PI-009: Seed Potato Tuber Inspection
Appendix 5: Scab
This page is part of the Guidance Document Repository (GDR).
Looking for related documents?
Search for related documents in the Guidance Document Repository
5a: Powdery Scab
Powdery scab is caused by the fungus Spongospora subterranea, which may survive for many years in the soil as resting spores. The fungus attacks roots, stolons, young shoots and tubers of the developing plant, and may penetrate tubers through lenticels or wounds. It is generally spread by planting infected tubers, moving or spreading contaminated soil or manure. Motile spores of powdery scab can also vectors of Potato Mop Top Virus (PMTV).
Powdery Scab symptoms initially appear on young tubers as purplish-brown sunken lesions, developing into faintly brown raised areas which occur singularly or in patches. Within approximately one week, under ideal conditions these areas enlarge to about 6 mm. When mature, these pustules become yellow-brown to black. At harvest and in storage, the pustules dry and break down leaving circular to oval pits which are filled with brownish spore balls. Sometimes these pits can enlarge to form cankers in the tuber, in which a dry sunken rot may develop.
5b: Common Scab
Common Scab, Surface Scab, and Russet Scab are all caused by common soil bacterium, Streptomyces spp. which may survive in the soil for long periods of time even in the absence of potatoes. The bacteria are generally spread in the same manner as Powdery Scab by planting infected tubers, moving or spreading contaminated soil or manure. The strains which cause scab infections produce a toxin which results in the formation of lesions on the tuber. Initial symptoms are small reddish-brown lesions around the lenticels of young tubers. In ideal conditions, the lesions increase in size, turn dark and form circular scabbed areas that are either isolated or in large corky masses. They may be superficial spots or greatly roughened blotches. The scab will appear as both smooth and rough, or may be cracked.
For tuber grade tolerance, Scab is treated in the same manner and under the same tolerance as Rhizoctonia. The total amounts are combined under the same tolerance when both diseases are present. In general, all Scab is treated equally for domestic and some export destinations depending on their current import requirements. In order to approximate the percentage coverage of the tuber, the surface can be divided into three dimensional quadrants as illustrated below (25); each half of the tuber represents 50% of the surface area. The size of quadrant the lesions would occupy if all lesions were pooled into a single area of the tuber will help to determine the disease level for scoring purposes. Examples of trace to severe coverage, based on the exposed side of the tuber, are given below (26).
Description for photo 25
This image is made up of three pictures of the same brown potato. The first picture shows a potato with approximately 22 scabs. The second picture shows the same potato where the scabs have been digitally cut out of the picture leaving the rest of the potato in the picture. The third picture shows the cut out scabs from the picture superimposed on a silhouette of the potato. The scabs have all been grouped in the top left hand corner of the silhouette to show that they cover 12.5 % of the surface area.
|Disease Level||Percentage Coverage|
|Trace||0% - 1%|
|Light||>1% - 5%|
|Moderate||>5% - 10%|
- Date modified: