i;8 



Larch and Spruce Canker. 



are very minute and of a dull yellow colour, and although by 

 no means rare, are apt to be overlooked unless specially 

 searched for (Figs. 2 — 3, PI. I.). 



Wounds for artificial infection were usually made by inserting 

 the point of a lancet quite through the bark to the cambium 

 zone, exercising at the same time a little lateral pressure so that 

 a small crack about two lines long was made. Into this wound 

 either ascospores or mycelium were introduced. When the weather 

 was very warm and dry the inoculated wounds were protected 

 for a week by a covering of oiled paper to prevent too rapid 

 drying. The first indication of the success of an inoculation was 

 usually manifested during the sixth to eighth week after the 

 experiment, when the outer dead bark became cracked and 

 raised, due to accelerated growth of the living cortex underneath ; 

 and at the expiration of about ten weeks a few pustules of 

 conidia, accompanied by a small number of usually imperfectly 

 developed ascophores, appeared if the weather continued moist ; 

 but, as a rule, it was not until the following year, in April or 

 May, that well-formed cups were produced. 



In addition to the kind of wound described above, I have 

 proved by repeated experiments that a pin-prick makes a wound 

 sufficiently large for the purpose of a successful inoculation, if 

 spores are placed in the drop of liquid oozing out of the 

 wound. 



May is the month during which artificial infection takes most 

 readily, and I imagine that the same holds good in a state of 

 nature, for the following reasons. Well-developed ascophores 

 are most abundant during this month, and there is a super- 

 abundance of sap which readily oozes to the surface through the 

 smallest puncture it is possible to make with a fine needle. In 

 this extruded sap the ascospores readily germinate and enter 

 the living tissues of the tree. 



The large percentage of instances where the canker appeared 

 only at the point of artificial inoculation proved that the disease 

 was in reality the outcome of such inoculation ; nevertheless, in 

 two instances where four-year-old larches, growing in pots, and 

 obtained from a locality where the disease was supposed to be 

 entirely absent, were inoculated, I was surprised to find that in 

 one tree the canker appeared simultaneous!)'' at three different 



