Reports on Plant Diseases

RPD No. 909 - Common Scab of Potato

October 1988

Common scab of potatoes is caused by Streptomyces scabies, a very prevalent, soil-inhabiting bacterium. This serious disease can be found in all potato-growing areas throughout the world. The scab organism sometimes occurs in soils where potatoes have never been grown. In most potato soils, however, scab was probably introduced with infected seed tubers. The major loss from common scab is lower market quality because tubers are unsightly or disfigured and have poor customer appeal. Where scab is severe, yields also may be reduced. The disease causes an annual loss of several million dollars in the United States.

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SYMPTOMS

The first symptom of common scab is the appearance of small, brownish specks or spots (lesions) on the young tubers. These spots (usually circular) soon enlarge, darken, and become corky. Certain lesions may merge to form large, irregular scabby areas. Some may be raised slightly above the surface of the tuber; others may be somewhat sunken. The lesions may be so numerous as to give a russeted appearance to the entire tuber. Under certain conditions the scab lesions become quite deep, pitted, and dark brown to almost black (Figure 1). Deep cracking and pitting of the potato surface is often aggravated by attacks of millipedes or the larvae of the potato flea beetle and garden symphylans, which apparently break down the cork (periderm) layers. This allows the pathogen to invade the tuber to a depth of about 1/4 inch. The causal bacterium also produces rather inconspicuous, tan to brown lesions on potato roots, underground stems, and stolons. If scabbed potatoes are stored, they tend to shrink and are often invaded by secondary, soft-rotting organisms.

Figure 1. Sunken scab lesions (courtesy R. Loria)

The scab bacterium is disseminated in infected tubers and fleshy roots or in soil. The bacterium overwinters on living roots of weeds and crop residue left in the field or garden. Wind, splashing rain, surface-drainage water, farm machinery, and tools all move soil from one location to another and thus spread the pathogen. The bacterium even survives passage through the digestive tract of animals and is distributed with manure. Once the soil becomes infested, it generally remains so indefinitely.

Scab can attack potato tubers grown in soil within a wide temperature range of about 50° to 88°F (10° to 31°C) with an optimum of 68° to 72° (20° to 22°C). The organism can grow, however, at temperatures as low as 41°F (5°C) and as high as 106°F (40.5°C) with an optimum of 77° to 87°F (25° to 30°C). Generally scab is more severe in dry soils, especially those deficient in organic matter, and becomes less severe in moist soils, although researchers have pointed out some exceptions. The fact that the Streptomyces bacterium is extremely variable may partly account for the difference in effectiveness of control measures at various locations and in different soil types. Certain strains or races of the bacterium cause scab on fleshy roots or tubers and damping-off or seedling blight of many kinds of plants. The list includes beets (garden and sugar), carrot, dahlia, eggplant, mangel, onion, parsnip, radish (Figure 2), rape, rutabaga, salsify, spinach, Swiss chard, turnip, and several species of weeds. A closely related species of Streptomyces causes soil rot or pox of sweet potato.

The Streptomyces bacterium is a saprophyte that can survive indefinitely either in its vegetative myceloid form or as minute spores (Figure 3, above) in most soils, except very acidic ones. The vegetative form consists of slender, branched mycelium with few or no cross walls. The spores are cylindrical or ellipsoid and are produced on specialized spiral hyphae that develop cross walls from the tip toward their base. As the cross walls constrict, spores are pinched off at the tip and eventually break away from the hypha. The spores germinate by means of one or two germ tubes, which develop into the vegetative myceloid form. Young, rapidly growing potato tubers are most susceptible. The pathogen can penetrate through natural openings (lenticels, stomata), mechanical or insect wounds, or usually occur under conditions when tubers are enlarging most rapidly. Following penetration the pathogen grows between or through a few layers of cells. The cells then die and the bacterium feeds on them as a saprophyte. The organism also secretes a substance that stimulates the living cells surrounding the lesion to divide rapidly and to produce several layers of cork cells (periderm) that isolate the bacterium and surrounding tuber cells (Figure 3, below). As the tuber cells–which are cut off by the cork layer–die, the pathogen subsists on them. The cork cell layers are pushed outward and sloughed off and the pathogen grows and multiplies in the additional dead cells which results in the development of large scab lesions.

Resistance is apparently associated with the effectiveness of the periderm, which underlies the scab lesions and walls them off from the tuber. In susceptible potato cultivars with deep scab lesions, successive layers of periderm form as penetration progresses. In resistant cultivars the lesions are shallow and a single periderm layer seems to prevent further infection. Different types of lesions may develop on the same tuber, which may reflect differences in pathogenicity between strains of the organism or in the maturity of the tuber surface at the time of infection.

Figure 2. Scab on radish roots.

Figure 3. Streptomyces scabies, the common scab of potato bacterium, as it would be seen under a laboratory microscope. (Above) Slender, branching
myceloid form and three minute spores; (Below) section of an old scab lesion showing a layer of periderm (cork) cells (drawing by L. Gray).

CONTROL

All of the following measures have been effective in some cases. Usually a combination of control methods should be adapted to the cultural practices used in your area.

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