A decade-long research effort led by Phillip Wharton at the University of Idaho is providing potato growers with updated management strategies to address the increasing incidence of black dot disease, a soil-borne fungal infection affecting tuber quality and storage performance.

The findings, published in Plant Health Progress on March 31, consolidate research conducted between 2011 and 2022, alongside more recent fungicide trials. The work responds to growing concern among Idaho producers over the rising prevalence of the disease.

Black dot, caused by the fungus Colletotrichum coccodes, manifests as dark lesions on tubers, stems, roots and stolons. While symptoms typically become visible late in the growing season or during storage, Wharton’s research confirms that infection occurs earlier in the field.

“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.”

The research highlights growing season length as a key control variable. Trials conducted on fresh-market potato varieties showed no measurable yield difference across harvest windows between 100 and 130 days after emergence. However, disease incidence increased significantly with later harvests. Potatoes harvested between 100 and 110 days showed infection rates of 0–17%, while approximately 55% of tubers harvested after 120 days exhibited symptoms, with greater surface coverage.

These findings support a shift toward shorter growing cycles as a practical method to limit disease severity without compromising productivity.

The study also confirms that black dot does not spread during storage, but rather develops from infections established prior to vine kill. As a result, storage management remains important for slowing symptom progression rather than preventing infection. Rapid cooling and controlled humidity in storage facilities were identified as contributing factors in reducing visible damage.

Fungicide timing emerged as another decisive factor. Field trials conducted between 2018 and 2022 demonstrated that applications made early in the crop cycle — from sprouting through early vegetative growth — were consistently more effective than later treatments. Results were comparable across registered fungicides, indicating that application timing plays a more critical role than product selection.

Wharton attributes the effectiveness of early interventions to the plant’s natural resistance during initial growth stages. Once canopy closure occurs and plants shift focus to tuber development, these defenses weaken, allowing the pathogen to infect stems and subsequently spread to developing tubers via soil.

The research also points to broader structural factors behind the disease’s increased impact, including possible fungicide resistance, varietal susceptibility and evolving agronomic practices.

Earlier diagnostic work, including a 2011–12 project supported by the Idaho Potato Commission, adapted UK-developed detection methods for use in Idaho soils. A subsequent 2012 survey found that nearly half of sampled fields in southern Idaho tested positive for black dot, underscoring its widespread presence.

Wharton indicated that further guidance will be made available through an Extension bulletin later this year, aimed at translating research findings into practical recommendations for growers.

The disease’s long persistence in soil — with viability of up to a decade — combined with its impact on tuber appearance and processing waste, continues to position black dot as an emerging management priority within potato production systems.