Rhizoctonia Solani (Black Scurf & Cankers): Potato Disease & Prevention Techniques

What Is Rhizoctonia Solani?

Rhizoctonia Solani And Its Effect On Potato Crops

A potato suffering from Black Scurf (Rhizoctonia Solani)Rhizoctonia Solani is a soil-borne fungal pathogen which targets and attacks potato crops the world over, including New Zealand.

Causing Sclerotia (Black Scurf) on Tubers, as well as Stem and Stolon lesions (Cankers) on younger plants, this fungal pathogen dramatically reduces the quality of both fresh and seed potatoes, and can even impact the establishment of new crops altogether.

Most prevalent in cool, wet soils, the effects of Rhizoctonia Solani have been shown to have a far-reaching, dramatic impact on growers, as well as the New Zealand economy as a whole, resulting in financial losses of up to 20% in some cases. This is most clearly demonstrated by a case in 2011, which saw Black Scurf – caused by Rhizoctonia Solani – significantly reduce the quality of certified seed used, and relied on, by the New Zealand potato industry.

Symptoms & Signs Rhizoctonia Solani May Be Affecting Your Potato Crops

The Signs & Symptoms Of Black Scurf

The initial signs that Rhizoctonia Solani may be affecting your crops is found below the ground, where it takes the form of Black Scurf. Also referred to as Sclerotia – a form of fungus – Black Scurf forms a hard, dark brown or black mass on the surface of the underground tuber, which can range in size from small and flat, to large, raised lumps.

These seemingly innocuous lumps are the resting bodies of the fungus, and are entirely superficial. That is, they don’t penetrate the skin or damage the crop in any way, even while these affected crops are kept in storage. No matter the size or shape of the mass in question.

Though these masses are harmless, they serve to further propagate and spread the disease, particularly if infected tubers are used as seed for future seasons. If used, the fungus proceeds to grow from the seed surface to the newly developing sprout, which can stunt the growth of future potato plants.

The Signs & Symptoms Of Stem & Stolon Cankers

A potato suffering from Black Scurf & Cankers (Rhizoctonia Solani)While Black Scurf may propagate the disease, Stem and Stolon Cankers are Rhizoctonia Solani’s more damaging form. These Cankers are found on the underground parts of potato plants at various stages throughout the season, where they attack germinating sprouts long before they emerge from the soil.

In the early stages, this can present itself as reddish and brown lesions found on stolons and roots, which transform into Cankers as they mature and turn rough, brown and present themselves as cracks and craters.

The effect these Cankers have on crops varies from superficial to severe. At its worse, these take the shape of large, sunken and necrotic lesions that can that can girdle the stem, and impact the normal flow of water as well as carbohydrates throughout the plant.

Cankers can also cause severe issues with the plant’s starch translocation, as infected stolons and stems interrupt the flow of starch from the plant’s leaves through to the tubers.

Small, green ‘aerial tubers’ can form on the stem above the soil as a result, and are usually an indicator that the plant has no saleable quality tubers below ground.

In some cases, these Cankers have also been found to form near the growing tips of these new sprouts, which can kill them outright, or at the very least result in a number of symptoms, including:

  • Poor and uneven stands.
  • Reduced tuber numbers and size.
  • Misshapen tubers.
  • Delayed emergence.
  • Weakened plants, stunted plant growth and reduced crop vigor.

Further signs of the disease are also present in Potato plants that are heavily impacted by these Cankers, which suffer from an inability to grow complete stems, instead producing stolons – also known as ‘No Top’ – with several small tubers attached.

Rhizoctonia Solani Prevention & Management Techniques

Ways To Prevent & Manage Rhizoctonia Solani In Potato Crops

The complete eradication and control of Rhizoctonia Solani

isn’t yet possible, but through the implementation of an effective cultural and crop protection strategy – one that takes an integrated approach to disease management, including educating yourself on each stage of the disease – you can limit its appearance and severity.

  • Use chemical products and controls including targeted, effective fungicide, ground and soil fumigation
  • Plant potato crops in warm, coarse, textured soil and cover with as little soil as possible to ensure crops emerge at the earliest possible time.
  • Increase the rate of crop residue decomposition, as this will – in turn – reduce the pathogen’s growth rate and ability to take hold.
  • Harvest crops as soon as possible: Sclerotia increases as time is left between vines being killed, and plants being harvested.
  • Reduce the amount of fungus and inoculum available to infect future crops by removing or burning leftover vines.
  • Avoid storing or disposing of infected crops on possible future planting sites like fields, as these can act as a new source for the disease.

Images via Down Garden Services & Pacific Northwest Handbooks

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