Major disease in Potato Management of Potato Scab

1. COLLEGE OF AGRICULTURAL TECHNOLOGY
(Affiliated to Tamil Nadu Agricultural University, Coimbatore-3)
Kullapuram (Po),Via Vaigai Dam, Theni-625 562
COMMON SCAB OF POTATO
Course teacher: Student:
Dr. Parthasarathy S, Sangeethagomathi R,
Asst. Professor, 2015021114.
(Plant Pathology).

2. Content
• History
• Occurrence and distribution
• Significance
• Symptom
• Lifecycle
• Management
• Reference

3. History
• Common scab of potato is caused by Streptomyces
scabies.
• The pathogen was first reported as Oospora scabies
by Thaxter in 1891.
• The name Streptomyces scabies is given by
S.A.Waksman.

4. Occurrence and distribution
 Common scab of potato, which has worldwide
distribution.
 This disease in more severe in high plain areas and
cool hilly tracts of the north.
 This disease widespread in the plains of Uttar
Pradesh, Haryana and Punjab.

5. Significance
• Common scab is soil borne disease occurs in most
areas where potatoes are grown.
• It is a major production problem that affects tuber
grade quality and has only a small effect on total
yield.
• In many fields the percentage of scab bearing tubers
has been 40% or more.

6. Symptoms
• Dark brown, pithy patches that may be raised and
“warty”.
• These lesions can affect just a small portion of the tuber
surface or may completely cover it.
• Sometimes the ridged portions are in broken concentric
rings.
• Affected tubers show shallow or deep scabs.

7. • Slight protuberances with depressed centres may
form and are covered with a small amount of corky
tissue.

8. Depressed centres surround with corky tissues

9. Dark brown pithy lesions on potato tubers

10. Disease cycle
• Common scab of potato is an efficient saprophyte
living on surface of tubers, soil and crop residues.
• Soil borne disease caused by the filamentous bacteria
Streptomyces scabies.
• The toxin that causes the common scab symptoms is
called thaxtomin.

11. • As the spore mature, they develope a grey or
melanized pigmentation.
• When a spore comes into contact with a suitable host,
it will germinate and the infection process begins.
• Optimum temperature is 25°-30°C.
• The pathogen invades lenticels and also grow on then
surface of wounds.

12. Pathogen
• The mycelium of the pathogen is slender, branched
with few or no cross walls.
• The spores are cylindrical produced on special
hyphae develop across the walls from tip and spores
are pinched off.
• S. scabies is a gram positive and aerobic in nature.

13. Systematic position
Kingdom : Monera
Phylum : Actinobacteria
Class : Actinobacteria
Order : Actinomycetales
Family : Streptomycetaceae
Genus : Streptomyces
Species : S. scabies

14. Favourable condition:
 Severe in neutral or alkaline soils. Disease increases
with increase in alkalinity.
 Warm and dry soil conditions, increases the
incidence.

15. Mode of spread:
 Soil borne, and spread through seed tubers, wind,
water.
 Also spread through digestive tract of animals.

16. Management
 Decrease the soil pH by adding elemental sulfur. The
disease is suppressed at soil pH levels of 5.2 or lower.
 Keep the soil damp for 2-6 weeks. This technique is
effective because high soil moisture promotes growth
of bacteria that can crowd out S. scabies on the potato
surface.

17.  Crop rotation with legumes are useful in reducing the
severity of the disease.
 Seed treatment with mancozeb provide control over
common scab.
 Green manuring of potato fields reduces the
incidence of common scab.
 Application of gypsum prior to planting can also
reduce the disease incidence.