ARBORICULTURE 



ARBORICULTURE 



37 i 



Heart-rot diseases. 



Most of the wood-rotting fungi belong to a class 

 known as the wound parasites. By far the most univer- 

 sally distributed and destructive of these is the shelf 

 fungus, Fames igniarius (Figs. 353, 354) , which causes the 

 white heart-rot of deciduous trees. Infection may occur 

 in any part of the tree where wood is exposed. The my- 

 celium, by dissolving the lignin of the woody elements, 

 first causes a brown decay that is later succeeded by a 

 whitish decomposition, at which stage very little of 

 the original woody structures is left. This decom- 

 position of the heartwood takes place rapidly and 

 often the tree is blown over. The fruiting bodies of this 

 fungus are produced usually at the point of infection. 

 They consist of large hoof-shaped bodies, carbonaceous 

 and rough on the upper surface, and brown beneath. 

 The under surface is full of small holes which are the 

 openings of long vertical tubes, on the inner surface of 

 which the spores are borne. These spores sift out of 

 the tubes and are distributed by the wind. 



Another fungus commonly met with, and acting 

 similarly, is Fames fomentarius. This is also at times 

 a wound parasite but works mostly in the sapwood 

 first and later spreads into the heartwood. A yel- 

 lowish white, soft decay is produced. Large sheets of a 

 chamois-skin-like character are often found in trees 

 rotted by this fungus. These sheets are compact masses 

 of the mycelium which have filled up the long cracks 

 produced by the shrinking of the diseased wood. The 

 sporophores are essentially different from those of F. 

 igniarius, being light gray and smooth on top and light 

 gray-brown beneath, with relatively large pores. Nu- 

 merous other shelf fungi occur as wound parasites and 

 do more or less damage where the bark, when wounded, 

 was not protected until the natural callus could form. 

 Some of the fungi which are wound parasites of living 

 trees continue a saprophytic life in structural timbers, 

 causing their ultimate destruction. 



Winter injury, sun-scald and drought 



Many misconceptions are prevalent concerning the 

 real effect of freezing and sun-scald on trees. The 

 injury by freezing in trees is due to the action of low 

 temperature upon the water in the cells of the tree or 

 to the tension pressures induced. As the temperature 

 appi caches freezing, the water in the cell begins to pass 

 out into the intercellular spaces and there accumulates. 

 The lower the temperature goes, the more water is 

 extracted from the cell and stored in the intercellular 

 spaces as ice crystals. When a certain temperature is 

 reached, the amount of water withdrawn is such that 

 the living elements of the cell can no longer survive, and 

 are killed. This specific minimum temperature varies 

 for different species of trees, accounting for the diffei ence 

 in susceptibility to winter injury. The living tissues 

 of the tree constitute the bark and sapwood, and when 

 areas of these tissues are killed, serious damage is done to 

 the tree. Either enough damage is done to the roots, 

 trunks, or twigs to cause ultimate death the coming 

 summer, or the areas that were killed serve as infection 

 courts for the various wound parasites like the heart- 

 rot or sap-rot fungi. In the case of sun-scald, the action 

 is exactly similar, with the exception that the water is 

 removed from one side of the tree by excessive heat 

 during the growing season, instead of by freezing. The 

 cracking and splitting of the bark of trees may be due 

 to the tension pressure set up in the bark by the con- 

 tracting of the woody cylinder in circumference during 

 periods of low temperature. 



Norway maple trees planted for shade are especially 

 susceptible to sun-scald because they are grown close 

 together in the nursery to a large size and then, when 

 planted out, they are often injured by the hot sun. 

 Care should be taken not to attempt to grow trees 

 farther north than their natural range, which is an 



indication of their ability to withstand the minimum 

 temperatures. 



Trees suffer in dry seasons from drought. Water 

 is transpired from the leaves of a large tree, in great 

 quantities. The hotter and dryer the atmosphere, the 

 more water is transpired. Thus it happens that during 

 continued droughts there is not sufficient soil-water 

 present for the roots to supply the requisite quantity 

 to compensate for this large loss, and the leaves are 

 killed by dessication. Dead tops or stag-head ultimately 

 result in cases in which such conditions continue year 

 after year. Artificial watering in such cases will relieve 

 this condition and largely prevent the trouble. 



Excessively wet soil will also act in a similar way. 

 Tree roots in saturated soil are unable to take up water 

 because they are suffocated by the lack of oxyeen. 

 Hence the tops suffer as from drought. 



Smoke and gas injury. 



In manufacturing districts in which large quantities 

 of smoke are produced, extensive injuries to trees as 

 well as other vegetation often result. The injurious 

 property of the smoke is mainly due to the sulfur 



353. Sporophore of Fomes igniarius var. nip;ricans. Spore- 

 bearing surface shown by the lighter color. 



dioxid that it contains. This gas, when dissolved 

 in the cell-sap of the leaf or the soil-water, produces 

 sulf uric acid, which is poisonous to the tree. 



The roots of trees are also injured by escaping sewer- 

 gas or illuminating gas, which either poisons or suffo- 

 cates the roots. Trees killed by escaping gas in the soil 

 usually shed their outer bark in large patches near the 

 base, the inner bark remaining tighly appressed to the 

 tree. 



Trees that come into contact with electric wires are 

 injured in proportion to the strength of the current and 

 the duration of the contact. Often trees are killed in 

 cities from this cause. 



Gases arising from recently laid creosoted paving- 

 blocks or macadam bound with tar often cause defoli- 

 ation of nearby trees, by the poisoning of the cell- 

 contents of the leaves. 



Sanitary measures for the prevention of diseases. 



(1) As a first means of preventing disease in trees, 

 one should buy clean stock from nurseries regularly 

 inspected by state agents. Stock should be bought from 

 nurseries in this country, owing to the great danger of 

 importing diseases which might become epidemic. (2) 

 In addition, one should use his influence in getting his 

 neighbors to eradicate diseased trees and diseased parts 

 of trees. Clean stock grown in a well-cared-for neigh- 

 borhood is the problem half solved. (3) The litter of 

 leaves or twigs under a diseased tree should be collected 



