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University
of Rhode Island GreenShare Factsheets
Potato
Diseases: Scab
Scab is a disease of potato tubers resulting in lowered
tuber quality due to scab-like surface lesions. There are no
above-ground symptoms. Two forms of scab occur. Common scab occurs
in all production areas and is most severe in soils with a pH
above 5.5. Another less common form, called acid scab, is important
in acidic soils (below pH 5.5).
Scab symptoms are quite variable. Usually roughly circular,
raised, tan to brown, corky lesions of varying size develop randomly
across tuber surfaces. Russet scab occurs as a rather superficial
layer of corky tissues covering large areas of the tuber surface.
Pitted scab occurs where lesions develop up to 1/2 inch deep;
these deep lesions are dark brown to black, and the tissues underneath
are often straw-colored and somewhat translucent. More than one
of these lesion types may be present on a single tuber. Although
scab symptoms are usually noticed late in the growing season
or at harvest, tubers are susceptible to infection as soon as
they are formed. Small brown, water-soaked, circular lesions
are visible on tubers within a few weeks after infection. Mature
tubers with a well-developed skin are no longer susceptible,
but existing lesions will continue to expand as tubers enlarge,
increasing disease severity throughout the growing season. Scab
is most severe when tubers develop under warm, dry soil conditions.
Coarse-textured soils that dry out quickly are therefore more
conducive to scab than are fine-textured soils.
Several other conditions can be confused with scab. White,
enlarged lenticles, which frequently occur on potato tubers harvested
from wet soil, can be mistaken for scab. Usually this condition
will disappear when tubers are dried. Patchy russeting, checking
or cracking of tuber surfaces caused by the fungus Rhizoctonia spp.
also may be confused with russet scab. A very different and uncommon
disease called powdery scab, caused by the fungus Spongospora
subterranea, causes very similar scab-like symptoms. Laboratory
examination may be necessary to identify these diseases.
Scab is caused by a group of filamentous bacteria called
actinomycetes which occur commonly in soil. In soils with a pH
above 5.5, Streptomyces scabies is usually responsible
for common scab and is capable of causing all the types of scab
lesions described above. It is commonly introduced into fields
on seed potatoes and will survive indefinitely on decaying plant
debris once the soil is contaminated. The organism can also survive
passage through the digestive tract of animals and be distributed.
Control in the vegetable garden for the most part involves
correcting the conditions most favorable for scab production.
• Soil pH and fertilizer choice are very important. Keeping the soil
pH at or below 5.2 will suppress scab. Sulfur can be applied to the soil to lower
the pH and make it more acidic. Acid-forming nitrogen fertilizers, such as ammonium
sulfate and diammonium phosphate, are more effective in reducing scab than ammonium
nitrate, while calcium and potassium nitrate can raise soil pH and favor scab
development. Fresh barnyard manure applied to the soil can create a scab problem
and the pathogen may persist in the soil for many years afterward.
• Follow a crop rotation schedule when scab is a problem. Plant
at least three years of nonsusceptible crops between potato crops. Scab infection
builds up following frequent crops of potatoes, beets, radishes, turnips, carrots,
rutabagas and parsnips. Rotations including rye, alfalfa or soybeans may reduce
scab severity.
• Keep soil moist during early
tuber development (for about 2 weeks after the plants emerge
from the soil). Avoid overwatering,
as it may cause rotting or poor plant growth.
• Varietal resistance: Plant certified seed potatoes of the
russet-skinned varieties, which are more resistant to scab. The variety Nooksack
is highly resistant, while Russet Burbank, Netted Gem and Norgold have moderate
resistance. A red variety, Red Norland, is also moderately resistant.
• Fungicide Seed Treatment: In situations where growers are planting
in ground not planted previously with potatoes or where the field is known to
be scab-free, treat seed tubers with a fungicide seed treatment to reduce scab
introduction through the seed pieces.
Adapted from the University
of Vermont Extension and the Ohio State University Extension,
2000
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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