30 BULLETIN 957, U. S. DEPARTMENT OF AGEICULTUEE. 



Rhoads ^" came to the conclusion that the disease spreads from the 

 original point of infection upward and downward at about equal 

 rates of progress. Posey and Gravatt, and Rhoads found that it 

 spreads at nearly the same rate laterally on both sides. Progress 

 upward and dowuM^ard is usually more rapid than it is sidewise. It 

 also appeared that in infections not yet bearing aecia, the point of 

 greatest swelling is where infection first took place and where secia 

 will be first produced. Very often a dead twig or a leaf scar shows 

 very plainly the original point of entrance of the fungus to the bark. 

 Rhoads concluded that infections on shaded lower branches do not 

 spread as rapidly as those on vigorously growing ones, but Posey and 

 Gravatt " find that the former are more likely to be attacked by 

 secondary fungi, which soon kill the branches. 



Posey and Gravatt " found that there are more or less frequent 

 instances in old infected areas of white pines where the infections on 

 lateral branches die out. The statement (131, p. 16; 141, p. 5) that 

 trees once infected with this fungus never recover was largely based 

 upon studies of trunk infections. Like all rules, it has its exceptions, 

 as here indicated. At Kittery Point, Me., Posey and Gravatt 

 studied one of the oldest outbreaks in North America. Trees of all 

 ages from a few years up to 50 or 75 were infected. Here it was found 

 that secondary fungi often kill the blister rust in an infected branch 

 and that increasing suppression of lower branches killed many of the 

 infected ones before the blister rust spread to the trunk of the tree. 

 It was found that about 15 per cent of all the infected trees in the area 

 studied recovered from the disease by the action of these two factors. 



THE PYCNIA AND PYCNOSPORES OF CRONARTIUM RrBICOLA. 



The pycnospores of the Uredinales have received comparatively 

 little attention, since it has been generally accepted that they are 

 apparently functionless (10, 21, 70, 110, 122, 155). The writer can 

 find but little data upon which this idea is based. Plowright (110), 

 Thaxter (158), Jaczewski (47), and Klebahn (68) are the only investi- 

 gators known to the writer who have actually inoculated plants with 

 the pycnospores of their rusts. It seems that the pycnospores should 

 be more thoroughly tested. 



The work with pycnospores of Cronartium rihicola in Europe seems 

 to be limited to that of Klebahn (68) , who made inoculations with them 

 upon young Pinus strohus trees without infection occurring. Even 

 in this case there was not a clear-cut result, such as is to be desired. 

 More recently CoUey (18) has shown the importance of the pycnial 

 spots, drops, and scars as symptoms of the blister rust in pine bark, 

 and still more recently (20) he has investigated their morphology and 

 cytology. 



i» Rhoads, A. S. Op. cit. 



11 Posey, G. B., and Gravatt, G. F. Field studies on the white-pine bUster rust at Kittery Point, Me. 

 Seen in manuscript. 



