Apparently this disease is very limited in its geographical range. Reports of it come from only three states, Georgia, North Carolina and Alabama. In this region, however, it is of considerable importance, and in Georgia more especially it is regarded as an important factor in commercial plum - growing. Japanese plums and their hybrids are particularly sensitive to wilt. The disease appears to have been known for several years.
Plum-wilt (Fig. 111) resembles Fire Blight and for some years was thought to be of a similar nature. Leaves on a branch or a whole tree suddenly wilt in the spring or early summer. Examination of the base of the wilted portion will reveal dark, dead bark. Frequently this dead area extends along one side of the trunk to the surface of the ground. Within a year from first wilting, the whole tree may die.
Before wilting, diseased trees may be located by the scorched appearance of their leaves. This is particularly noticeable in dry weather in late summer. The edges of the leaves become dry. This, however, is not a specific sign of the wilt disease inasmuch as this appearance may be induced by any factor which cuts off the water - supply. Trees growing in a dry, hard soil as well as those with diseased roots are very liable to exhibit symptoms similar to those of the early stages of wilt.
From those wilted trees which die in the early summer, gum is exuded. Later beetles bore holes in the bark through which gum flows freely. Death of the tree follows in the fall and winter, and no leaves are put out the next spring. Old trees are said to suffer more than young ones.
Recent investigations have shown conclusively that plum-wilt is due to the fungus Lasiodiplodia Triflorce. The pathogene gains entrance into the tree by means of wounds. Observations show that at least 33 per cent of the infections occur through borer wounds. Borers prefer Japanese varieties, probably because they are budded on peach stock in which the borers make their attack. Wounds made by the Black Spot pathogene, Bacterium Pruni, are also very common points of entrance for the plum - wilt fungus. In this connection it is to be noted that Japanese plums are frequently attacked by Bacterium Pruni, which fact may, in part, explain their susceptibility to wilt. The wilt fungus may also enter the tree by way of wounds made by cultivating and pruning.
Fig. 111. - Plum - wilt.
On infecting the plum-tree the fungus spreads out in the bark. Later the medullary rays and sap-tubes are invaded. In the latter the mycelium passes from one duct to another through the pits in the walls. The wood is not destroyed to any noticeable degree. But through the ducts the fungus spreads rapidly; it may pass from a branch into the trunk and thus kill the whole tree. The attacked bark, medullary rays and cambium are killed and finally turn black. During the invasion of the ducts, gum is produced. This partially plugs the vessels and the sap-flow is checked; thus the leaves show a tendency to wilt. Around the margin of an infected area may be found a layer of gum-filled tissue. This sharply limits the fungus for a time. Older trees are not able to limit the fungus in this way, hence they suffer more. Trees eight to ten years old develop gum more readily, yet not in quantities sufficient to prevent rapid spread of the mycelium. While gum stops the fungus temporarily, the deposit in the ducts injures the host. When the attack results in gum deposition throughout a cross - section of a trunk or limb, the affected member dies, apparently from a lack of water.
From the foregoing the following points should be borne in mind where control measures are necessary: (1) the disease is most serious in Georgia. It occurs also in North Carolina and Alabama. Warmer climates seem to favor it. (2) The causal fungus gets into the tree only through wounds. Of these wounds, those made by the peach-tree borer, the Black Spot pathogene, cultivating and pruning are most prominent. Borers and the Black Spot organism prefer Japanese plums, because they are budded on peach stock. It is therefore suggested that if some stock other than peach could be satisfactorily substituted, much will have been done toward the control of plum-wilt. This line of treatment may offer permanent relief. One should avoid making wounds of any sort. Care should be taken to clean and dress all wounds wherever feasible. Control measures against the Black Spot pathogene (Bacterium Pruni) are to be emphasized in this connection (see page 311).
Higgins, B. B. Plum wilt. Georgia Agr. Exp. Sta. Bul. 118: 3 - 29.
1916. Higgins, B. B. Some serious diseases of plum and peach trees.
Georgia State Hort. Soc. Proc. 1915: 42-45. 1915. Stucky, H. P., and Temple, J. C. The plum wilt. Georgia State Hort. Soc. Proc. 1911: 68 - 72. 1911.