196 MASS. EXPERIMENT STATION BULLETIN 170. 



This fungus causes defoliation, and sometimes a great deal of injury results. The 

 mildew (Uncinula macrospora Pk.) is found on elms, and Taphrina ulmi Johan. is 

 found on Ulmus montana and Ulmus campestris. 



The American elm is very susceptible to drought and winterkilling of roots. 

 Frost cracks are also rather common on the elm, and from these and injury from 

 borers the elm bleeds rather freely. 



Sun scald, sun scorch, "bronzing" and various types of winter injury, - 

 such as root killing, death of buds, twigs and branches, frost blisters and 

 frost cracks, drought effects, "staghead" from various causes, and 

 many other troubles not caused by organisms, are quite commonly found 

 on trees. 



There is also a sooty mold that grows in the "honeydew" secreted 

 some years quite abundantly by aphids on various species, which some- 

 times causes considerable retardation of growth. The honeydew is usually 

 washed off the leaves by rains before it does very much harm, but occa- 

 sionally, in periods of drought, the concentrated sticky covering remains 

 on the leaves long enough to plasmolyse the cells, causing a mottled 

 appearance of the leaves. 



Wood-destroying Fungi. 



There are a great number of fungi that may be found on dead wood 

 following various injuries caused by sun scald, insect work, fires, illum- 

 inating gas, oil sprays and other agencies. Some of these parasites attack 

 the dead bark and penetrate living tissues of the host, destroying the 

 cell structure, and others are found in the heartwood. By far the largest 

 number of wood-destroying fungi, however, are saprophytic in nature, 

 and find congenial conditions only on dead tissue or that which has 

 become weakened from some cause. These fungi produce different 

 chemical and mechanical effects on the tissues, depending upon the nature 

 of the host and of the attacking organism. 



A great many of the fungi that attack wounds are capable of pro- 

 ducing cavities, although the heartwood fungi are the chief offenders in 

 this direction. These wood fungi are the most insidious enemies of trees, 

 and quite often no trace of their work is discovered until a great deal of 

 injury has been done. They penetrate the tissues slowly and persistently, 

 and the decay is usually so well hidden from sight that the damage does 

 not appear until the injured tissues are removed with mallet and chisel. 



While the wood-destroying fungi are responsible for much injury to 

 trees, fortunately it can be prevented by the antiseptic treatment of 

 wounds; and if the decay has progressed until cavities are formed, these 

 should be thoroughly cleaned and disinfected. The great amount of tree 

 work done during the past few years has demonstrated that the careful 

 removal of infectious material from cavities, followed by thorough anti- 

 septic treatment of the cavities, has been very successful in arresting 

 decay and preventing further injury. 



