April 10, 2003

 

NDSU Plant Pathologists Focus On Black Dot

Increased irrigated potato acres in North Dakota and Minnesota are favorable for producing potato varieties demanded by processors but they are also favorable for soil borne pathogens, such as the black dot fungus.

According to Neil Gudmestad, North Dakota State University plant pathologist, "Economic loss due to black dot in North Dakota and Minnesota in 2002 was very high. There is a great need to develop management programs that will provide relief to producers." In 2002, an estimated $15-25 million was lost in the two states based on the processed value of potatoes. North Dakota and Minnesota each have two potato processing plants creating a greater demand for the production of potato varieties suitable for processing.

NDSU is currently looking at the biology of the organism and studying the pathogen diversity. "We can not breed for resistance until we understand the fungus and the genetic diversity," Gudmestad says. Work is also being done to determine the factors that affect infection, which parts of the plant contribute to the most economic loss after infection, the importance of each inoculum source and the effects of cultural practices on disease development. Until conclusions are made concerning this research Gudmestad recommends management strategies for producers to control the occurrence of black dot:

• Avoid seed with greater than 15 percent infection
• Maintain crop residues or cover crops in sandy soils to avoid wounding from soil blowing early in crop development
• Avoid stress to the crop which would include low nitrogen, herbicide injury or moisture stress
• Use a strobilurin based fungicide before row closure and every 14 days for at least four applications during the season alternated with another fungicide of differing chemistry

Gudmestad says that black dot has always been a concern for potato producers, but its occurrence fluctuates greatly from year to year depending on the environmental conditions in any one growing season. An increase in irrigated potato acreage has caused an increase of the disease in recent years. "Growers have increased their acres of potatoes without being able to expand their acres of irrigated land suitable for potato production," Gudmestad says. This has resulted in more crops of potato on the same land and soil borne organisms, such as the black dot fungus, have increased in population over time.

The fungus Colletotrichum coccodes causes black dot. The disease is difficult to detect, as the symptoms are similar to more common potato diseases. Black dot is particularly devastating because it affects all parts of the plant. On potato foliage the symptoms are nearly indistinguishable from early blight. As black dot infects the tubers it produces blemishes that are easily mistaken for silver scurf. Black dot infects the roots of the potato causing them to rot and it is also capable of invading the vascular tissue causing wilt. Black dot is most devastating when it infects potatoes in conjunction with another organism such Verticillium or Fusarium. Black dots (black sclerotia) form on the stems injuring them and giving the disease its name. "When black dot is present, the plant wilts and dies much more rapidly than with other diseases causing severe economic losses," Gudmestad says.

Weather conditions are key to the development and survival of the disease. According to Gudmestad, soil saturation prior to row closure promotes high levels of the pathogen to germinate and infect. This can occur from heavy precipitation or irrigation. Later in the season, higher air temperatures cause the plant to collapse more rapidly cutting production.

In addition to black dot, NDSU is currently conducting research on ring rot, Potato virus Y, late and early blight, Rhizoctonia canker, Verticulum wilt, pink rot and leak.

For more information on black dot and other potato diseases contact Neil Gudmestad at (701) 231-7547 or neil.gudmestad@ndsu.nodak.edu.

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Source: Neil Gudmestad, (701) 231-7547, neil.gudmestad@ndsu.nodak.edu
Writer: Deana Oe, (701) 231-8371, deana.oe@ndsu.nodak.edu
Editor: Tom Jirik, (701) 231-9629, tjirik@ndsuext.nodak.edu