Introduction During the summer of 1905 many of the trees in the Wau- sau trial orchard were found to be suffering from some disease. In the case of many of the Northwestern Greening trees the foliage on one or more of the larger branches appeared dwarted and wrinkled and later turned brown but did not fall. On examination it was found that these branches were gird- led at the base or point of union with the trunk. The bark in the ecrotches was brown and sunken, the injury extending through the cambium to the sap wood. Further investigation in the orchard showed hundreds of trees to be affected, some like the Northwestern in the crotches, others having large patches of dead bark on the trunks. It was not until the close of the growing season that it was definitely determined that the trouble was caused by the apple tree bight canker. It has since been found that the disease is widely prevalent in the state, having been found in twenty-six counties and probably exists in the majority of young orchards throughout the state, attacking the crotches and trunks of young apple trees. The apple tree bight canker and methods of treatment were fully deseribed by H. H. Whetzel, plant pathologist, Cornell University, at the 1906 annual meeting of this Society and the address printed in the 1906 Annual Report. That a better knowledge of the disease and the means of com- bating it may be had the article is here reprinted. F. CRANEFIELD, Secretary. Fig. 1—Typical blight canker on Fig. 2—Typical canker showing main limb of young tree. the crack about the margin where the diseased bark has dried away from the healthy tissue. Notre — Figs. 1 to 13 inelusive from Bulletin 236, Cornell University, Ithaca, N. Y. Figs. 14, 15, 16 and 17 from photographs of trees in the Wausau Trial Orehard. The Blight Canker of Apple Trees H. H. WHETZEL. During recent years several kinds of cankers occurring espe- cially upon apple and pear trees have been described and fig- ured in bulletins from different experiment stations in this ecuntry. By careful inoculation experiments these various eankers have been shown to be due to different species of either fungi or bacteria. Growers very generally even at the present time attribute such injuries to ‘‘sun seald”* or ‘‘freezing.’’ Lack of knowledge cf the nature of fungous and _ bacterial growths, together with the ease with which responsibility may be shifted upon the weather, has made this opinion the common and natural ene. Not only have experiment station workers shown that these injuries are usually due to the attacks of liv- ing organisms rather than to the results of weather conditions, but they have demonstrated that the different forms of these eankers are due to distinctly different organisms. The term ‘‘eanker’’ then has come to be a very general one and is applied to diseeses which cause the death of definite areas of bark en the limbs and bodies of trees. At least six distinct canker diseases of apple and pear trees have been de- seribed and figured in recent years. Each of these cankers have been proven to be due to distinetly different kinds of fungi. In this paper, however, we shall deal with a canker disease, not eaused by a fungus and differing strikingly from those already mentioned. To this disease I have given the name of blight eanker. THE DISTINGUISHING CHARACTERS AND APPEARANCE OF THE CANKER. The blight canker while it may occur on trees of almost any age is most destructive on young trees just coming into bearing, trees from 8 to 15 years old. Old trees weakened by age and 6 Wesconsen State Horticultural Soczety neglect may suffer seriously from its attacks and the dead limbs protruding here and there from the green foliage in old or- chards are often to be attributed to the ravages of this canker. In young trees with smooth bark the cankers are easily de- tected even in their first stages. They appear as discolored and Fie. 4—Typieal pit cankers in the body of a tree from which the diseased bark has dropped out. The upper one is exuding sap, —‘‘ bleeding.’’ Fie. 3—Cankers in body of tree resulting from the renewal of activity sea- son after season. Observe that these started from small pit cankers which partially healed each season. The disease the following season destroyed all the bark growth, leaving only the little bit of wood formed in the callus. This still persists. somewhat shrunken areas, the margin along the advancing front being usually slightly raised or blistered. The tissue in ac- tively spreading cankers is of a darker green than the healthy bark and is very watery or sappy. On damp cloudy days drops of a milky, sticky fluid exude from the cankered tissues through The Blight Canker of Apple Trees 7 the lenticles or pores in the bark. After a short time the dis- eased tissue begins to turn brown and dry out. Unless in a very active state of progress the margins are quite distinct, marked by a crack where, in drying, the diseased tissue has sep- arated from the healthy bark. The older cankers are brown, somewhat darker than the healthy bark. They are distinctly sunken. The surface is smooth, never checked or roughened or beset with pustules or pimples, except in the old cankers where after a time rot fungi gain entrance and thriving in the already dead tissue produce their fruit bodies on the surface. The prog- ress of the spreading canker depends largely upon the continua- tion of favorable weather conditions, which seems to be a humid atmosphere and cloudy days. With the return of bright, suany weather the active spread of the canker is checked abruptly, eften to be resumed again with the return of favorable condi- tions. This checking and renewing of activity sometimes re- sults in large eankers with concentrically arranged eracks within the eankered area. This renewal of activity may take place during the same season or the canker may partially heal over to spread anew the following year. A large per cent of the eankers are active during but one season. There are always some, however, in which the disease is perennial, living through the winter to become active again the following spring, spread- ine and enlarging the original limits of the eankered area. These ‘“‘hold over’’ eankers, as they are called, are the danger- ous ones. They are the source of infection for the following year. The diseased bark is usually killed to the wood, to which it clings tenaceously the first season. It gradually de- cays, however, and falls out, leaving the wood bare and exposed. In small eankers, the cone of diseased bark may be quickly forced out by the rapidly forming calluses which heal and close the canker wound. In some eases the canker is superficial, never reaching the cambium except perhaps in a limited area at the point of infection. Such wounds heal quickly beneath the dead bark which clings to the tree as a sort of scab. The cankers vary in size from half an inch in diameter to as much as a foot or more in length and several inches across. On 8 Wesconstn State Hortecultural Soczety healthy, vigorous trees they are small and more or less cireular in outline. They form funnel-shaped wounds with the small end at the cambium. These I have designated as ‘“‘ pit eankers.”’ Often the dead bark remains as a sort of lid to the pit but it is easily removed with the finger or a knife blade. I have seen young trees with limbs and bodies literally covered with these Fig. 5—Large body canker result- Fic. 6—Large body canker near base ing from successive seasonal attacks of tree, often referred to as ‘‘collar rot.’’ of the disease. Originated at the The tree has made repeated attempts to pruned stub. heal this wound, The Blight Canker of Apple Trees 9 pit cankers in all stages of healing over. Aside from affording an entrance to rot fungi such eankers unless they enlarge do not seem to seriously affect the health of the tree. In many cases these pit cankers do not heal properly or at all and the disease spreading the same or the following season forms the large and dangerous ‘limb’’ or ‘‘body cankers.’’ “Crotch cankers’’ usually appear in the crotches where the main limbs arise from the body but may also appear in the see- ondary erotehes well up in the tree. In general characters they are similar to the limb and body cankers. Owing to their pe- euliar position water is retained more readily in the dead bark, thus affording the very best of conditions for the entrance and erowth cf rot fungi. These find easier access to the heart wood at the erotch than on the limbs. It was observed that these erotech cankers heal much less readily and suecessfully than do the limb and body eankers. Crotch cankers unless promptly attended to means the almost certain destruction of the trees. The large eankers at the bases of young trees frequently re- ferred to by growers as ‘‘collar rot’’ are in many cases very probably due to the same cause as that of the eankers on the upper parts of the tree. HOW THE DISEASE AFFECTS THE TREE. The effect of the blight canker upon the tree is to lower its vitality to a greater or less degree by cutting off the food sup- ply to the roots and thus indirectly reducing the flow of sap to the branches and leaves. In other words, it acts the same as ‘‘oirdling.’’ The ‘‘eollar rot’’ and ‘‘erotch eankers’’ seem to be the most fatal to the tree. The effects of the canker are first evidenced in the foliage. If there is a large body canker the entire tree may show the effects of the trouble. More often the first symptom noted by the grower is the peculiar appear- ance of the foliage on one or more of the limbs. Either these branches fail to leaf out at all in the spring, or if they do the leaves never fully expand but remain undersized and curled or inrolled. They never take on the dark green color of healthy 10 Wresconsin State Horticultural Socvety foliage, remaining pale and gray. trees as having ‘‘mouse ear > leaves. Growers often refer to such As the season advances and the eankers spread, the leaves often die and dry up on the Fig. 7—Canker on pear tree resulting fo) from inoculation with bacteria active canker on apple tree. from fia. 8—Pruned stub canker that has spread down side of limb during early spring of second year. Diseased tissue cleaned away, treated with corrosive sublimate and painted. Good calluses formed. Canker ceased to spread. The Blight Canker of Appie Trees 11 branches. Sometimes badly affected trees will pull through un- til autumn or even live for two or three seasons. Such trees have seanty foliage, blossom profusely and frequently set a heavy crop of fruit. This fruit falls prematurely or is small and inferior im quality. Such affected limbs and trees, as if in anticipation of their ap- proaching death, seem to devote their expiring energy to one Fig. 9—Canker on limbs of apple which originated through blighted spurs. erand and final effort to reproduce themselves. As already stated, where the trees are strong and vigorous they frequently succeed in promptly healing the canker wounds. The dead bark of the canker makes, however, an excellent infection court for the entrance into the tree of ‘‘heart rot’? and other decay inducing fungi. Moisture so necessary to the germination and erowth of the spores of fungi, is retained for a considerable time in the dead tissue. This is more especially true of crotch eankers. No doubt these rot fungi are often to blame for the final death of the tree. The heart wood of badly affected limbs and trees is commonly found to be soft and rotten with only a thin rim of sound sap wood surrounding it. 12 Wesconsin State Hortecultural Soczety THE CAUSE OF THE CANKER. A miecroseopie examination of the viscid milky drops that ex- ude from freshly cankered surfaces on moist, cloudy days will show them to be composed almost entirely of minute rod-shaped bacteria. The diseased tissue within the bark will also be found to be alive with these minute plants. By their rapid growth and multiplication within the cells of the bark they cause its Fie. 10—Recently blighted shoot on limb of Greening apple tree with well marked active canker about its base. death. They are not carried along in the sap but slowly work their way from cell to cell. When the canker dries down they die and disappear so that examination of the tissue of old eankers does not show them. That they are the direct cause of the disease was proven in the following manner: Bacteria from the cankered tissue was introduced into the bark on the body of a healthy apple tree and also into the bark of a healthy The Blaght Canker of Apple Trees 13 pear tree, with the result that typical cankers appeared in both cases. Blossoms and growing twigs of both pears and apples were also inoculated with bacteria from this same canker. These nearly all developed good cases of blight in about ten days, while twigs and blossoms punctured with a sterile needle gave no infection. This last experiment was twice repeated during Fig. 11—Pruned stub canker. In- Fie. 12—Canker formed about the fected at time of pruning, probably mouth of the burrow of a borer near by the saw. Note the color of dead base of an apple tree. bark. the summer with pure cultures of the bacteria from the apple tree canker. The blight resulted in practically every ease. Young fruits of both the pear and apple were also inoculated and gave well developed cases of the disease. By a compara- tive study in various culture media of the bacteria from cank- ers, twigs and fruits of both pear and apple obtained from dif- ferent orchards about Ithaca the organism of the canker was shown to be identical with that of the well-known ‘‘fire blight’’ of the pear and ‘‘twig blight’’ of the apple, Bacillus amylov- orous. 14 Wresconstn State Flortecultural Soczety HOW TREES BECOME INFECTED. Only those ways of infection which personal observation has discovered are here recorded. No doubt the bacteria enter the bark in still other ways than those I have observed. Early in my investigation I came to the conelusion that the bacteria frequently get into the bark of the limbs and body by way of short spurs and watersprouts. The opinion was fully confirmed later in the season. Twig blight became very preva- lent during July and August, especially in the region about Ithaca, N.Y. It was then an easy matter to find blighted spurs and watersprouts with active cankers about their bases. Where these watersprouts grew out from the trunks as is often the ease in young trees, typical body cankers were formed. The infection of the sprout itself is generally attributed to the work of insects which after visiting freshly eankered spots or blighted twigs introduce the bacteria into the succulent tissues of the rapidly growing healthy shoots. The blighted watersprout soon dries up and falls away, leaving often a very indefinite scar in the cankered area so that the following season it is usually im- possible to te'l with certainty the manner cf infection. Obser- vation of a large number of trees, during the past season, con- vineces me that the blighting of adventitious shoots on trunk and limbs is responsible for a majority of the eankers in such lceations. A number of ecankers were produced in this way by artificial inoculation. Another source of infection was found to be the pruning knife. Along one side of an orchard of some 350 trees which was under observation throughout the season, it was early no- ticed that the pruned stubs of 1904, especially, showed collars of dead bark often two or three inches in width. Instead of forming a callous and healing over the wound, as would nor- mally oeeur, the tissue had died and shriveled up but still elung to the stub. In most eases the bacteria which had caused the death of the bark had died out the first season. In a few in- stances, however, the canker was observed to be active early in the spring, extending down the side of the adjoiming limb. ‘Two The Blight Canker of Apple Trees 15 badly diseased trees on this side of the orchard seem to have been the source of infection. Owing to their diseased condition they had been severely pruned the previous season and very probably the knife or saw had carried the bacteria to the healthy trees. Flies which were observed to constantly follow the pruner to suck up exuding sap may have been the direct agents in many cases in transferring the bacteria. The knife itself may convey the disease, as is shown by the following in- Fig, 13-—-Body canker cleaned out, treated and painted about eight weeks after treatment. Good healthy calluses formed, which with proper care will completely heal this wound. eident: While making inoculations into the body of an apple tree on the station grounds, I had ceeasion to remove from near the base a large sprout of several years’ growth. This I did with my knife which I had but shortly before used to eut bark from a fresh eanker.