Agriculture: Potato Pest Management Guidelines

Stem and Stolon Canker (Black Scurf)

  • Rhizoctonia solani
  • Symptoms and Signs

    Aboveground symptoms of stem canker include uneven stands, weak shoot growth, and aerial tubers. Foliage may develop yellowing, purpling, and upward curling of leaves. Aboveground symptoms alone are not diagnostic, however, because other diseases can cause similar symptoms. On belowground stems and stolons, Rhizoctonia solani typically causes reddish-brown lesions that often develop into sunken cankers. Stolons can be girdled and killed, resulting in a pruning effect and malformation and abortion of tubers. If tubers in affected fields are left in the ground after vine death, they often develop black scurf, an accumulation of irregular black sclerotia of R. solani on the tuber surface. This is especially visible on fresh market red and white-skinned varieties and is an economically important problem for organic growers.

    Comments on the Disease

    The strain of R. solani that typically affects potatoes, called AG-3, is specific for potatoes and generally does not cause damage in or reproduce on other species. Close relatives of potato, such as black nightshade and tomato, may be exceptions. Rhizoctonia solani is a soilborne fungus, but inoculum of the fungus on seed tubers (visible as dark, irregularly shaped sclerotia) is sometimes more important for disease development than inoculum in soil. In Kern County, AG-3 types of R. solani apparently do not survive in soil between crops of potatoes; whereas in Tulelake districts, R. solani AG-3 may overwinter in soil. The fungus only infects juvenile tissue. Disease development is favored by relatively wet, cool (55° to 60°F) soils.


    Where R. solani AG-3 survives in soil between potato crops, rotate out of potatoes for two to three years to reduce soilborne inoculum. Sugarbeet, however, has been associated with increased severity of stem canker in subsequent potato crops.

    Reduce initial inoculum by using certified seed tubers that are free from sclerotia of the fungus.

    Fungicide treatment of seed tubers can reduce R. solani inoculum borne on the seed pieces. The significance of this benefit may be small in fields where heavy soil infestations of R. solani AG-3 persist between potato crops.

    Rhizoctonia stem canker can be further reduced by favoring rapid emergence, such as warming seed tubers before planting, planting tubers at a relatively shallow depth, avoiding early planting dates when soil temperatures are cool and avoid overwatering.

    Apply fungicides to seed pieces and/or in-furrow at planting to reduce the disease incidence on belowground stems and stolons. Research indicates that fungicide applications after planting can reduce tuber blemish on daughter tubers. Daughter tubers may also be protected by harvesting quickly after vine desiccation rather than holding tubers in soil for extended periods. However, this may lead to insufficient skin formation and increases the risk of bruising and skinning problems.  

    Common name Amount per acre REI‡ PHI‡
    (Example trade name) (hours) (days)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
      (Moncut) 0.71–1.1 lbs per acre 12 NA
      MODE OF ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitors (7)
    COMMENTS: For in-furrow use only. Do not apply through irrigation system. Medium to high risk for resistance development.
      (MonCoat MZ) 0.75–1 lb/100 lb cut seed pieces 24 NA
      MODE OF ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitors (7) / dithiocarbamates and relatives (M3)
      (Vertisan) 0.7–1.6 fl oz/1000 row feet 4 7
      MODE OF ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitors (7)
    COMMENTS: For in-furrow use and after planting. Medium to high risk for resistance development.
    (Maxim PSP) 0.5 lb/100 lb cut seed pieces 12 0
    MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylpyrrole (12)
    (Quadris) 0.4–0.8 fl oz/1000 row feet 4 14
    MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitors (11)
    COMMENTS: For in-furrow use and after planting. High risk for resistance development.
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
    1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action group number.
    NA Not applicable
    Text Updated: 03/19
    Treatment Table Updated: 03/19