Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER III BODY AND BRANCH DISEASES AND INJURIES Many diseases and injuries of the bark and wood are more or less common to all kinds of trees. Extremes of temperature, lightning, mistletoes, lichens and some species of fungi are some of the causal agents which affect trees in general. As in the case of the diseases and injuries common to the seedlings and leaves of many kinds of trees, the primary causal agents of the diseases affecting bark and wood are often difficult to determine. It is only by a careful analysis of the conditions surrounding the tree in question and by close ob- ‘ servation of other trees in the vicinity that clues can be obtained. Also, in such cases, the knowledge of similar tree troubles may be essential in determining the exact cause. Many of the diseases and injuries of the branches and trunk affect the appearance of the leaves before direct attention is called to the primary injury. In general, the presence or absence of fruiting-bodies of fungi on the bark is of but little value in determining whether or not the trouble is caused by a specific fungus. This is due to the fact that numerous species of fungi find excellent conditions for growth in dead bark and some species follow very closely any injuries to the bark. A trained pathologist must be closely observant to assert even tentatively that a fungus found on dead areas of bark is responsible for the lesion. The only sure way of proving the fungus to be the cause of disease is the usual procedure of isolating and growing the fungus in pure cultures and inoculating it into healthy plants. Often, however, an active patho- gene shows characteristic parasitic tendencies, especially to the trained eye, which make diagnosis more certain. A comparison of the edge of a canker on chestnut caused by E…