able to combat the development and spread of such viruses by detecting the presence of viruses in seed potatoes before they can be utilized to grow more crops.

He said that farmers can help to prevent potato virus Y during the seed potato process by using insecticides and spraying minerals, helping to prevent the presence of aphids — small sap-sucking bugs whose presence is one of the primary ways in which potato virus Y can spread — on the crops.

Despite the popular understanding of the word “virus,” Plant said potatoes containing potato virus Y are not harmful to humans.

“It makes potatoes unmarketable,” Plant said. “The effects on humans are ‘geez, that doesn’t look good to eat.’”

He said the high number of rejections, along with similar results across the country, would reduce the nationwide seed potato supply over the next few years.

Plant said growers will likely be able to find “alternative markets” for the rejected potatoes, such as selling them to be used in french fries or potato chips by potato processors, but would probably take a cut in profits as those ventures are less marketable than seed potatoes.

The post-harvest test is an intricate and multi-step process conducted in the basement of the Maine Potato Board in Presque Isle. The lab tests potatoes from across Maine, with new technology that makes it far easier to spot the presence of viruses within crops.