432 ROBERT F. PATTON 



Breeding for resistance to white pine blister rust, incited by 

 Cronartium ribicola J. C. Fischer ex Rabenh. , appears to be the most 

 practical approach toward long-term control of this disease. Several 

 programs are now actively directed toward the development of blister rust 

 resistant white pines in both eastern and western United States and in 

 Canada. So it is appropriate to consider our present knowledge of the 

 biology of this rust fungus in relation to resistance expressed by the 

 pine hosts. Emphasis in this paper is placed primarily on selected 

 elements of the infection process and the relationships of these and 

 critical factors in the genetics of the pathogen to the expression of 

 resistance by white pines. 



THE BASIDIOSPORE 



Since the basidiospore, or sporidium, is the infecting agent of the 

 pine, considerable attention has been given to conditions influencing its 

 production and germination. These conditions are probably most important 

 in nature from an epidemiological standpoint, but also are important to 

 the tree breeder in progeny testing, both in test plots with exposure to 

 natural inoculum and under conditions of artificial inoculation. Although 

 spore germination is a prerequisite for infection, unfortunately in 

 progeny testing infection does not always follow spore germination. 

 Variability in germination behavior of basidiospores and the influence 

 of subtle microclimatic effects at the site of spore germination are some 

 of the more important aspects of basidiospore behavior still to be 

 clarified. 



PRODUCTION OF BASIDIOSPORES 



Some variation occurs in the reports of conditions necessary for 

 teliospore germination and formation of basidiospores (Bega, 1959) . 

 York and Snell (1922) reported that basidiospores were fully developed 

 5-6 hours after teliospores were exposed to favorable conditions for 

 germination. At the opposite extreme, Van Arsdel, Riker, and Patton 

 (1956) found that 36 hours were required for germination of teliospores 

 formed at a constant 16°C, and 42 to 48 hours for germination of basidio- 

 spores and infection of pine. Hirt (1942) calculated 11-1/2 hours as a 

 minimum time for infection after basidiospores were cast on pines but 

 more infection resulted during longer periods of favorable conditions. 

 The variations in techniques used by various workers, from floating 

 excised telial columns on water to exposing intact plants with infected 

 leaves in a mist chamber, undoubtedly influence the time for production 

 of basidiospores. However, the inherent variability of the fungus is 

 perhaps readily expresssed by exposure to a variety of environmental 

 conditions from the period of telial formation to casting of basidio- 

 spores. About 6 to 14 hours (Bega, 1959; Hirt, 1935) seem to be the 

 range of time most commonly encountered for basidiospore formation. 

 Moisture at 96 to 100% relative humidity (Hirt, 1935), or as a free 

 water film, is necessary for germination in the optimal temperature 

 range of about 12 to 20°C. 



Viability of teliospores is affected by environmental influences, 

 and marked variations in spore casts are produced by changing meteoro- 

 logical conditions (Hirt, 1942). In particular, the temperature at which 

 the telial column was formed has a marked effect on the subsequent germi- 

 nation of the teliospores (Van Arsdel, Riker, and Patton, 1956). These 

 effects could be of importance in progeny testing by artificial inoculation. 



