This disease is known to growers as Rough Bark. It attracted more than usual attention in Virginia in 1909. The more careful observers have noted the disease on the Yellow Newtown; the trouble is so common in Virginia on this variety that at times the roughness is regarded as a character, and is used as a mark of identification. While not common or serious on any other varieties, it is known to affect the York and Winesap. It has recently been found on apples and pears in California.
The Rough Bark disease is most serious on the smaller branches of old neglected trees. The one-year-old twigs are very susceptible, while the current year's growth escapes. Sometimes larger branches and even the trunks are affected. The diseased bark sinks in definite areas; these spots are dark, cracked, and have ragged margins (Fig. 39). These characters give rise to the name Rough Bark. Some of the lesions spread uninterruptedly until the whole branch is involved. Small branches are thus occasionally killed as a result of girdling. Other affected areas are halted and the wounds are occluded. As a result of the above abnormalities the foliage assumes an unhealthy aspect; it becomes pale and cannot function properly. The leaves are never directly affected under natural conditions.
Only recently has it been shown that the rough bark of the Yellow Newtown is a disease and not a normal characteristic. The trouble is now known to be due to the fungus Phomopsis Mali. It gains entrance to the bark tissues through wounds in the earlier part of the growing-season. While the fungus can be induced under artificial conditions to develop on the fruits and foliage of the apple, yet the lesions on these organs are not regarded as economic phases of the disease. Shortly after the fungus attacks the bark, pycnidia develop from the mycelium. From these fruiting pustules spores ooze forth in cream-colored tendrils. A moisture-period of some duration is essential to this process. These spores are capable of initiating new lesions on the bark. The fungus hibernates in cankers which have been developed throughout the growing - season. In the spring the pathogene renews activities, thus perpetuating itself from year to year.
Fig. 39 - Rough Bark canker of apple.
Spraying is said to offer little hope for the control of Rough Bark. In most cases it would be a tedious operation to attempt eradication of all diseased twigs. On the other hand, the decision in this regard must rest with the grower who is especially desirous of cultivating the Yellow Newtown in those apple regions where the disease is most prevalent. The susceptibility of the above-named variety should be borne in mind in contemplation of orchard planting; the experience of the local growers of this variety should be the guide in determining whether it can be profitably grown in spite of Rough Bark.
Roberts, J. W. The Rough Bark disease of the Yellow Newtown apple.
U. S. Agr. Dept. Plant Indus. Bur. Bul. 280: 7-15. 1913. Roberts, J. W. A new fungus on the apple. Phytopath. 2:263 - 264.