378 ROBERT F. PATTON 



INOCULUM CONSIDERATIONS 



Source of Inoculum 



The production of a reasonably homogeneous inoculum is necessary for 

 any large-scale progeny testing program, and for more detailed quantita- 

 tive studies on the infection process and resistance. Compared with the 

 difficulties and problems encountered by workers who must produce and 

 maintain inoculum of obligate parasites on host plants, growing faculta- 

 tive saprophytes on common laboratory media is a convenience to be 

 jealously regarded by rust workers. But according to Ellingboe (1968), 

 if uniformity of inoculum is a consideration, the natural host-parasite 

 relationship is an advantage, for there seems to be greater uniformity 

 in inoculum produced on the natural host than in inoculum produced on 

 agar media. 



Numerous early investigations on blister rust did not come up with 

 any conclusive proof that pine infection from inoculum originating on 

 different ribes species varied in a qualitative manner. Snell (1942) 

 speculated that there was a "threshold" or "quantum" relation between 

 basidiospore production and infection. For artificial inoculation, the 

 concern has been to have as large a production of telia and basidiospores 

 as possible, and this was most easily accomplished by using Ribes nigrum 

 L. For many of the inoculations in Wisconsin's Blister Rust Nursery, 

 infected leaves of R. nigrum plants still present in some private gardens 

 provided an abundance of inoculum. These had to be supplemented in other 

 years by collections from the native ribes species growing in the wild. 

 The supply of abundant inoculum can be one of the most worrisome problems 

 in large-scale progeny testing programs. 



The approach taken by Heimburger in Ontario was to select a strain 

 or strains of R. nigrum that retained rust -infected leaves late into the 

 season and to propagate these in a lath house. Rust infection could be 

 initiated by aeciospore inoculations in the spring, if necessary, and 

 uredial intensification provided for an abundance of telial inoculum in 

 the late summer and fall. A similar procedure is being followed for the 

 resistance breeding program conducted by Region 9 of the U. S. Forest 

 Service. Cuttings of Heimburger' s leaf-retaining strains were obtained 

 and these are being propagated for establishment of a "rust garden" to 

 serve as an inoculum source for progeny tests. 



Storage of Inoculum 



The amount of inoculum produced in the wild, or in disease gardens, 

 varies and is subject to the vagaries of the weather. The information 

 that Van Arsdel, Riker, and Patton (1956) presented on the effect of high 

 temperatures, particularly during the telial formation period, on 

 fertility of teliospores possibly helps to explain the occasional poor 

 results experienced in some past inoculations and should be considered 

 in future evaluations of test results. Dependence upon fresh inoculum 

 also makes timing of the actual inoculation subject to weather influences 

 that could affect initial infection and the course of disease development. 

 In the Lake States and southern Ontario where most inoculation tests 

 with eastern white pine have been made, hot dry weather in late summer 

 or early fall has often made it extremely difficult to provide favorable 

 conditions for inoculations, and has probably influenced the viability 

 of the inoculum itself. 



