REVIEW OF FUSIFORM RUST INOCULATION TECHNIQUES 353 



Results from recent studies of C. fusi forme which support the above 

 approach are: (1) the possible existence of racial variation in the 

 pathogen (Snow, Powers and Kais, 1969), (2) in the field, significant 

 differences between 3 and 5 year rust ratings occur (LaFarge and Kraus, 

 1967), and (3) preliminary data indicate a close agreement between 

 artificial (screening-shed) and natural inoculations with regard to 

 relative susceptibility of test trees (Dinus, 1969 and this proceedings). 



Although this intensive screening procedure appears feasible, it is 

 designed to detect only a "supertree", i.e., one that is resistant at 

 age 1 month through 6 years to different sources of inoculum and on a 

 variety of sites. Because screening-shed inoculations subject very 

 young seedlings to high levels of inoculum, we might be discarding seed- 

 lings that would be resistant at an older age when they are normally 

 exposed to the fungus in the field. After all, we can protect young 

 seedlings in the nursery. If resistance increases with age, perhaps more 

 emphasis should be placed on steps (2) and (3) above. In regard to field 

 testing, we know from the seed source studies that some sources perform 

 best in given areas and, therefore, probably will not be planted on all 

 sites. Is it realistic to test this material over a large range of 

 sites? 



A most important consideration in any rust resistance program and 

 one that is related directly to inoculation technique and evaluation of 

 the results is that of horizontal versus vertical resistance. Assuming 

 racial variation in C. fusiforme exists, it would be a mistake to utilize 

 a selection scheme that would overlook or discard horizontal resistance. 

 For, unless a phenomenon such as "stabilization selection" (van der Plank, 

 1968) is operating in the forest and can be utilized, there are obvious 

 dangers involved in the management of plantations containing only one or 

 several vertically resistant trees. Although with existing heterozygosity 

 and open pollinated seed orchards, pine genotypes will probably not become 

 as homozygous as other agricultural crops. 



A critical question in any consideration of inoculation techniques 

 is that of the effect of inoculum density on disease development. The 

 relationship between numbers of sporidia and numbers of foliar lesions 

 and subsequent stem galls needs clarification in respect to genetic and 

 environmental variability. This information would be of primary importance 

 to individuals investigating disease resistance, especially those concerned 

 with screening programs. 



At present, the best selection criterion for fusiform rust resistance 

 is the number of cankers per infected tree (Goddard and Strickland, 1966; 

 LaFarge and Kraus, 1967). These authors found a significant positive 

 correlation between the number of cankers per infected tree and the percent 

 of diseased trees within progeny lines. Needle symptoms are not always 

 indicative of subsequent gall formation. Resistant shortleaf pines {Finns 

 echinata Mill.) develop needle symptoms but no galls (Henry and Jewell, 

 1963; Jewell, 1966; Hare, these proceedings). Possibly, resistant 

 varieties of slash pine (P. elliottii Engelm. var. elliottii) or loblolly 

 pine (P. taeda L.) would react in a similar manner. To the contrary, 

 susceptible slash pine seedlings can develop cotyledonary symptoms with- 

 out subsequent gall formation (Powers, 1968); this further complicates 

 the use of foliar lesions as an indication of resistance. 



