April 2010
Reining in Rhizoc

Rhizoctonia solani, more commonly known as black scurf, is a common fungus that attacks potatoes in the early season of their development. The pathogen is present in all potato growing areas.

The fungus creates black splotches, spore bodies, on the tuber skin.

The splotches, called sclerotia, are more of a cosmetic problem that can be washed off during a tuber’s early stage of development.

The fungus does not require a host potato to survive. It can persist in the soil for years. Ideal conditions for rhizoctonia are cool temperatures and moist soil.

Bingham County Extension educator Bill Bohl, out of Blackfoot, Idaho, recommended a minimum soil temperature of 45 F for planting.

Ideally, 50 F is even a better soil temperature,” Bohl said, “but you know good and well a farmer has to make the best use out of the equipment. If he has one planter and lots of acres he’s going to start planting when it’s a lot cooler.”

Seedpiece treatments combined with in-furrow treatments for black scurf is probably the most aggressive approach to the problem, said Phil Nolte, Universtiy of Idaho potato seed specialist wrote in an email response to questions about black scurf.

Green, emerged stems are far less susceptible than white, below grounds stems before emergence. Anything that encourages rapid emergence, shallow planting with hilling later helps reduce rhizoctonia, Nolte said.

Harvesting soon after the vines are killed can also minimize black scurf. Harvesting as soon as possible after skin set significantly reduces tuber scurf. Sclerotia do not form and grow in storage.

Ironically, certified seed may or may not help as far as rhizoctonia is concerned. Seed is rarely, if ever, rejected due to black scurf, Nolte said.

Crop rotation of three to four years can also greatly reduce soilborne rhizoctonia, said Nolte.

Other research advises growers to avoid growing sugar beets as a rotation crop, that sugar beets tend to increase rhizoctonia problems and to avoid a rotation with buckwheat prior to planting potatoes because rhizo colonizes mature buckwheat stems.

Growers are advised to manage rhizoctonia by monitoring their fields, to keep records of rhizoctonia existence and severity.

A Washington state study in 2003 found that mustard green manures might offer farmers an equally effective and less expensive alternative to fumigants.”

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