DANGEROUS INTERNATIONAL FOREST TREE DISEASES 79 



of the rust ; whereas P. grandidentata is out of host range at least for 

 this rustrace. 



Some years afterwards the number of infections caused by the para- 

 site decreased and finally ceased, although the situation remained 

 the same in some parts of the nursery, except that pines as well as 

 poplars became older, reaching the age of about 13 to 15 years. Con- 

 sequently there is circumstantial evidence that infection chance in 

 young stands is distinct from those in older stands. 



In a related species, Melampsora larici-populina, we are often struck 

 by the fact that needle infection of the larch is extremely abundant 

 on branches hanging closely to the ground near old leaves carrying 

 telia of the rust. The higher side branches show diminishing numbers 

 of infections and after a certain distance from the ground the needles 

 remain free from them. 



This implies a limited spread of these very minute, hyaline, thin- 

 walled basidiospores, which is in contrast to the uredospores which are 

 thick- walled, spiny or warty, and suitable for dispersal by men, ani- 

 mals, and wind over long distances. 



In case of Melampsora pinitorqua, explanation as to the disappear- 

 ance of infections on pine seems to be basidiospore dispersal. 



Spread of the leaves carrying the telia over long distances is only 

 possible by wind. After establishment of this inoculum the telia may 

 germinate- in next spring and infection of the alternate host is only 

 then realized when : 



1. A susceptible host is available. 



2. A close proximity of this host is available (probably infec- 

 tion is only possible within a few meters) . 



3. When the host has a certain physiological condition (devel- 

 opment of the shoot) making infection possible. 



Range : Distribution throughout all countries of Europe and the west- 

 ern part of America. 

 Hosts : 

 Pinaceae — 



Very susceptible : 

 Pinus syVoestris L. 

 P. montana Mill. 

 P. pinaster Sol. 



P. ponder osa Laws, (in Canada) 

 Resistant : 

 Pinus laricio Poir. 

 P. nigra Arn. 

 P. rigida Mill. 

 Salicaceae — 

 Populus alba L. 

 P. canescens Sm. 

 P. tremula L. 

 P. tremuloides Michx. 

 Literature : 

 Moriondo, F. Richerche sulla Melampsora pinitorqua in Italia. 



Accad. Ital. Sc. Forest. 1956. 

 Murray, J. S. Rusts of British forest trees. Leaflet Forestry 

 ^ Comn., London, 4. 1955. 



Sylven, N. Om tallens Knackesjuka. Medd. Stat. Skogsforsoksan- 

 stalt. 1917. 



