Black dot (Colletotrichum coccodes)

Black dot: biology

The black dot fungus has a wide range of hosts and is commonly found in most soils where potatoes are grown. The fungus is both tuber- and soil-borne, and persists as sclerotia on plant debris in the soil or on tubers. It enters potato plants through a wound. If it enters via underground stems, it will move upward in the plant; if it infects injured leaves or stems, it will move downwards into stems and roots.

Black dot is often mis-diagnosed as it is similar in appearance to early blight, Verticillium and nutrient deficiencies. Black dot causes top leaves in the potato canopy to turn yellow and wilt in mid to late summer. Primary diagnostic symptoms include tiny dot-like sclerotia on stems above and below ground on lower stems, tubers, stolons and roots. During harvest, remnants of infected stolons may remain attached to tubers and infected tubers exhibit greyish surface lesions that resemble silver scurf. Infected stolons, roots and stem pith may turn reddish-purple. Black dot is easily identified by silver-black lesions on lower stems and petioles at the end of the season.

While black dot will not kill plants outright, it will affect plant health. It also allows other diseases to impact the plant. Direct impacts from black dot include:

• Losses in yield and quality of tubers
• Decreased number of large tubers
• Reduction in the weight of tubers
• Foliar symptoms may contribute to premature death of potatoes