ARTIFICIAL VS. FIELD INOCULATION WITH FUSIFORM RUST 335 



Increases in the total number of infections per plant occurred each 

 year and were consistently larger for progenies of rust-infected selec- 

 tions. However, the frequency of new infections differed greatly between 

 years and also between locations in any one year. In addition, con- 

 siderable variability existed within families. Coefficients of variation 

 decreased with years of exposure, but were never less than 41%. However, 

 differences among families have been significant since the second year 

 and family ranking has changed only slightly since the third year. At 

 Gulfport after 5 years of exposure, progenies of 18-40 and 18-62 had 

 more infections than those of any of the other four selections (Table 1) . 

 Progenies of 18-41, 18-27, and 11-6 had fewer infections, but more than 

 those of 8-7. A similar pattern emerged at Bogalusa after 4 years 

 (Table 1). Progenies of 8-7 and 11-6 had fewer infections than any 

 others. The simple correlation between the total number of infections 

 and the proportion infected was significant at the 0.01 level. 



Annual increases in numbers of newly infected plants varied between 

 years and locations (Fig. 2). However, family rankings have been similar 

 at both locations since the third year. Initially, variation within 

 families was large, but coefficients of variation have been less than 

 30% since the third year. Consequently, differences which were evident 

 among families within 2 years of planting have become more definite. 

 Regardless of location, fewer progeny of 8-7 were infected than of any 

 other selection (Fig. 2). On the other hand, progenies of 18-62 and 

 18-40 were by far the most frequently infected. Progenies of one rust- 

 infected (18-41) and two rust-free selections (18-27 and 11-6) , exhibiting 

 intermediate levels of infection, differed significantly from both the 

 least and most heavily infected families. When plants that were previously 

 infected but had recovered were deducted from totals at the last examina- 

 tion, results were essentially the same (Fig. 2). 



DISCUSSIONS AND CONCLUSIONS 



Resistant families were apparent within 3 years of planting regard- 

 less of location or index of infection. However, variability within 

 families was such that at least 4 or 5 years of field exposure were 

 required for accurate evaluation. The agreement of family rankings at the 

 two field locations indicates that the genetic control of resistance is 

 stable and little affected by interaction with these two environments. 

 Kinloch (1968) reached a similar conclusion for loblolly pine (P. taeda 

 L.) planted on a larger number of sites. The three indices of infection 

 gave the six families similar ratings and were significantly correlated. 

 However, the proportion infected seemed the most reliable in view of the 

 large error terms encountered in analyses based on the other indices. 

 Results from the proportion of plants infected paralleled those obtained 

 after adjustment for plants previously infected but not showing infection 

 at the last measurement. This agreement demonstrates that a single 

 scoring 4 or 5 years after planting can provide a realistic estimate of 

 field performance, at least in well-replicated test plantings. 



Combined analysis of the two artificial inoculation tests demon- 

 strated that the technique yields consistent results. Progenies of 8-7 

 and 18-27 resisted artificial inoculation and performed better than 

 average in the field (Fig. 3). While not best in the field, progeny of 

 18-27 were significantly more resistant than the worst families. Since 

 both these selections demonstrated heritable resistance in both types of 

 test, it appears that they can be used safely in breeding programs as 



