PATHOLOGY AND GENETICS OF WHITE PINE BLISTER RUST 



437 



He noted that, if entry depends on access to open stomata, a relatively 

 large number of stomata are open to some degree at all times of the day 

 or night and fungal invasion would be permitted at any time. Germ tubes 

 grow at random on the needle surface without any apparent regard for 

 stomata (Hirt, 1938; Patton, 1967). Although Hirt called attention to 

 the waxy plugs in the outer stomatal pits, he did not determine whether 

 they influenced infection. In observations of germinating spores on 

 needle surfaces, we saw numerous germ tubes growing across the wax plugs. 

 Similarly, on microtome cross sections of needles embedded in paraffin. 

 we observed germ tubes that had crossed the stomatal pit, although the 

 wax plug had been dissolved in processing. No germ tubes have ever been 

 seen penetrating solid wax plugs, but have been observed growing into 

 large crevices in the plug, or into stomatal pits lined with a wax layer 

 but not completely plugged. 



To determine whether wax plugs were related to age, we compared the 

 numbers of stomatal pits occluded with waxy deposits in cryostat- 

 sectioned fresh needles from different sources (Table 1) . In this sample 

 a much larger percentage of pits were open or partially open in primary 

 needles of 2-1/2-month-old seedlings than in secondary needles of three 

 other sources. This relationship corresponded well with a similar com- 

 parison of vesicles formed in needles of trees of different age and 

 susceptibility (Patton, 1967). Thus, it appears that wax plugs often 

 prevent growth of germ tubes down into the outer stomatal pits, and 

 consequently reduce the number of opportunities for infection even of 

 susceptible needles. This effect may well be one influence of age, but 

 is not considered a maior mechanism of resistance. 



Table 1. Occlusion of outer stomatal pits by wax deposits in 

 needles from trees of different ages 





Needle 



Age and type 



of host 



Total No. 



of pits 

 observed 



Percentage 



of 



pits 



type 



Open 



Partial 

 open 



iy 



Plugged 



Primary 

 Secondary 



2-1/2-month-old 

 seedlings 



4-year-old seedling 



654 

 699 



69.1 

 10.0 



17.5 

 15.8 





15.4 

 74.2 



Secondary Graft of 40-year-old 



susceptible tree 1,254 



5.1 



7.9 



89.0 



Secondary Graft of approximately 

 50-year-old resistant 

 tree 901 



2.0 



91.2 



Another aspect of stomatal influence on infection in relation to age 

 was intial growth of germ tubes into the outer stomatal pits . We found 

 that the number of germ tubes that entered the stomatal pit and grew 

 down to the guard cells, without further penetration into the substomatal 

 chamber, was considerably less in secondary than in primary needles 

 (Patton, 1967). The chances for a germ tube to enter a stomatal pit may 

 be reduced by wax plugs in secondary needles. But the failure of germ 



