University of Idaho research tackles a growing black dot threat with timing-based management tools

MOSCOW, Idaho — A University of Idaho scientist has published management recommendations based on a decade of research to help Idaho potato farmers protect their crops against an increasingly troublesome fungal disease called black dot.

Phillip Wharton, an associate professor of plant pathology, compiled findings from several black dot studies conducted from 2011 to 2022 into a single paper. The journal Plant Health Progress published the paper online on March 31.

Black dot is caused by the fungus Colletotrichum coccodes and is named for the abundant black dots that form on tubers, roots, stems and stolons, which are horizontal plant stems, commonly known as runners, that form clonal plantlets. Symptoms appear late in the season, after the crop canopy fully covers the rows, a stage known as row closure. Stem lesions typically form around the base of leaf petioles, which are the slender stalks that attach a leaf blade to a plant stem, and they begin as small brown spots and can grow together into larger lesions that may surround the stem. As lesions mature, they develop centers that range from circular to irregular in shape and appear white or straw-colored.

Based on his research, Wharton advises potato farmers to shorten their growing season to reduce black dot infections that appear in storage. His research shows this can be done without sacrificing yields.

“For growers, the big takeaway is that black dot has to be managed in the field,” Wharton said. “By the time you see it in storage, the infection has already happened.”

In response to rising demand among farmers for information about black dot, Wharton reexamined previously unpublished research and combined those findings with results of replicated fungicide field trials in 2021 and 2022. The paper covers soil testing, cultural control methods, growing season length, storage practices and fungicide timing.

Wharton believes several factors may be causing increased problems for growers, including potentially fungicide-resistant strains, new potato varieties and outdated management and storage practices.

Black dot reduces potato quality, mars their appearance and can increase processing waste. It tends to be uniformly distributed throughout a field, and the fungus can remain viable in the field for up to a decade. It produces spores on dead plant material in the soil during the spring and spreads via water droplets that splash on emerging plants.

“In the past couple of years, I noticed that I was being asked to present my work on black dot a lot more often, which suggests it’s becoming more of an issue,” Wharton said. “This is something we want to address: Why is it becoming more of an issue?”

In a 2011-12 project funded by the Idaho Potato Commission, Wharton adapted diagnostics developed in the United Kingdom for black dot detection for use in Idaho soil testing.

Wharton also began trials to determine how adjusting the length of the growing season affected how often black dot occurs and how severe it becomes. He planted fresh-market potato varieties and harvested them between 100 and 130 days after half of the plants had emerged.

Though he saw no noticeable yield difference between the harvest dates, black dot was more common and more severe in the potatoes harvested after 120 days. Among potatoes harvested between 100 and 110 days, 0-17% had noticeable levels of black dot, with symptoms covering 5-10% of the tuber surface. By contrast, about 55% of potatoes harvested after 120 days had black dot, with the disease covering 10-20% of the tuber surface.

In a 2012 soil survey of southern Idaho fields, nearly half of samples tested positive for black dot.

From 2012-22, Wharton conducted trials for managing black dot in the field and in storage. He and his research team discovered black dot does not spread in storage. Instead, potatoes that show symptoms in storage were infected before vine kill in the field. Wharton advised growers to promptly cool potato cellars after loading them and carefully manage humidity to slow development of symptoms.

Based on data from his field trials conducted from 2018 through 2022, Wharton concluded fungicide applications early in the season are much more effective than those applied later. He found growers who apply fungicides between sprouting and when plants are 6 to 8 inches tall see better disease control and fewer storage problems. Wharton saw similar results with all fungicides registered for black dot control, suggesting timing matters more than the specific product used.

Based on his many studies of Colletrotrichum diseases in potatoes and other crops, Wharton suggests plants can naturally resist infection during early growth stages. After row closure — when plants stop growing and focus on tuber development — these defenses weaken. This allows the pathogen to grow, infect stems and produce material that washes into the soil and infects developing tubers.

Wharton plans to share his black dot research in an Extension bulletin later this year.

Media contact  

Phillip Wharton
Associate professor Department of Entomology, Plant Pathology and Nematology
208-844-6317
pwharton@uidaho.edu