RUST RESISTANCE OP OAT VARIETIES. 7 



On the upper leaf blades of the plants inoculated at the time of 

 heading, where one would normally expect the ultimate production 

 of telia, the resistant plants seemed to produce them at a remarkably 

 early stage. These early telia were produced, in all cases observed, 

 only, on leaves infected with the crown rust. It is the belief of the 

 writer that such a hastening in the completion of the life cycle of 

 the fungus is entirely comparable to the well-known instances in 

 the seed plants of the influence of unfavorable conditions, such as 

 drought, poor soil, and injury, in hastening the period of blooming 

 and the maturing of seed. 



The teliospores are to be considered necessarily as the final stage 

 in the year's life cycle of the rust. These spores do not serve to 

 spread the infection during the current season, but provide a means 

 for the reproduction of the disease another season or when conditions 

 are again favorable. 



It may be argued that the ability which the fungus in these par- 

 ticular varieties possesses to produce this final spore form is the best 

 possible indication of extreme susceptibility. Nevertheless, it is 

 certainly true that if a variety is able to prevent the formation of 

 the summer-spore stage of the rust, the spread of the disease will be 

 checked in localities where only such varieties are grown. 



This unusual occurrence of telia on seedling leaves is thought to be 

 an additional evidence of resistance. It has been used sometimes in 

 making the distinction' between resistant and susceptible individual 

 plants. Whether this interpretation is accepted or not, the presence 

 of telia on seedling plants of some and their absence on other strains 

 grown under identical conditions is very good evidence of real dif- 

 ferences in the protoplasmic reaction of the two hosts, for the prog- 

 ress of the rust fungus following infection is quite different in the 

 two instances. The same stimuli, whether they are chemical, enzymic, 

 or osmotic, which cause the formation of flecks in some varieties 

 and not in others may exercise a rather direct influence on the ability 

 of the parasite to produce a particular spore form at any given time. 



Norton (10) reports the abundant occurrence of aecia of the 

 asparagus rust on plants which were resistant to the uredinial stage 

 of the rust. 



Smith (11), in studies of the water relations of asparagus rust, has 

 found that " a very direct relation exists between atmospheric mois- 

 ture and the prevalence of the several spore forms of the rust," and 

 that " the teleuto stage may occur in asparagus beds little affected by 

 the rust, and apparently not preceded by any trace of the other 

 spore stages." He concludes that "the teleuto stage is then to be 

 regarded as a provision for surviving any condition unfavorable to 

 the fungus, whether of food supply, moisture, temperature, or resist- 

 ance by the host, without regard to season." 



