scab [skab]
Scab
in agricultural plants, a dangerous disease caused mainly by microscopic pathogenic fungi and, sometimes, by actinomycetes and bacteria. Scab affects the superficial tissues of the leaves, fruits, flowers, shoots, tubers, and roots. It is characterized by scaling of the cuticle or skin and by the formation of spots, pits, pustules, or warts. Many agricultural plants are affected. Most serious are apple scab, pear scab, potato scab, and citrus scab. Less damaging are sugarbeet scab and cherry scab.
Apple and pear scabs. Apple and pear scabs are widespread throughout the world. Apple scab is caused by the ascomycete Venturia inaequalis (conidial stage of Fusicladium dendriticum). The causative agent of pear scab is V. pirina (conidial stage of F. pirinum). Both diseases are characterized by the formation of a brownish olive, olive-green, gray-black, or almost black velvety bloom on the leaves, fruits, and flowers. Small, round, bubblelike swellings form on the cortex of shoots, the skin cracks, and tiny pits appear. Affected leaves and flowers fall prematurely, and the fruits dry out, grow unevenly, and often crack. The bloom, which covers the spots and lines the pits and cracks, is the conidial spore carrier of the fungus, by means of which numerous repeated infections occur through the summer. Primary infection in the spring is caused by conidia or by ascospores that develop in special receptacles, or perithecia, on overwintering diseased leaves. The disease is especially destructive in years with much precipitation. Trees that are severely affected with scab are less frost resistant and produce a smaller yield of lesser quality. Methods of control include the use of the most scab-resistant varieties (among apples—the Saffron Pippin, English Winter Gold Pearmain, and Wagener; among pears, Beurré Ligel, Beurré Clairgean, Beurré Bosc, and Cure); destruction of fallen foliage and affected shoots; and repeated spraying of trees with fungicides during the growth period.
Potato scab. The causative agents of common potato scab are various species of actinomycetes, including Actinomyces scabies, A. tricolor, and A. cretaceus. Common potato scab is particularly destructive to potatoes growing on light, sandy soils or on soil with a high limestone content. Small star-shaped scabs appear on the surface of the affected tuber. At harvest, the tubers may be covered with a deposit of thin mycelial filaments, which later break up into small bacillus-shaped spores—the sources of infection. Large quantities of manure and excessive moisture aggravate the disease. The most resistant potato varieties are the Berlichingen, Kameraz, Detskosel’skii, and Priekul’skii.
Powdery scab of potatoes is caused by the fungus Spongospora subterranea, which is distributed principally on heavy clayey or clayey-loam soils. The fungus is widespread on peat soils in the northwestern USSR. The tubers in the soil become covered with warts, which dry very quickly upon exposure to air. The skin of the nodule bursts, and an open star-shaped pustule is formed, which is filled with a powdery mass—the spores of the causative agent. Somewhat resistant varieties are Parnassiia, Iubel’, Majestic, Lorkh, and Cardinal.
Silver scab, or silver scurf, affects potatoes in northwestern regions and in the Far East. The causative agent is the imperfect fungus Spondylocladium atrovirens. The tubers are marked by silvery patches. During harvest the spots are only slightly noticeable, but by the end of the storage period (closer to spring), the spots deepen, acquire a characteristic silvery sheen, and are covered with a dark deposit of the conidial spore carrier. No potato varieties are resistant to the disease.
Another scab, caused by the imperfect fungus Oospora pustulans, affects potatoes in northwestern regions and in the Far East. No varieties are resistant to the disease.
Black scab, or Rhizoctonia disease, is most common in northern and middle parts of the USSR. It is caused by the fungus Rhizoctonia solani. There are no resistant potato varieties.
Potato scabs lower the commercial value of the tubers, increase waste from food potatoes, and shorten the storage life. Control measures include crop rotation, the use of resistant and slightly susceptible varieties, and the planting of undiseased or disinfected tubers.
Citrus scab. Citrus scab, which affects lemons and oranges, is caused by the imperfect fungus Sphaceloma fawcetti. Small, rounded, predominantly yellow spots appear on the leaves and fruits. The spots later form small warts or scabs. The fruits develop poorly and are deformed; when the ovaries are affected, the fruits fall off the tree. Diseased shoots are retarded in growth. The fungus spreads by means of conidia. Control measures include the removal and burning of affected parts of the plant before the vegetative period and spraying with fungicides.
Sugarbeet and cherry scabs. Sugarbeet scab is caused by actinomycetes (Actinomyces scabies, A. cretaceus, A. albus) and bacteria (Bacterium scabiegenum). It is characterized by the formation of warts on the roots. Cherry scab is caused by the fungus Karaculinia cerasi. With early infection, the fruits wrinkle and dry out.
REFERENCES
Peresypkin, V. F. Sel’skokhoziaistvennaia fitopatologiia. Moscow, 1969.Kartofel’. Edited by N. S. Batsanov. [Moscow, 1970.]
M. I. DEMENT’EVA
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