223 



and others. The greatest of these values is that reported by Pritchard, 

 whose statement is here quoted: "The rust was abundant within 25 yards 

 of the barberry bushes, but practically disappeared at a distance of 60 

 yards. The most persistent searching was required to discover a single 

 pustule beyond SO yards." Another observation by Pritchard 23 indicated 

 that urediniospores of /'. graminis are carried only short distances in suffi- 

 cient number to cause an epidemic. We have the published record, how- 

 ever, of rust spores having been blown as much as a mile and producing 

 infection ; and, strange as it may seem, the spores in question are the 

 smallest, most delicate ones in the life cycle of the rusts if we omit the 

 non-functional pycniospores, namely, the basidiospores. E. T. Bartholo- 

 mew-- gives a table which shows that 59.1% of the leaves on apple trees 

 near cedars were infected with rust. A quarter of a mile away it was 

 55.4% and a. mile away it was 6.5%. All this does not show, of course, that 

 rust spores are not carried by the wind for long distances in a vigorous 

 condition, but it does show that the distance for abundant infection from 

 any spore producing center is not great. 



With this as a basis it should be possible to obtain an idea of the 

 maximum distance a rust might be expected to progress in a season. The 

 greatest distance recorded above is one mile, but those spores would doubt- 

 less travel a mile and a half farther (or two and a half miles) and pro- 

 duce infection. As a factor of safety, let us double this value ; and as a 

 lurther factor of safety, let us double this latter value. This gives us ten 

 miles. A rust generation, according to Freeman and Johnson, 23 takes eight 

 to twelve days, and more in cold, bad weather; and our own results at the 

 Purdue I^xperiment Station agree very well with those figures. Assuming. 

 then, ten days for a rust generation, ten miles of migration per generation, 

 a growing season from the middle of April to the middle of October, ap- 

 proximately ISO days, and good weather with no interruption to the growth 

 of the fungus, we should expect it to migrate for a distance of ISO miles. 

 This value will be used presently in comparing the telial distribution of 

 some of the rusts with their possible recial distribution. 



PROPAGATION OF INDIVIDUAL SPECIES. 



The black rust of grasses was among the first to be observed living, 

 and apparently thriving, at long distances from any of its iccia. There 



-'Bot. Gaz. 52:184 191.1. 

 ^Phytopathology 2:255-0. 1!I12. 

 -•Bur. Plant fnd. Bull. 216:45. 1011. 



