ARBORICULTURE 



ARBORICULTURE 



369 



In brief, the general action of the fungi that cause 

 tree diseases is as follows: The fungus exists in two 

 stages, the vegetative and the fruiting. The vegetative 

 stage consists of a mycelium, which is an aggregate of 

 long much-branched microscopic threads of fungous 

 cells. This mycelium penetrates and grows in the body 

 of the tree, extracting nutriment from it by dissolving 

 the different structures it can utilize. In this process 

 the tissues of the tree are destroyed and the fungus 

 obtains food for its further development. In time, 

 after sufficient food has been stored in the mycelium, 

 the fungus forms its fruiting bodies. In the case of the 

 larger number of the heatt- and sap-wood rots, this 

 fruit body consists of a shelf-like or toadstool-like 

 structure on the exterior of the tree. In these fruiting 

 structures are borne millions of microscopic spores or 

 seeds of the fungus, which are distributed largely by 

 the wind. If by chance one of these spores finds lodg- 

 ment on an exposed wound in the bark of a tree which 

 it is capable of infecting, it will under proper condi- 

 tions germinate, producing a mycelium that will 

 penetrate the tissues of the new host. 



Seedling diseases. 



The young seedlings in the nursery beds are often 

 attacked by certain soil organisms that cause a decay 

 of the stem at the surface of the ground, allowing the 

 plant to wilt and fall over. This type of disease is called 

 "damping off;" in many cases it is caused by the 

 fungus Pythium de Baryanum. This disease is largely 

 induced bj' verj' moist soils, a moist atmosphere, high 

 temperature, and little ventilation. By growing seed- 

 lings in light sandy soils, well watered but not exces- 

 sively, and by aerating the beds well to reduce the 

 humidity of the atmosphere around the plants, this 

 trouble is practically avoided. Soils in which it has 

 once occurred should be abandoned, however. In 

 greenhouses in which this disease is often troublesome 

 and the precautionary measures mentioned above 

 difficult to maintain it may be necessary to sterilize 

 the soil with formaldehj-de or by steam heat immedi- 

 ately before sowing the seed. 



Leaf diseases. 



Very little serious damage is caused by most leaf 

 diseases of trees other than the unsightly appearance 

 produced. There are a few leaf diseases, however, 

 that are of more or less importance in the United 

 States. Conspicuous among these is the anthracnose 

 disease of oak and sycamore leaves, caused by the 

 fungus Gnotnonia vetieta. Throughout northeastern 

 United States in recent years the plane-tree (Plakinus 



occidentalis) has been 

 almost universally af- 

 fected and the trees 

 often completely defoli- 

 ated before the first of 

 July. Early in the sea- 

 son the disease becomes 

 manifest by the pro- 

 duction of rapidly en- 

 larging brown blotches 

 which are usually pro- 

 duced along the veins 

 of the leaf. If examined 

 closely, the numerous 

 spore clusters can be 

 seen. These clusters are 

 composed of innumer- 

 able microscopic con- 

 idia which serve for the dissemination of the fungus. 

 The twigs are commonly affected also and on these, as 

 well as the fallen leaves, the fungus winters over. The 

 same fungus produces a similar disease on oaks, especi- 

 ally the white oak, and is often epidemic in certain 

 regions. As a measure of prevention, all fallen leaves 



24 



348. Maple leaf affected by the tar- 

 spot fungus, Rbytisma acerinum. 



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and twigs from diseased trees should be carefully col- 

 lected and burned. All diseased twigs should be pruned 

 off and burned. If desirable, the trees may be sprayed 

 with standard bordeaux mixture from the time the 

 leaves begin to unfokl, at periods of two or three weeks, 

 until the first of July. 



The tar-spot disease of the maple 

 caused by Rhytisnm acerinum is a con- 

 spicuous disease but seldom causes 

 serious damage to the tree. Fig. 348. 

 The diseased .spots appear as blotches 

 of tar on the upper surface of the 

 leaves. Burning the diseased leaves 

 will materially lessen the chances of 

 attack the coming year since the 

 pathogen winters in them. 



Maple trees, at least in the north- 

 eastern states, have suffered during 

 the past few summers from a physio- 

 logical trouble called leaf-scorch. The 

 leaves of one side or the whole of the 

 tree turn brown between the veins and 

 finally dry up. This disease has been 

 ascribed to over-transpiration of the 

 leaf -tissues at a time of year when the 

 roots cannot furnish enough water to 

 the tree to compensate the loss. Thus 

 the leaf-cells are killed by desiccation 

 and the brown blotches appear. It is 

 advised that artificial watering during 

 hot, dry weather will materially lessen 

 the severity of the trouble, especially 

 under city conditions. 



Canker diseases. 



A diseased condition of the limbs 349. canker 

 or trunk of a tree that results in the disease of the 

 death of an area of the bark is termed chestnut pro- 

 a canker. These cankered areas may duced by the 

 be either sunken or enlarged, and the p'^f^iuca° °The 

 terms necrotic and hypertrophy cank- conidial tendrils 

 ers are used respectively to designate are produced 

 these characters. Not all cankers are f™" the numer- 

 produced by parasitic organisms ; some °"^ ruptures 

 are the result of malnutrition, winter ba^t.° '" * 

 injury, sun-scorch, insects, and the like. 



By far the worst epidemic disease of trees in America 

 is the chestnut-canker now prevalent in many of the 

 eastern states. In all, a total loss of .$2.5,000,000 has 

 been ascribed to this disease between the time it was 

 first noticed, in 1904, on Long Island, and the year 

 1912. The cankers produced on the chestnut are 

 caused by the fungus Endothia parasitica. Fig. 349. 

 At present, the question of the origin of this disease in 

 the United States is undetermined. The fungus works 

 rapidly in the bark and sap-wood, killing the tissues. 

 Fruiting bodies of two types are formed on the dis- 

 eased bark, a conidial stage that appears as long 

 yellow tendrils of gelatinous matter and the reddish 

 yellow to dark brown cushion-like stromata that contain 

 the ascospores. These latter spores are ejected in large 

 numbers at every rainy period throughout the spring, 

 summer, and fall and are carried by the wind, readily 

 causing new infections. On the individual tree, the 

 fungus is rapidly spread by the washing down of the 

 conidia into wounds in the bark. The cankered areas 

 soon cause the death of the parts above by girdUng 

 the limb and, in most cases, complete death of the tree 

 follows in two or three years. The state of Pennsyl- 

 vania has taken steps to eradicate tliis disease by seeking 

 out diseased trees and compelling owners to cut all 

 affected trees in the western half of the state. The 

 extremely virulent nature of the disease, however, has 

 led more conservative pathologists to question the 

 advisability of expending large sums of money with so 

 slight a chance of ultimate or complete success. The 



