WHITE-PIXE BLISTER RUST, 57 



were protected by surrounding trees or other objects, the rust spread 

 Httle or not at all. In other words, where moisture was plentiful 

 through the season, the distance of spread by urediniospores was 

 governed by factors controlling the free access of the wind. In 

 Essex County, N. Y., drought prevailed through July and August 

 in 1918. Here Pennington ■"' worked with spore ti-aps and Snell (128) 

 gave special attention to a study of the spread of the uredinial stage 

 on Ribes. The Ribes of this section were largely Rihes rotundifolium 

 and R. glandulosum. Here the rust spread in most instances only 

 to adjacent leaves from those first infected on a given bush. The 

 spore traps here caught urediniospores only at a very short distance, 

 50 feet or less. 



In 1919, York (cited in Spauldmg, 146) caught urediniospores up 

 to a distance of 3,400 feet in an open location, but they did not 

 germinate. Urediniospores caught at 3,200 feet did germinate. 

 Pennington ^^ caught urediniospores up to 156 feet. 



AGENTS DISSEMINATING THE I'REDINIOSPORES. 



Wind has been supposed to be the principal agent distributing 

 the urediniospores of Cronartium rihicola. While this supposition is 

 correct in the main, other agents are concerned in the matter. 



Hennings (53) says that sprinkling diseased Ribes plants with a 

 strong stream of water carries urediniospores from plant to plant. 

 Rain accompanied by high wind is kno\vn to carry spores of some 

 plant diseases (38, 44) . It is entirely possible for this to occur with 

 any spore capable of wind distribution, as in the present case. 



In 1917 Gravatt and Marshall (45) made studies in the experi- 

 mental greenhouse at Washington, D. C. They found that weevils, 

 snails, slugs, and sow bugs fed on the uredinia and telia of Cronartium 

 rihicola on the diseased plants. The ingested urediniospores lost 

 their viability to a large extent, but not entirely. 



In the same year Gravatt and Posey (46), working at Kittery 

 Point, Me., found that gipsy-moth larvae feed quite freely upon leaves 

 of Rihfis hirtellum and R. vulgare and that in some cases the only 

 infected leaves were those which had been partially eaten by insects, 

 indicating that they carried the spores which infected the leaves. 

 The insects were found carrying viable urediniospores on their bodies. 

 There can be no doubt that these insects play an important role in 

 the local distribution of this fungus within the gipsy-moth infested 

 area. 



Studies by Snell (127) at Lewis, N. Y., m 191S, show that a large 

 number of insects visit Ribes plants during the season when the rust 

 is present upon the leaves. The spore-laden insects were inclosed 

 in chambers with the tips of Rihes glandulosum plants, and infection 



36 Pennington, L. H. Op. cit. 



