TESTING FOR FUSIFORM RUST RESISTANCE IN SLASH PINE 



Ronald J . Dinus 

 Southern Institute of Forest Genetics 3 Southern Forest 

 i iment Station a Forest Service 3 U.S. Department of 

 Agriculture , Gulfvort, Mississippi, U.S.A. 



ABSTRACT 



Similar results were obtained in artificial and field tests for 

 resistance to the fusiform rust fungus {Cronartium fusiforme) 

 in open-pollinated progenies of six slash pines (Pinus elliottii) 

 En two artificial inoculation tests, resistance was reliably 

 identified in 9 months. At least 4 years were required to 

 distinguish resistant from susceptible lines in two field tests. 

 The proportion of plants infected was a better measure of field 

 performance than numbers of stem infections or total numbers of 

 infections. One selection performed poorly under artificial 

 conditions but better than average in the field. Its form of 

 resistance apparently differed from that of selections that 

 performed well in both tests. 



INTRODUCTION 



Since fusiform rust, caused by Cronartium fusiforme Hedge, and Hunt 

 ex Cumm. , is the most serious disease of slash pine [Pinus eLliottii 

 Engelm.), identifying resistance to it is vital to the success of tree 

 improvement programs for slash pine. Tests to locate resistant selections 

 can be performed by exposing progenies to uniform, abundant supplies of 

 inoculum under artificial conditions or to natural levels of inoculum 

 under field conditions. Jewell (1960) devised an artificial inoculation 

 technique in which telia-bearing leaves are suspended over pine seedlings 

 in the cotyledon stage under controlled conditions of temperature and 

 moisture. Subsequently, the capacity and efficiency of this method have 

 been increased (Jewell and Mallett, 1964, 1967). This and related 

 approaches (Arnold and Goddard, 1966; Davis and Goggans , 1968) may be too 

 severe a test, however, since selections having slight but useful resis- 

 tance may be eliminated. Resistance to some diseases varies with age 

 (Patton, 1961), so tests on juvenile material could be misleading. 

 Furthermore, testing at only one age with local inoculum could result in 

 selection for specialized rather than generalized resistance (Smith, 1968) 



While some of the potential shortcomings of artificial inoculation 

 are not experienced in field trials, the latter are costly and time con- 

 suming, and they too may be unreliable. In some cases, infection rates 

 under field conditions have fluctuated widely from location to location 

 and year to year (Henry and Jewell, 1963; Kinloch and Kelman, 1965; 

 LaFarge and Kraus , 1967). 



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