58 BULLETIN 957, U. S. DEPAETMENT OF AGRICULTURE. 



resulted in due time. Many of these insects are, of course, accidental 

 visitors, but quite a number feed or breed upon the Ribes plants. 

 All of those which frequent the Ribes bushes by preference may carry 

 many spores of both uredinia and telia. Such insects would be 

 most likely to spread the disease, since upon leaving one Ribes plant 

 they would seek another, thus scattering the spores exactly where 

 they could start new colonies of the disease. But the accidental 

 visitors, in a locality where Ribes bushes are abundant, could also 

 spread the spores locally, but in a much more indiscriminate manner, 

 so that but a very small percentage of the spores would ever reach 

 leaves of Ribes. 



Aside from carrying spores on their bodies, some insects feed 

 directly on the uredinia and telia and a few of the excreted spores 

 are known to retain their viability. Still other insects may be leaf 

 eaters and ingest the rust spores only accidentally. These would be 

 of minor importance in spreading the disease. 



At Lewis, N. Y., where the Ribes bushes overhang narrow cattle 

 paths which wind through a heavy cover of blackberry, raspberry, 

 and other low shrubs, observations by Pennington and Snell indicate 

 that cattle, sheep, horses, dogs, and berry pickers may carry the ure- 

 diniospores from an infected bush to neighboring healthy ones. 



The remarks on the carrying of seciospores by automobiles, steam 

 trains, and electric cars on page 36 apply also to some extent to the 

 urediniospores. 



GERMINATION OF THE UREDINIOSPORES. 



The urediniospores of Cronartium ribicola have been generally 

 found to be erratic in germinating. At one time excellent germina- 

 tion occurs; at another, none at all. In the greenhouse experiments 

 it seems that urediniospores produced in newly formed uredinia have 

 greater infective power than those produced later in the same ure- 

 dinia. Such early urediniospores seem to give as good results as 

 fresh aeciospores. The former are usually produced in limited quan- 

 tities while the latter are usually abundant. This results in a more 

 liberal use of the latter, so that a fair comparison of the two is not 

 possible from the usual inoculation work. Gravatt made compara- 

 tive tests and concluded that seciospores and urediniospores from 

 newly open sori were about ecjual in infective power. More such 

 tests should be made before any conclusive statement is made on 

 this point. 



Gravatt and York found that newly matured urediniospores pro- 

 duced out of doors were decidedly more viable than older ones. 

 This agrees with general experience in making inoculations in the 

 greenhouse. 



Gravatt found that cooling the Urediniospores on ice stimulated 

 germination. Uncooled spores gave 15 to 23 per cent germination 



