370 



ARBORICULTURE 



ARBORICULTURE 



350. Stem of white pine tree attacked by 

 the white pine blister-rust fungus, Cronartium 

 ribicola. The fruiting stage is here shown in 

 the process of rupturing the bark. 



problem of saving individual trees in affected regions 

 has not been solved. The very nature of the disease 

 precludes spraying, and tree surgery methods have not 

 given anything more than temporary success. Incipient 

 cankers are so difficult to locate and, by producing conidia 



when only three 

 or four weeks 

 old, serve to 

 spread the fun- 

 gus so rapidly 

 over the tree that 

 human efforts so 

 far have failed. 



Rust diseases. 



The most de- 

 structive and 

 important rust 

 diseases of trees 

 are those caused 

 by different spe- 

 cies of the Peri- 

 dermiums on 

 coniferous trees. 

 These rust fungi 

 attack the nee- 

 dles, twigs, or 

 limbs of conifer- 

 ous trees, often 

 producing de- 

 foliation or com- 

 plete death of 

 the tree. Notable among these diseases is the white-pine 

 blister-rust caused by the Peridermium stage of the 

 fungus Cronartium ribicola (Fig. 350) which has its Cro- 

 nartium stage on species of currants and gooseberries. 

 The fungus is not native to this country but has recently 

 been imported from Europe. In Europe the disease has 

 practically exterminated the white pine (Pinus Strobus) 

 in many localities, and in this country the danger of 

 a similar destruction of this species is now being faced, 

 if the fungus acts with the same virulence as in its 

 native environment. In 1912 the Federal Board of 

 Horticulture placed a quarantine on most of the coun- 

 tries of Europe, which prevents anyone bringing white 

 pine trees from the-se countries into the United States. 

 The fungus attacks white pine from one to twenty 

 years old, the mycelium growing in the bark and usu- 

 ally producing a swelling. 

 The orange - yellow f ru it ing 

 bodies burst through the bark 

 in one to five years after in- 

 fection. The spores from the 

 pine can affect the gooseberry 

 or currant leaves only. On 

 these hosts other kinds of 

 spores are borne in the fall 

 which can infect the white 

 pine only and thus the yearly 

 cycle is completed. The black 

 currant (Ribes nigrum) is by 

 far the most virulently af- 

 fected in this country, and it 

 is advised that plants of this 

 species are therefore ex- 

 tremely dangerous to the 

 white pine industry and their 

 culture should be discouraged. 

 By keeping currants and 

 gooseberries at least 500 feet 

 away from white pines, a 

 partial safeguard is provided 

 against the passing of the 



toe, Arceuthobium pusil- fu "g us f f m , one h st * ti 

 lum, causing witches' other. All plants affected by 

 brooms. this fungus should be imme- 



diately destroyed. Consult Bureau of Plant Industry, 

 U. S. Dept. of Agric., Bulletin 206 for further details. 



Witches' broom diseases. 



Excessive branching at any point on a limb, forming 

 a compact cluster of short stubby branches, is often 

 caused by numerous agents, both insect and fungus, 

 as well as by the mistletoes. In the South, especially, 

 trees of many species are disfigured by these witches 

 brooms caused by the American mistletoe, Phoraden- 

 dron flavescens and related forms. In the northern states 

 the dwarf mistletoe, Arceuthobium pusillum (Fig. 351) 

 causes similar brooms on the black spruce (Picea 

 Mariana). Witches' brooms on red cedar (Juniperus 

 virginiana) ere caused by the rust Gymnosporangium 

 nidus-avis. 



Root diseases. 



The principal root disease of trees that is widely dis- 

 tributed throughout the United States is that caused 

 by the mushroom Armillaria mellea. Fig. 352. The 

 mycelium of this fungus grows in the soil on decaying 

 wood as a saprophyte, but can also become parasitic, 

 killing trees over large areas. Almost any of our indig- 

 enous trees may be affected 

 with this disease. Fruit trees 

 often suffer in certain re- 

 gions. The mycelium pene- 

 trates the roots and grows in 

 the bark and wood, working 

 up into the base of the tree. 

 The destruction of the root- 

 system is so rapid, however, 

 that two or three years after 

 infection the tree often dies, 

 before the fungus has spread 

 any great distance up the 

 trunk. In the older decayed 

 parts of the 

 root and in the 

 soil, the myce- 

 lium is aggre- 

 gated into long 

 black, flat- 

 tened strands 

 called "rhizo- 

 morphs," or 

 more com- 

 monly termed, 

 "shoe-strings." 

 In the case 

 of conifer- 

 ous trees, an 

 abundant exu- 

 date of resin- 

 ous sap occurs 

 at the base of 

 affected trees, 

 which forms 

 the surrounding soil into a cake. The disease may be 

 recognized by the gradual yellowing of the foliage, 

 decay of the rooks, presence of the "shoe-strings" in 

 the soil, and the appearance of the honey-yellow mush- 

 rooms around the base of the tree. No practical method 

 of treatment is known. Diseased trees and the mush- 

 rooms should be destroyed. 



Other soil organisms may enter through wounds in 

 roots and cause diseases. Such a disease may, after 

 becoming established in the roots, continue to spread up 

 the trunk. Thus wounded root-systems are as danger- 

 ous a source of infection as wounded branches. Roots 

 often suffer from winter injury largely because the wood 

 ripens from the top of the tree downward, and when 

 severe conditions are undergone early in the winter 

 before the new wood of the roots has ripened, it is 

 killed. 



352. Base of young white pine tree attacked 

 by Armillaria mellea. Note the cake of earth 

 around the tree due to the ezudate of resin, also 

 the white mycelium felts within the bark and 

 the enlarged resin vesicles induced by the 

 fungus. The slender strands clinging to the cake 

 of earth are the "shoe strings." 



