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Distinguish between Powdery - and Common scab

Powdery scab and Common scab are potato blemishes that both result in tubers being rejected or downgraded on the markets and certain levels of seed infection result in rejection of seed batches. The symptoms of the diseases look similar, but the pathogens and epidemiology of the diseases differ substantially and thus require contrasting management strategies. This article will help distinguish between the two diseases.

Common scab

Powdery scab

Causal organism(s)

-Various species in the Streptomyces genus, with the most common being S. scabiei.
-Actinomycetes – bacteria that resemble fungi due to their threadlike growth habit
-Can survive on manure and decomposing material.

-Spongospora subterranea f.sp. subterranea.
-Protozoan – lower fungi, known as slime moulds that produce zoospores.
-Obligate parasite (needs host to grow and reproduce).

Environmental conditions favourable for infection

-Young, rapidly growing tuber tissues are most susceptible to infection. The pathogen primarily infects through young lenticels.
-Warm, dry soil conditions with soil temperatures above approximately 25˚C are conducive for disease development, especially during tuber initiation or periods of rapid tuber development.
-The disease is usually more severe in dry soils with a pH of 5.2-7.0, although S. acidiscabiei can cause disease in soils with a pH of 4.5.

-The pathogen can infect root hairs and roots within two weeks after planting. Stolon, young shoot and tuber infection occurs throughout the growing season, although tubers are most susceptible to infection early in tuber formation when tissue is unsuberized. Infection occurs through young lenticels.
-Infection occurs at soil temperatures between 9 and 17˚C and requires high soil water content.

Survival in soil

-The organism can persist saprophytically in soil on decomposing organic matter and manure.

-Resting spores, the survival structures of the pathogen, can survive in the soil for 10 – 20 years.

Control

-High soil moisture inhibits growth of the pathogen and provides a measure of control of the disease.
-Lowering of soil pH can reduce the levels of scab in some cases but may negatively affect fertilizer absorption and yield.
-Soil and seed treatments are not entirely effective and are costly.

-If conditions are unfavourable for infection, the tubers may escape infection.
-Excessive irrigation can lower soil temperatures but also create high soil water content; irrigation should thus be applied judiciously.
-There are currently no effective soil or seed treatments for the control of powdery scab.

Symptoms

-All underground plant parts can be infected, but symptoms are usually seen on haulms, roots or stolons.
-Tubers: There are three main types of common scab lesions on tubers namely raised, pitted and superficial. The severity and type of symptom depends on a combination of the potato variety, environment and the pathogen strain infecting the tuber. The pathogens produce phytotoxins that cause cells to swell, become corky and eventually die. S. scabiei usually causes corky lesions with raised margins and slightly sunken centres. Lesions are brown to dark brown and roughly circular in shape. Under favourable environmental conditions, lesions may coalesce to cover large areas of the tuber surface. Pitted scab lesions are raised with pitted or crater-like centres. Symptoms of superficial scab lesions include netted scab and russeting. These two symptoms can also easily be confused with elephant hide, caused by Rhizoctonia solani. Common scab lesions are often star-shaped and none have powdery centres.

-Haulms: No above-ground symptoms
-Roots: Milky-white galls form on roots of susceptible cultivars. Galls are irregular in shape and usually 0.5 – 2.0 cm in length. Powdery scab galls can be confused with those of root-knot nematodes. However, powdery scab galls appear to develop on the outside of the root, while those of RKN are more like swellings of the root itself.
-Stolons: Infection of stolons can occur and sometimes results in development of small galls.
-Tubers: Scabs on tubers result from infection of lenticels in unsuberized tissue of developing tubers. Development of infection goes unnoticed to the naked eye for several weeks, after which purple-brown pimple-like swellings (2mm diameter) (not seen with common scab) can be observed on the tuber surface. These then expand, turn brown and eventually erupt to reveal a powdery mass of sporeballs within the lesions. The skin at the edges of lesions forms loose flaps (not seen on common scab lesions). In severe cases lesions merge to cover large areas of the tuber.
Powdery scab lesions may serve as entry points for secondary organisms, which can cause tubers to rot.

To distinguish between symptoms of the two diseases, symptoms on tubers on a number of plants should be observed as typical symptoms of a disease do not necessarily occur on one or two tubers.

Credits
CHIPS Magazine South Africa ; Jan. - Feb 2013 Issue pg. 26.

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