Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Potato (Solanum tuberosum) -- Silver Scurf
 
Cause: A fungus, Helminthosporium solani, which is spread by infected seed pieces and potato debris in soil. The disease can worsen in storage, particularly following 3-4 months of storage and when humidity is above 90%.
Symptoms: Small, light brown circular spots that tend to grow together and cover large areas of tubers. Sometimes scurf is an olive color from the fungal spores. When lesions are wet, they take on a silvery color that gives the disease its name. Symptoms frequently are found on or near the tuber's stem end. The disease frequently found along with Rhizoctonia (black scurf) disease on tubers. This disease is only a surface problem but never-the-less can cause substantial losses by disfiguring the tuber surface.

Note the small, light-brown circular spots.

Close-up of the affected area, showing conidia borne in whorls.

Close-up of conidia.

 
Cultural control:
  1. Use disease-free seed.
  2. Harvest tubers soon after they mature.
  3. Dry tubers with nonhumidified air when they enter storage.
  4. Storage below 40°F slows disease development but may lower quality of processing potatoes.
  5. Russet-skinned cultivars are damaged less than red or white cultivars.
  6. Rotate out of potatoes at least 3 years before replanting to potatoes.
Chemical control:
  1. Dithane DF Rainshield at 1.25 lb/50 gal water used as a dip for seed piece decay helps control this disease, though it is not labeled for silver scurf.
  2. Evolve Potato Seed-Piece Treatment (Tops MZ-Curzate) at 0.75 lb/100 lb seed pieces.
  3. Maxim or Maxim MZ Potato Seed Protectant at 0.5 lb/100 lb seed (tubers).
  4. Quadris Flowable at 0.4 to 0.8 fl oz/1000 row feet applied in a 7” or less band either in furrow at planting or as a directed spray at cultivation. 4-hr reentry.
  5. Tops MZ at 0.75 to 1 lb/100 lb seed pieces. Tops MZ-Gaucho is registered for use by commercial seed treaters only.
Biological control: Bio-Save 110 and 10LP (Pseudomonas syringe) are registered for control of silver, limited research in Montana and Idaho have shown this biological control to substantially reduce losses. Store product at minus 70oF or on dry ice until used. Add 250 grams of product to 40 gal of water and drip or spray overhead prior to storage.
References:
Strand, L.. 2006. Integrated Pest Management for Potatoes in the Western United States, 2nd edition. UC Agriculture and natural Resources Publication 3316. 167 pp.

Jacobsen, B. 1999. Montana Crop Health Report, Montana State University Extension Service Vol. 12, No. 8.

Content edited by: Phil Hamm and Cynthia M. Ocamb on January 1, 2010
 
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