436 ROBERT F. PATTON 



in the field and have some significance to infection and expression of 

 resistance. 



VARIATION IN HOST SUSCEPTIBILITY TO INFECTION 



The influence of age has many ramifications in a breeding and progeny- 

 testing program. Patton (1961) found that susceptibility of eastern white 

 pine to infection decreased with age. One factor in this is that the 

 number of needle penetrations decreases markedly with increasing age of 

 the tree (Patton, 1967). The greatest difference is noted between primary 

 and secondary needles, but also the number of infections in secondary 

 needles of inherently susceptible trees decreases as the age of the tree 

 increases. The influence of age upon the expression of inherent resis- 

 tance also has been shown in artificial inoculation tests of progenies 

 from crosses of resistant parents (Patton and Riker, 1966) . Inoculated 

 1-0 stock died within 2 years after inoculation whereas up to 45% of 

 4-year-old seedlings from the same progenies were resistant to infection. 



The influence of age is also expressed through effects on maturation 

 of plant parts. Van Arsdel (1968) showed that susceptibility of young 

 stems decreased with age of the shoots. Straib (1953) suggested that 

 formation of telia early in the season in one geographical area may enable 

 infection of pine at a more susceptible stage than in another area where 

 telia formed later in the season when pine tissues are more mature and 

 presumably less susceptible. Although direct infection of young stems 

 apparently is unimportant in nature, it might play a role in artificial 

 testing programs. Conceivably, susceptibility of needles might change 

 during the season. This change, along with differences in shoot suscepti- 

 bility, might well be considered in comparisons of inoculation results 

 from different areas, as in international test programs and where stock 

 was submitted to cooperators in other regions or countries for testing. 



One expression of host resistance is its effect on the number of 

 needle penetrations. Penetration of the germ tube into the substomatal 

 chamber may be reduced almost to zero in very highly resistant selections 

 (Patton, 1967). In testing progenies of resistant selections it could be 

 important to determine whether this was the only resistance mechanism or 

 whether others might act at a later stage in disease development. 



The influence of vigor on susceptibility of hosts to infection by 

 obligate parasites is well known. Boyer (1967a) showed that susceptibility 

 of white pine seedlings to infection by C. ribioola was related to vigor 

 as indicated by dry weight. Seedlings of low vigor grown on two poor 

 soils developed fewer infection loci than more vigorous seedlings that 

 grew on richer soils. Awareness of such a quantitative response is 

 important in evaluating results of artificial inoculations in progeny 

 testing. 



STOMATAL RELATIONS TO INFECTION 



There is a body of observational evidence for the influence of age 

 upon host susceptibility and the expression of resistance, but the 

 mechanisms of this influence are unknown. Since entry to the needle is 

 gained through the stomata, some attention has been given to stomatal 

 influences. Hirt (1938) concluded that it was difficult to determine 

 what relation, if any, stomatal activity may have to needle penetration. 



