Black dot disease can lead to yield and quality losses in potatoes and is more widespread than one may expect. Black dot may not be top-of-mind for farmers as it has been under-studied, can be difficult to recognize when it is present in their fields, and is often mis-diagnosed.

Based on soil surveys conducted in 2019 and 2020¹, black dot was shown to be present in all major potato growing provinces. “It’s time to consider management strategies to protect your potato crop from black dot” says Bill Moons, a Market Development Agronomist at Bayer. Moons highlights three key steps to combat the damaging effects of black dot on your potato crop: prevention, stem protection and identification.

Know what to look for and identify it early

Black dot is caused by the fungus Colletotrichum coccodes, which can infect potato roots, tubers, stems and foliage. Black dot gets its name from the small, black sclerotia that form on infected plant tissue. Symptoms are similar to verticillium wilt and many other common potato diseases. Unlike these other diseases, however, black dot multiplies much faster and can take down crops much quicker.

Black dot can be seed-borne but it’s the soil-borne pathogen that does the most damage, as these spores are easily spread by wind. The pathogen can enter the plant from underground (seed, roots and stems) or above ground (stems and leaves). Black dot spores can survive on plant debris in the field or move field to field on infected seed tubers and on blowing grains of sand.

“It tends to appear first on stems,” says Moons. “When you look closer, you can see the dark brown lesions filled with tiny sclerotia. Later in the season you’ll see all these bleached out, white stems that are not feeding the tuber. They’re a big contributor to the Early Die complex,” he adds.

Black dot lesions appear early in the season
Large black dot lesions result in collapsed stems 
Left: Black dot lesions appear early in the season. Right: Large black dot lesions result in collapsed stems. (Both images courtesy of Laura Aarts, Bayer CropScience). These images were taken in southern Ontario.

Black dot can also cause the crop canopy to yellow or wilt in mid- to late-summer, so if you see this symptom don’t assume it’s verticillium wilt. Scout weekly and thoroughly to look for those black dot lesions.

Start with the soil to prevent black dot

Black dot gets its start early in the season, where the fungus overwinters as sclerotia either on tuber surfaces or plant debris. Your first line of defense should be an in-furrow application of Velum® Prime to provide early-season protection and avoid early foliar spray timing concerns.

If you see sclerotia on stems before canopy closure, apply a foliar fungicide to mitigate damage to the stems of healthy plants.

Stem protection key to combating black dot

You need to take action to combat the threat of black dot. Moons emphasizes the importance of scouting weekly and closely examining random plants to look for black dot lesions, particularly where soil is sandy, or drainage is poor.

Foliar fungicides should be applied before canopy closure to get good coverage down onto the stems and provide early protection–spraying before 70 per cent ground cover is a good target. Anything later may deliver diminishing returns.

Proline® GOLD fungicide, now registered on potatoes in Western Canada, and Propulse® fungicide, now registered on potatoes in Eastern Canada, are excellent fungicide choices for protection against black dot, as well as control of early blight, brown leaf spot and white mould. Both active ingredients in the Proline GOLD and Propulse formulations contribute to suppressing black dot.

Protect your investment through prevention

Awareness and proactivity are essential to managing black dot. If you know when black dot is a threat and what to do when you find it, you can protect your investment in your potato crop.

The best way to minimize black dot is through good crop rotation, crop fertility and soil health. “If you have a field you consider to be ‘tired’, meaning that previous potato crops did not live up to expectation, it’s likely that black dot is present in the soil, amongst other potential pathogens” says Moons. In those fields, it’s important to monitor the crop closely and consider curative treatments.