The first important notice of this disease in the United States dates back about fifteen years. It is a condition more or less common to cherry and peach - trees in old neglected orchards, particularly in central and eastern America. The trouble also prevails in Germany and Australia. A similar disease caused by a closely related variety of the causal fungus affects the plum.
Some varieties of peaches are said to be injured more than others, although none is free from Die Back. It is thought that the varying local conditions are responsible for such differences in more cases than can be attributed to varietal susceptibility. Affected twigs and branches are killed back; even large limbs are often severely injured.
The pathogene attacks only the woody portions of the tree. Rough black cankers, sometimes large affected areas, appear on limbs. This condition is popularly referred to as sun-scald. The lesions center about a bud or a wound. At times there is an enlargement at the point of injury. Evidences of the disease are noticeable in late winter or early spring months. Affected twigs are at first purplish, but later scarlet, and leathery. Still later the bark becomes drab-colored, loose and wrinkled. Finally black pustules of the pathogene appear under the bark in these grayish areas. These fruiting bodies break through the bark and become covered with a silvery-white coat, thus dotting the whole diseased portion in a characteristic manner. This condition is often spoken of as silver-twig. Twigs and water sprouts are killed back in mid - winter or later. This killing occurs repeatedly until finally the affected tree is given a ragged appearance. During the growing season an affected limb or twig may be girdled, in consequence of which the foliage assumes a yellowish aspect, then suddenly wilts and dies.
While the causal factor is designated as the fungus Valsa leucostoma var. cincta, the general opinion prevails that other factors such as frost, unfavorable soil, lack of cultivation and other neglect play an important role in the production of the Die Back disease. Indeed, some prominent authorities hold that the prime cause is the action of low temperatures. The fungus may in any case be regarded as an exciting cause at least, and in its absence the other above-named factors would doubtless exhibit less influence in bringing about the trouble. The pathogene characteristically attacks trees in a weakened condition, like those injured by frost and fire. And this is a real source of danger, inasmuch as such trees when taken at this disadvantage suffer injury far in excess of that induced by frost alone. Furthermore the semi-parasitic nature of the fungus allows it to live indefinitely about the orchard on dead limbs, ready to attack any trees that are in poor condition. A warm spell in the spring followed by a freeze renders the tissue of the peach - limbs favorable to the growth of the fungus.
In the spring, March and April, conidia coil out in long, reddish brown masses from the fruiting bodies already mentioned. The process is favored by quiet, damp weather, and it takes place in a few hours following the advent of favorable conditions. The spores are scattered by the wind, rain and probably by birds. Within twenty-four to forty-eight hours germination occurs, and the germtube enters the bark through a bud or through a wound of any sort. The germtube soon develops a copious growth of mycelium which is found between the outer bark and the wood in grayish mats. Numerous gum pockets are formed in the cambium and inner bark; many of these unite to form larger ones. The gum thus formed exerts a pressure on the bark, which is ruptured, and an exudation follows. This gum-flow, sometimes referred to as gum-mosis, is characteristic of the peach and other stone - fruit trees when injured (see page 303).
The fungus may grow down the twigs into the branches, and in this manner large limbs and even trunks become infected. On all affected parts pycnidia are developed in abundance. They occur most abundantly, however, on twigs, while perithecia are most common on the limbs and trunks; the two kinds of fruiting bodies may be intermingled on trunks and larger limbs. Ascospores are discharged from January to April, while conidia, as noted above, are disseminated in March and April.
Sometimes the advance of the fungus is halted, and affected parts may outgrow the disease. Frequently, however, the fungus remains active until late in the fall; its progress is then interrupted by the first freezing weather. With the return of favorable weather the fungus renews activities and the mycelium spreads.
The fungus is ever-present on stone-fruit trees, and shows a marked preference for trees already in a poor condition as a result of Frost Injury and other detrimental factors. It would therefore appear essential that trees be given the best of care; they should be in a mature condition before winter. The control measures involved in Frost Injury are discussed on page 43.
Spores of the fungus are formed on all affected parts; these spores cause infections. It is therefore advisable that diseased limbs and other parts be removed. Their destruction would seem necessary on account of the ability of the fungus to live as a saprophyte until such time as the trees are in a condition favorable to attacks by the pathogene.
It has been shown that spraying to protect susceptible parts is not wholly reliable. However, the application of bordeaux mixture and lime sulfur to the bark greatly reduces the disease, and is believed worthy of further trial. Fall and early spring applications, when no foliage is present, are considered highly desirable where the disease is troublesome. When spraying for Leaf Curl, San Jose scale or other enemies, the bark may be coated for Die Back.
Rolfs, F. M. Winter killing of twigs, cankers, and sun scald of peach trees. Missouri State Fruit Exp. Sta. Bul. 17: 9 - 101. 1910.
Rolfs, F. M. A disease of neglected peach trees. Missouri State Bd. Hort. Rept. 2: 278 - 283. 1908.