Methods in Rust Investigations 247 



house, the spores are collected by fastening paraflfined paper loosely 

 around the infected parts of the plants in such a way that there are 

 several openings for ventilation. The ripe spores which fall in this 

 loose paper bag are scraped off and applied with a scalpel to the plants 

 under investigation. Where sufficient inoculum is collected, it is dusted 

 by using the glass blower described by Durrell and Parker (1920). 

 Oftentimes in the greenhouse the peridia of aecia do not open but pro- 

 ject out as a long cylindric column from which no spores are shed. If 

 some of the infected plant parts bearing such aecia are placed upon 

 waxed paper in a moist chamber over night, the peridial cells will fall 

 apart and the mass of spores can be scraped from the paper for inocu- 

 lating. 



Teliospores. In those rusts where the teliospores are deciduous, 

 dusting can be employed very effectively, as, for example, in Kunkelia 

 nitens. However, in most species of the genus Puccinia, the teliospores 

 are not deciduous and therefore must either be scraped off and applied 

 with a scalpel or the plant parts bearing the teliospores laid on or 

 around the plants to be inoculated in such a way that the basidiospores 

 will reach the parts to be infected. Where sufficient material is avail- 

 able, the latter method probably is the better, as Melhus, Durrell, and 

 Kirby (1920) point out that the teliospores of Puccinia graininis germi- 

 nate better if not detached from the straw. 



6. Condition of Plant Inoculated. In general, the plants to be in- 

 oculated should be in the best of growing conditions. It is a mistaken 

 idea that weaker plants are more subject to rust. Plants which are 

 in a weak and stunted condition, poorly developed from any cause do 

 not readily infect or if infection takes place the rust does not develop 

 well, the uredinia are fewer and smaller and often paler in color than 

 when the host is developing vigorously. This also is true for parts of 

 the same plant. Usually the young rapidly-developing plant parts are 

 most readily and vigorously infected, the older, especially the senescent 

 leaves and stems, showing much less infection. Not only is this true 

 in case of the development of uredinia, but especially in the case of 

 aecia. 



The aecia of Puccinia Clematidis developing on the young, rapidly- 

 growing stem of Cleinatis virginiana often produce marked hypertro- 

 phies and galls with the production of a large number of pycnia and 

 aecia. Infection on very old leaves of the same plant usually produces 

 only a few pycnia or possibly also a few aecia. On the other hand, 

 telia usually develop when metabolism and growth are slowed up as 

 in the case of ripening grain, under poor conditions of light and growth 

 during midwinter in the greenhouse, or on old senescent leaves. How- 

 ever, the species of rusts vary considerably in this regard. Thus while 

 Puccinia viontanensis upon seedling grasses in the greenhouse in mid- 

 winter goes to telia so readily that it is sometimes difficult to keep it 

 in culture, Puccinia triticina upon wheat seedlings has never been seen 

 by the writer to produce telia under such conditions even on old dying 

 leaves, but does produce some telia on old leaves when the wheat plants 

 approach maturity. 



