INOCULATION" OF WESTERN WHITE PINE WITH BLISTER RUST 363 



of air space per sq ft of ground space) which when thoroughly soaked have 

 fair capacity to withstand sudden rises in temperature without correspond- 

 ing drops in relative humidity; (2) hand misting during critically warm 

 periods; (3) good inoculum; and (4) a 72-hour inoculation run including 

 12- to 16-hour periods where near optimal conditions are maintained. 



VARIATION IN INTENSITY OF INOCULATION 



Results from inoculating almost 80 different, control lots (presumably 

 non-resistant seedlings) over 11 different seasons are given in Table 1. 



It is interesting that Control RR, used in four different seasons, 

 is the least infected (75-96%) control during the four seasons in which it 

 was used. The seed came from a residuum of seven infected trees in a very 

 heavily infected natural stand where most western white pines were already 

 i dead or dying. The seedlings were partly resistant. 



r The year-to-year variation in infection we have experienced following 

 inoculation of 2-year-old plants ranged from 75 to 89% following inocula- 



, tion of 1-year-old plants the variation of infection ranges between 76 and 

 99% (see Table 1, column 8). This is a fairly wide variation, but the 

 over-all level is certainly adequate for progeny testing if the number of 

 seedlings and replicates per progeny is kept large. 



There was no obvious relationship between number of basidiospores 

 trapped per sq mm (in 72 hours on vaseline-coated slides placed at 

 seedling level in center of inoculated seedbeds) and the intensity of 

 infection of control seedlings. Estimating that a 2-year-old seedling 

 had 400 lineal cm of secondary needles, 1 mm wide (a 4,000 sq mm 

 target), then according to the spore-cast data of columns 5 and 6, Table 1, 

 the interception of sporidia by a 2-year-old seedling would range between 

 11,000 and 48,000 sporidia, with 3,200 to 9,600 of these spores known to 

 be germinable. Corresponding estimates for 1-year-old seedlings with 

 100 lineal cm of primary needles and cotyledons (1,000 sq mm target) 

 would be 3,700 to 12,500 sporidia intercepted per seedling with 1,400 to 

 3,200 germinable spores. It was indeed surprising that variation in 



I degree of infection did not accompany such 3- to 4-fold increases in 

 exposure, sometimes involving differences of tens of thousands of sporidia 



• per plant. Other environmental factors inside the inoculation chambers 

 must have exerted over-riding influences . 



VARIATION IN UNIFORMITY OF INOCULATION 



While we may have consistently secured heavy infection of susceptible 

 control plants, we still have serious problems securing uniform inoculation 

 throughout any large experiment where several nursery beds are inoculated 

 simultaneously. 



In our earliest inoculations (1953) we were using canvas inner and 

 outer inoculation tents that straddled two nursery beds, along with con- 

 tinuous misting from a line of nozzles suspended over the aisle between 

 the beds (Fig. 4). Apparently, we were floating away basidiospores 

 collected on foliage of the inner rows of seedlings nearest the nozzle 

 line, yet allowing other basidiospores to dry and die on seedlings in the 

 1 outer rows near the drier canvas wall. Seedlings in portions of these 

 inner and outer rows (including some controls) averaged only 35 to 55% 

 infected, while infection in five control lots of the same test averaged 

 77%. 



