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canker,small sore on the inside of the mouth. A canker appears as a shallow, whitish ulcer surrounded by a thin, red area. It is tender, sometimes painful, and may occur singly or as one of a group of sores. Cankers develop on the inner surfaces of the lips or cheeks, on the gums, under the tongue, or on the roof of the mouth. The cause is unknown, but cankers have been associated with friction, injury, allergy, and viral infection. They generally heal by themselves in a few days but can be recurrent.
a disease of cultivated and wild plants characterized by extreme, irregular proliferation of stems, branches, roots, and, less commonly, other organs, resulting in the formation of excrescences and tumors. Bacteria and fungi are the most common causative agents of canker. Some cankers heal very slowly or not at all. The use of the term “canker” is arbitrary. The most widespread and harmful cankers affecting crops are potato wart, crown gall, apple black rot, and bacterial canker of tomatoes.
Potato wart is a serious quarantine disease caused by an intracellular parasite—the pathogenic fungus Synchytrium endobioticum of the class Phycomycetes. The disease is characterized by the formation of fleshy nodular growths on the tubers and stolons and, less commonly, on the stems and leaves. The growths are sometimes larger than the tubers. A severely infected tuber is commercially worthless, and, as a rule, infected tubers cannot develop at all. The disease sharply decreases crop yield.
The causative agent of potato wart overwinters in the soil and plant residues in the form of spores. The spores germinate in spring, forming monociliated zoospores that penetrate the plants. The causative agents are spread by the tubers, postharvest residues, and manure; the spores remain viable even after passing through the intestinal tract of animals. Control measures include cultivation of potato varieties resistant to the disease, such as Kameraz, Berlichingen, and Priekul’skii Early. Other measures include keeping the fields in bare fallow, controlling solenaceous weeds, feeding affected potatoes to cattle only after they have been boiled, and disinfecting the soil with fungicides.
Crown gall, which affects fruit crops, is most dangerous in nurseries, especially when young plants are infected. The causative agent is the rodlike bacterium Bacterium tumefaciens, which can infect not only pomes and drupaceous plants (apple, pear, cherry, apricot, sweet cherry) but also willows, roses, chrysanthemums, beets, tomatoes, and sunflowers. The bacterium penetrates plants at the sites of injury to the root system and releases growth substances, such as indoleacetic acid and gibberellin-like substances, that intensify cell division and give rise to primary galls. Secondary galls often appear some distance away from the primary ones; they are usually sterile, since they result from the metabolic products of the bacteria moving through the tissues. The galls hinder the movement of juices, especially when the main root or the root collar (crown) is affected.
Control measures include establishing nurseries on plots where plants susceptible to crown gall were not grown for two or three years, growing healthy planting stock, discarding and burning seedlings with large galls on the main roots and root collar, and pruning away growths on the lateral roots and disinfecting the cut portions with copper sulfate or naphthenate.
Apple black rot, or apple canker, is a very dangerous disease. It most seriously affects weakened pomes (apple, pear, quince, medlar); less commonly, it affects drupaceous plants, persimmon, English walnut, and other plants in old neglected orchards. The causative agent, the pycnidial fungus Sphaeropsis malorum, penetrates plants at the sites of injury and attacks the bark, leaves, flowers, and fruits. Brown sunken spots appear on the bark, which gradually spread in concentric circles around the branches and trunk, causing them to wither. The affected areas resemble gooseflesh, owing to the formation of minute pycnidia under the epidermis. Apple canker is manifested on the leaves in the form of cinnamon-brown spots and on the fruits as black rot. Severely infected plants die in three or four years. The scion and stock must be compatible if the disease is to be prevented. It is recommended that disease-resistant varieties (Bel’fler-kitaika, Pepin Shafrannyi, Borovinka, Papirovka, Pepinka Litovskaia) be grafted into the crowns of vigorously growing stock with powerful root systems.
Control measures include uprooting dead trees, pruning overgrown branches, trimming away affected bark and then applying fungicides, applying tree coating to the wounds, collecting and burning rotted fruits and infected leaves, and coating boles and boughs with lime water.
Bacterial canker of tomatoes is caused by the bacterium Corynebacterium michiganense, which may be transmitted by seeds, infected residues, wind, and insects. Pruning during the growing season also causes the disease to spread. Canker of tomatoes attacks the fruits, stems, and vascular system, causing the branches and leaves to wither. Control measures include treatment of seeds, disinfection of soil with fungicides, thorough cleansing of the young crops before pruning, destruction of postharvest residues and replowing plots.
Cankers affecting trees may be caused by the bacterium Pseudomonas remifaciens (poplar and ash canker), the bacterium Pseudomonas pini (pine canker), the rust fungi Cronartium flaccidum and Peridermium pini, the ascomycetous fungus Dasyseypha willkommii (European larch canker), and the bacterium Erwinia multivora (blight canker).
REFERENCESPeresypkin, V. F. Sel’skokhoziaistvennaia fitopatologiia. Moscow, 1969.
Zhuravlev, I. I., and D. V. Sokolov. Lesnaia fitopatologiia. Moscow, 1969.
Pospelov, S. M., M. V. Arsen’eva, and G. S. Gruzdev. Zashchita rastenii. Leningrad, 1973.
M. I. KHOKHRIAKOV