Potato XXII

Common Scab

Howard F. Schwartz and David H. Gent

Identification and Life Cycle

Common scab is caused by at least three species of filamentous bacteria in the genus Streptomyces, and occurs in most potato production areas of the world. The pathogens resemble fungi in culture, but unlike most bacteria, produce spores. S. scabies is probably the most important species causing common scab in the High Plains region. Disease is initiated when soilborne or seedborne bacteria infect tubers through lenticels and produce toxins that cause excessive cell growth and death. Disease is most severe in warm, dry soils. The common scab pathogens can infect many diverse seedlings, but the role of this infection is unclear in the epidemiology of the organism. The pathogens survive between potato crops in seed tubers, soil, and perhaps pathogenically on other plants.

Plant Response and Damage

Common scab symptoms occur almost exclusively on tubers, but Streptomyces spp. reportedly infect stolons as well. Early disease symptoms on tubers are browning and excessive cell growth, but mature lesions are 0.2 to 0.4 inches and circular to irregular in shape. Lesions often grow together with time. At harvest, lesions may be superficial (russet scab), slighty raised (erumpent scab), or sunken (pitted scab). The types of lesions that develop are probably related to the strain of Streptomyces, environmental conditions, and timing of infection. Common scab does not reduce yield, but reduces tuber quality and grade.

Management Approaches

Biological Control

No biological control practices have been developed for common scab.

Cultural Control

Plant high quality seed free from common scab. A three to four year rotation to other crops provides some disease suppression, but does not eliminate the pathogens from soil. Varieties vary widely in their disease reaction, and resistant or less susceptible varieties should be planted if available. Avoid high pH soils and amendments that increase soil pH. Maintaining high soil moisture (near field capacity) for four to six weeks beginning at tuber initiation will reduce disease incidence and severity.

Chemical Control

Seed treatments provide some control of common scab.

Common/Trade Name	Rate	Remarks
Maneb
Maneb 75 DF	1 lb per 10 gal of water	Dip whole or cut tubers, spread treated seed pieces in a cool place if held before planting.
Maneb 80 WP	1 lb per 10 gal of water	Dip whole or cut tubers, spread treated seed pieces in a cool place if held before planting.
Seed Treatment for Potatoes LD (plus Streptomycin Sulfate)	1 lb per 100 lb of seed	Apply for thorough coverage to whole or cut seed.
Mancozeb
Dithane DF	1.25 lb per 50 gal of water	Dip whole or cut tubers, spread treated seed pieces in a cool place if held before planting.
Dithane F-45	1 qt per 50 gal of water	Dip whole or cut tubers, spread treated seed pieces in a cool place if held before planting.
Dithane M-45	1.25 lb per 50 gal of water	Dip whole or cut tubers, spread treated seed pieces in a cool place if held before planting.
Dithane WSP	1.25 lb per 50 gal of water	Dip whole or cut tubers, spread treated seed pieces in a cool place if held before planting.
Manzate 75 DF	1.25 lb per 50 gal of water	Dip whole or cut tubers, spread treated seed pieces in a cool place if held before planting.
Penncozeb 75 DF	1.25 lb per 50 gal of water	Dip whole or cut tubers, spread treated seed pieces in a cool place if held before planting.
Penncozeb 80 WP	1.25 lb per 50 gal of water	Dip whole or cut tubers, spread treated seed pieces in a cool place if held before planting.
Potato Seet Treater 6%	1-1.3 lb per 100 lb seed	Apply for thorough coverage to whole or cut seed.
Potato Seet Treater PS	1 lb per 100 lb seed	Apply for thorough coverage to whole or cut seed.
Metiram
Potato Seed Treater	1-1.5 lb per 100 lb seed	Apply for thorough coverage to whole or cut seed.
The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.

Categories: Potato, Disease, Common Scab

Date: 03/29/05