University of Wisconsin–Madison

Potato Silver Scurf

Silver scurf is a fungal blemish disease of potato tubers caused by the fungus Helminthosporium solani. The pathogen causes tan-to-gray circular lesions typically initiating on the stem-end of the tuber surface, often appearing shiny and silver when wet. Infection reduces both the visual appeal and quality of potato tubers, as the pathogen causes damage to the periderm or skin which enables the onset of other pathogens or enhanced desiccation. This disease is primarily a concern for stored, commercial fresh market potatoes, but the disease can also render potatoes for processing (ie: potato chips) less useful due to increased depth of peeling to remove blemish damage.  Additionally, the resulting reduced moisture in potato tubers can impact frying quality.  Silver scurf can co-occur with and/or sometimes be misidentified as another fungal potato disease, black dot, caused by Colletotrichum coccodes.

Primary Source: Infested soil, debris-borne, infected seed potatoes, infected tubers in storage
Spread: Rain or irrigation washes spores through infested soil during initial spread. Once in storage, warm temperatures and high humidity allow sporulation to occur. Airborne spores can then infect new stored tubers.
Favorable Conditions: High humidity ( > 90%), high temperatures

Disease Cycle

The silver scurf disease cycle begins with primary infection, which can occur when overwintered spores (conidia) are washed through infested soil and plant debris and onto tubers. Other sources of primary inoculum are infected seed tubers, which can spread the pathogen to daughter tubers. The pathogen infects and causes lesions on the tuber, which can produce more conidia that are released into the soil. At the end of the growing season, H. solani conidia can overwinter in soil, and fungal parts can subsist in soilborne crop debris for several years. Once harvested and in storage, the lesions on infected tubers can enlarge and again produce spores (sporulate) in moderate temperatures and high humidity. These spores can then spread and cause secondary infection in stored tubers.

Management

The following practices can help reduce inoculum and spread for this disease:

  • Plant certified seed
  • Rotate away from susceptible crops (2-3 years)
  • Rogue volunteer plants
  • Assess seed prior to planting, and daughter tubers prior to storage
  • Disinfest storage spaces between uses
  • Store at low temperature and humidity
  • Avoid leaving potato tubers in the field for >2 weeks post vine kill
  • Store potatoes in environments with good airflow and temperature control

Late-season management tips

  1. Seed health: Planting certified seed without pathogen is the best management. The use of seed-applied or in-furrow fungicides can reduce the silver scurf pathogen in the potato crop.
  2. Harvest promptly: Delayed harvest after vine kill gives the pathogen more opportunity to infect tubers in the soil.
  3. Handle tubers carefully: Wounds and bruises increase disease entry points and speed blemish development in storage.
  4. Post-harvest fungicides: can be used to reduce viability of the silver scurf pathogen to limit progress and spread of the disease. Some fungicides with documented efficacy include salts of phosphorous acids (ie: Phostrol) applied post-harvest, and Stadium (Syngenta; pre-mix of azoxystrobin, fludioxonil, and difenoconazole).
  5. Sanitation: Clean storage facilities thoroughly to remove residual inoculum.
  6. Storage management: Cool tubers quickly, good airflow, maintain high humidity without free moisture, and store at recommended temperatures to slow disease development.
  7. Monitor storability: Keep close watch on lots destined for longer-term storage, as these disease symptoms can exacerbate on tubers over time. Silver scurf can spread and create new infections. Black dot is not known to spread via spores in storage, but symptoms can worsen. While these diseases may not be as dramatic as late blight or early dying, their subtle but persistent effects can significantly impact storage success and market returns.

Chemical Control

Fungicides can be applied during the production season as well as post-harvest to limit silver scurf on potato. Seed-applied and in-furrow applied fungicides can be of particular usefulness in reducing silver scurf in potato.  Up-to-date Wisconsin-specific conventional seed, in-furrow, and postharvest fungicide information and recommendations can be found in Commercial Vegetable Production in Wisconsin. Or, for home garden fungicide recommendations, see Home Vegetable Garden Fungicides (D0062), a fact sheet available through the UW Plant Disease Diagnostic Clinic website. Always follow label directions carefully.

Resources

  • Commercial Vegetable Production in Wisconsin (A3422). This guide offers the latest recommendations for disease, insect, and weed management in Wisconsin’s most common commercial vegetable crops. Also included are lime and fertilizer recommendations as well as insect identification information and keys.
  • UW Plant Disease Diagnostics Clinic. The University of Wisconsin-Madison/Extension Plant Disease Diagnostics Clinic (PDDC) provides assistance in identifying plant diseases and provides educational information on plant diseases and their control.

References

Silver scurf lesions on a potato tuber
Silver scurf lesions on a potato tuber. Photo Credit: Amanda J. Gevens, UW Extension.
Disease Cycle of Potato Silver Scurf
Disease Cycle of Potato Silver Scurf. Amanda Gevens, UW-Plant Pathology

For diagnostic and management support in Wisconsin, please consider contacting Dr. Amanda Gevens (gevens@wisc.edu) or our Plant Disease Diagnostic Clinic (pddc@wisc.edu).

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