The phenomenon of gum-flow is common to stone and citrus fruit-trees. It results from stimulation produced by foreign factors of one kind or another. The flow of gum, gumming, or gummosis, is not a disease, although in many cases it is a sign of disease. In other cases it is an indication of injury. The term gummosis, then, is used broadly to designate any disease or injury which is accompanied by a gumming, or a flow of gum. The remarks presented here refer in most cases to a particular bacterial disease of the cherry, plum, peach and apricot which is commonly accompanied by gum-flow. So far as is known at present this disease, called Bacterial Gummosis of cherry, occurs only in western Oregon and Washington. Time may show, however, that this Bacterial Gummosis prevails in other sections of the United States. It is to be noted in this connection that the flow of gum in the cherry in the states already referred to is frequently caused by factors other than the bacteria under consideration. In the Pacific Northwest the disease is usually called cherry gummosis, but the common designation Bacterial Gummosis is preferable in that it is more specific.
Among the plants affected by this disease the cherry is most susceptible. Sour cherries are slightly or not at all attacked. Sweet cherries, on the other hand, are very susceptible to bacterial gummosis. The Bing and Napoleon (Royal Ann, Ox Heart), two very desirable cherries in the Northwest, suffer considerably from Bacterial Gummosis. Certain sweet varieties, however, like the Lambert and May Duke, are comparatively resistant.
This disease is most serious on trees which have been set for three or four years. In many cases they may be killed. On trees more than ten or twelve years old the damage is largely a blighting of the smaller branches, spurs and buds, and it is usually not serious.
Often there is little indication of the disease until whole trees or branches fail to leaf in the spring, or until there is a sudden wilting in the growing - season. Careful examination of cherry trees showing such symptoms will usually reveal a girdling of a limb or trunk. Sometimes there is no gumming, but ordinarily a more or less copious flow of gum accompanies such a condition (Fig. 50). The amount of gum exuded from lesions is no indication of the amount or severity of the disease. On cutting into the bark at affected points the inner tissues will be found to be brownish and the appearance will indicate approaching death. A sour odor may accompany this condition. The lesion produced on the woody parts is a canker. At first the bark is discolored. The discolored area enlarges and the surface of the bark becomes depressed. Then gum exudes from the margin of the canker. Subsequently the extension of the canker ceases and a callus develops about its margin. Enlargement of the cankered area takes place in the winter and early spring, but ceases by the last of March. The callus is formed in the spring and summer. The next winter the callus may be attacked and the lesion extended. In the spring a second callus is formed. This process may be repeated annually for several years. Finally an affected branch or trunk may be girdled, as a result of which the foliage becomes yellow, then brown, and finally withers and dies. Such foliage hangs on the tree for some time. Below a girdled trunk, suckers may spring up from the healthy part.
In addition to the cankers just described another phase of Bacterial Gummosis is found in a blighting of the spurs and buds. The first indication of this is usually not noticed until spring, when some of the buds fail to swell and open when others unfold. A drop of gum often exudes from such buds. Sometimes affected buds do unfold in the spring in an apparently normal fashion, but before the blossoms open, these buds wilt and become dry.
Fig. 50. - Cherry twigs affected with Bacterial Gummosis; cankers about the base of blighted spurs. Gum - flow evident.
The Bacterial Gummosis of cherry is caused by Bacterium Cerasi. Before proceeding with a discussion of this particular disease, brief reference will be made to the causal nature of gummosis in general. As already indicated, the trouble may be due to one or more of several factors. These factors act in a stimulative manner. Among such causal agents may be noted: (1) mechanical and insect injuries; (2) unfavorable soil and climatic conditions; (3) fungi; (4) bacteria.
Mechanical injuries such as those produced by bruising, or by cultivating tools, may induce gum - flow. Pruning wounds often exude gum. Insects, such as borers, produce injuries through which gum commonly exudes.
The relation of soil and climatic conditions to the formation and flow of gum is not well understood. Trees in low places where the soil is excessively wet are often more subject to gummosis than others. On the other hand, trees on well - drained soil may exude gum. Not infrequently frost induces gummosis.
Fungi are common causal agents in gum-flow. Some of the more important ones will be noted: (1) the blight fungus, Coryneum Beijerinckii; (2) the Die Back pathogene, Valsa leucostoma; (3) the Black Knot fungus, Plowrightia morbosa; (4) the Brown Rot fungus, Sclerotinia cinerea; (5) the Root Rot fungus, Armillaria mellea. Doubtless many other fungi induce gumming.
It will be seen that gummosis is associated with a variety of conditions of the plant, but it is often difficult or impossible to attribute a given case of gum - flow to any one cause. Gum is formed internally and may not always exude. It is formed in pockets which are not visible on the surface of the bark. Usually, however, the bark ruptures and the gum flows out.
With reference to bacteria as the cause of gummosis, the development of Bacterium Cerasi will now be considered. Many facts in the life - history of the organism are lacking. The bacteria apparently lie dormant in the bark through the late spring, summer, fall and early winter. In the case of limb and trunk cankers the bacteria may or may not become active again. In the event of renewed activity in the winter the bacteria spread at the edge of the old cankers, thus enlarging these lesions. The bacteria probably lie dormant also in the buds. The effect of their action is not ordinarily observed until the buds fail to open in the spring or until, after opening, they suddenly die. From the cankers, bacteria may possibly be carried by insects in the summer to new points where infections result. The bacteria attack the outer bark, then the phloem and cambium. These affected elements all turn brown. Brown streaks are found in the bark (between the phloem and outer cortex) extending above and below a canker. Gum pockets are formed under the bark, which splits and allows the gum to ooze forth.
The theory of gum-formation has created no little interest. It is now generally held that gum is formed through the transformation and liquefaction of the walls of certain cells. Such cells are formed abnormally by the cambium as the result of stimulation by parasites or other factors already enumerated. In Bacterial Gummosis of the cherry this stimulating factor is Bacterium Cerasi. Probably it produces an enzyme which dissolves the walls of the cells, with the result that gum is formed in pockets made in turn by the dissolution of the cell-walls in a local area. In order that gum may be produced, an abundance of water seems necessary. It also seems essential that the tree be in a growing condition for gum - production. It may then be understood why young trees exude gum more often than do old trees.
Recommendations for the control of the Bacterial Gummosis of the cherry are made in the Northwest along three lines: (1) the removal of cankers; (2) the protection of susceptible trunks and limbs with coarse cloth or burlap; (3) the use of resistant seedlings and the growing of resistant varieties.
Cankers on old trees are rare, so that surgical methods apply to young trees only. The removal of diseased and dead bark has several advantages. Such operations should prevent the further spread of the bacteria in a given canker. The elimination of dead bark from cankers in which the bacteria have ceased activity will permit more rapid healing of the wound. The removal of diseased bark also means the removal of a source of the trouble. The trees should be inspected late in the winter and early in the spring for new infections. All discolored bark should be removed as advised for Fire Blight cankers (page 23). All wounds should be disinfected with corrosive sublimate 1-1000, and a wound - dressing should be applied. These measures should give effective results.
It is suggested that trunks and limbs be wrapped with burlap or coarse cloth until the young tree passes the danger - point. This method is as yet in the experimental stage and should be used with this fact in mind.
The use of mazzard seedlings on which desirable varieties may be grafted is strongly advised. These stocks show striking resistance to gummosis. The buds should be set in the limbs and not in the trunks; this prevents the spread of the bacteria from one limb to another. The grower is cautioned concerning mazzard seedlings: various sweet cherry seedlings are probably sold under the name of mazzard. Some growers raise their own seedlings. The feasibility of this practice must be determined by the grower. (See also general discussion of Gummosis under Peach, page 303.)
Barss, H. P. Cherry gummosis. Oregon Crop Pest and Hort. Bienn.
Rept. 1911-1912: 199 - 217. 1913. Barss, H. P. Bacterial gummosis or bacterial canker of cherries.
Oregon Crop Pest and Hort. Bienn. Rept. 1913-1914: 224 - 240.
1915. Rees, H. L. Bacterial gummosis of cherries. Washington Agr. Exp.
Sta. Monthly bul. 3:2:12 - 16. 1915.