This group of diseases includes mainly diseases that affect below-ground parts of plants and whose symptoms consist of more or less localized scabby lesions affecting primarily the outer tissues of these parts. The scab bacteria and the diseases they cause are:
Streptomyces, causing the common scab of potato and of other below-ground crops (5. scabies), and the soil rot or pox of sweet potato [S. ipomoeae).
Pseudomonas, causing scab of gladiolus [P. marginata).
The scab bacteria survive in infected plant debris and in the soil and penetrate tissues through natural openings or wounds. In the tissues, the bacteria grow mostly in the intercellular spaces of parenchyma cells but these cells are sooner or later invaded by the bacteria and break down. In typical scabs, healthy cells below and around the lesion divide and form layers of corky cells. These push the infected tissues outward and give them the scabby appearance. Scab lesions often serve as points of entry for secondary parasitic or saprophytic organisms that may cause the tissues to rot.
• Common Scab of Potato
It is caused by Streptomyces scabies and occurs throughout the world. It is most prevalent and important in neutral or slightly alkaline and light sandy soils, especially during relatively dry years. The same pathogen also affects garden beets, sugar beets, radish, and other crops. The disease, by its usually superficial blemishes on tubers, roots, etc., reduces the value rather than the yield of the crop, although severe root infection may reduce yields, and deep scabs increase the waste in peeling.
The symptoms of common scab of potato are observed mostly on tubers and at first they consist of small, brownish, and slightly raised spots, but later they may enlarge, coalesce, and become very corky.
Frequently, the lesions extend below the tuber surface and, when the corky tissue is removed, 3 to 4 mm deep pits are present in the tuber.
Sometimes the lesions appear as small russeted areas and are so numer
ous that they almost cover the tuber surface, or they may appear as slight protuberances with depressed centers covered with a small amount of corky tissue (Fig. 176).
FIGURE 176.
Early (A) and advanced (B) symptoms of common scab of potato caused by Streptomyces scabies. (Photos courtesy U.S.D.A.)
The pathogen, S scabies, is a hardy saprophyte that can survive in
definitely either in its vegetative mycelioid form or as spores in most soils except the most acidic ones. The vegetative form consists of slender (about 1 μ,πι thick), branched mycelium with few or no cross walls. The spores are cylindrical or ellipsoid, about 0.6 by 1.5 μηι, and are produced on specialized spiral hyphae that develop cross walls from the tip towards their base and, 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 mycelioid form (Fig. 177).
* M Z D O0^ Spore s
Spore infectin g tube r through lenticel , stoma , wound o r directl y Sporogenous hyph a
breaks int o spore s
Pathogen overwinter s i n th e soil o n infecte d plan t tissu e
Potato tuber s wit h differen t kind s of commo n sca b symptom s
As th e firs t cor k laye r i s penetrate d a ne w one form s furthe r i n
FIGURE 177.
Disease cycle of the common scab of potato caused by Streptomyces scabies.
The pathogen is spread through soil water, wind-blown soil, and on infected potato seed tubers. It penetrates tissues through lenticels, wounds, stomata and, in young tubers, directly. Young tubers are more susceptible to infection than older ones. Following penetration the pathogen apparently grows between or through a few layers of cells, the cells die and the pathogen then lives off them as a saprophyte. In the meantime, however, the pathogen apparently secretes a substance that stimulates the living cells surrounding the lesion to divide rapidly and to produce several layers of cork cells that isolate the pathogen and several plant cells. As the cells that are cut off by the cork layer die, the pathogen subsists on them. Usually, several such groups of cork cell layers are produced, and as they are pushed outward and sloughed off, the pathogen grows and multiplies in the additional dead cells and thereby large scab lesions develop. The depth of the lesion seems to depend on the variety, soil conditions, and on the invasion of scab lesions by other organisms, including insects. The latter apparently break down the cork layers and allow the pathogen to invade the tuber in greater depth.
The severity of common scab of potato increases as the pH of the soil increases from 5.2 to 8.0 and decreases beyond these limits. The disease develops most rapidly at soil temperatures of about 20 to 22°C but it can occur between 11 and 30°C. Potato scab incidence is greatly reduced by high soil moisture during the period of tuber initiation and for several weeks afterwards. Potato scab is also lower in fields following certain crop rotations and the plowing under of certain green manure crops, probably as a result of inhibition of the pathogen by antagonistic micro-organisms.
Control of the common scab of potato is through the use of certified scab-free seed potatoes or through seed treatment with pentachloronit-robenzene (PCNB) or with maneb-zinc dust. If the field is already in-fested with the pathogen, a fair degree of disease control may be obtained by using certain crop rotations, bringing and holding the soil pH to about 5.3 with sulfur, irrigating for about 6 weeks during the early stages of tuber development, and using resistant or tolerant potato varieties.
SELECTED REFERENCES
Jones, A. P. 1931. The histogeny of potato scab. Ann. Appl. Biol. 1 8 : 3 1 3 - 3 3 3 . KenKnight, G. 1941. Studies on soil Actinomyces in relation to potato scab and its
control. Mich. Agr. Exp. Sta. Tech. Bull. 178:48 p.
Labruyere, R. E. 1971. Common scab and its control in seed-potato crops. Versl.
Landbouwk. Agr. Res. Rept. 767:71 p.
Lapwood, D. H. 1973. Irrigation as a practical means to control potato common scab (Streptomyces scabies): Final experiment and conclusions. Plant Pathol.
2 2 : 3 5 - 4 1 .
Lapwood, D. H., and M. J. Adams. 1975. Mechanisms of control of common scab by irrigation, in "Biology and Control of Soil-Borne Plant Pathogens/7 G. W.
Bruehl (Ed.). The Amer. Phytopathol. S o c , St. Paul, Minn., pp. 1 2 3 - 1 2 9 . Nelson, R. 1948. Diseases of gladiolus. Mich. Agr. Exp. Sta. Special Bull. 350:63 p.
Person, L. H. 1946. The soil rot of sweet potatoes and its control with sulfur.
Phytopathology 3 6 : 8 6 9 - 8 7 5 .