PROBLEMS IN PREVENTING PLANT-DISEASE EPIDEMICS 65 



not yet been propagated on anything except living plants. It also is generally 

 known that stem rust is heteroecious ; like many other rusts it requires two 

 distinct kinds of plants to complete its full life cycle. Like other fungi, stem 

 rust multiplies and reproduces by means of microscopic spores, one one- 

 thousandth of an inch or less in size. Stem rust produces two kinds of spores 

 on wheat, the urediospores, or summer spores, of the red stage, and the 

 teliospores, or winter spores, of the black stage. The urediospores can germi- 

 nate immediately after they are formed; they can infect wheat and produce 

 successive generations about once a week on growing plants. As the wheat 

 begins to ripen, however, teliospores, or winter spores, are formed. They 

 require exposure to cold weather, especially alternate freezing and thawing, 

 before they will germinate. They therefore survive severe winters and germi- 

 nate in the spring. On germination, the teliospore sends out one or two germ 

 tubes, each of which produces four sporidia, a third kind of spore. The 

 sporidia are forcibly shot off from the tubes, or promycelia, on which they 

 are formed, are disseminated by wind, and can infect only certain kinds of 

 barberry, on which a fourth and fifth kind of spore are produced, the pycnio- 

 spores and the aeciospores, or cluster-cup spores. The pycniospores are re- 

 stricted to a sexual function, but the aeciospores are forcibly shot out from 

 the cups, in which they are formed in long, closely packed chains. The aecio- 

 spores are disseminated by the wind and can infect only wheat or certain 

 other wild or cultivated grasses, resulting in the formation of the uredial 

 stage. It is only the uredial stage, then, that can produce successive genera- 

 tions of spores on wheat and that enables the rust to spread rapidly from 

 wheat to wheat. 



The basic reason why stem rust can become quickly and widely epidemic 

 is that it can multiply so rapidly from small beginnings. A urediospore may 

 germinate in less than an hour, send out a germ tube that grows along the 

 epidermal surface of wheat, enters through a stoma, and is inside of the plant 

 within six hours or even less. The rust tubes, or hyphae, then branch and 

 grow parasitically between the plant cells, form an extensive network, or 

 mycelium, about 5 mm. in extent, which then produces a new crop of 50 

 thousand to 450 thousand urediospores within a week or ten days. Each of 

 the new spores can then repeat the process, and this can go on and on as 

 long as wheat is green and growing. On a single barberry bush in northern 

 United States there may be about 70 billion aeciospores by the middle of May. 

 If only 1 per cent caused infection and produced small uredial pustules on 

 wheat, the progeny would be 70 thousand billion urediospores. On an acre 

 of moderately rusted wheat there are about 50 thousand billion urediospores, 

 each one capable of surviving a long air journey and starting infection many 

 miles from the place where it was produced. 



The astronomically large numbers of rust spores can be quickly dissemi- 

 nated far and wide by the wind. Only the teliospores and pycniospores are not 



