414 ALLAN KLINGSTROM 



point of attack is the annual shoot. Needles on seedlings 2-4 weeks old 

 (Regler, 1957) or older (Klingstro'm, 1963) can carry aecidia. In the 

 latter case the rust establishes itself on the needle tip. Younger pines 

 are damaged more noticeably by the fungus. As the pines increase in age, 

 injuries of a fatal nature decrease, but the number of injuries which 

 lead to deformation increase (Bo'hner, 1952; Regler, 1957). Under Swedish 

 conditions, older pines can also be attacked (Kardell, 1966). 



The germination of basidiospores has been studied in laboratory tests 

 (Klingstrom, 1963, 1969) and has also been used for bioassay in studies of, 

 among other things, inhibitors in Pinus silvestris L. (Klingstro'm, 1969). 

 However, a detailed study of infection biology in the field remains to be 

 made. Usually pycnia develop before a caeoma with aeciospores is formed, 

 but the function of the pycnia is unclear. There is, however, reason to 

 make comparisons with tests on other species of Melampsora, in which 

 exchange of pycnial fluid is necessary (Ziller, 1965) . 



The establishment of the fungus on pine usually takes place during 

 periods when a fortuitous coincidence of suitable weather for the forma- 

 tion and distribution of basidiospores occurs with the axial extension 

 of succulent pine shoots. Substances' with an inhibitory effect on 

 basidiospore germination can be traced to the surface of pine shoots, e.g. 

 to leachates in drops of water. Other substances in the pine shoots 

 which have a similar inhibiting effect on spore germination undergo a 

 considerable decline in conjunction with axial extention (Klingstro'm, 

 1969) . Experiments have included both studies of Melampsora basidiospore 

 germination and Avena straight growth test. In general these have given 

 similar results. 



RESISTANCE BIOLOGY 



Weather conditions, biologically active surface deposits, possibly 

 in conjunction with leaching, and a decline in the amount of inhibitors 

 in pine shoots during axial extension are possible factors influencing 

 the natural occurrence of M. pinitorqua. There have also been individual 

 reports about differences in the frequency of Melampsora in different 

 pine material. Rennerfelt (1954), Bergman (1954), and Klingstro'm (1963, 

 1969) have examined the differences in the attack frequency between 

 different plus tree clones. Eklundh Ehrenberg (1963), Schtitt (1964, 

 1965), Hattemer (1965), Illy (1966), and Klingstro'm (1969) have presented 

 results concerning differences in the attack frequency between progenies. 

 Certain germ plasm useful for resistance breeding seems to be available, 

 although there is as yet no breeding work using such resistant pine 

 material. But it is too early to discuss inheritance, genes involved, 

 heritability, combining ability and so on. The works cited indicate that 

 differences in attack frequency exist between clones and between progenies 

 and, further, that general combining ability may hold with certain parents 

 It should be possible to puf these results to practical use in pine seed 

 orchards with suitable clones. However, further tests are necessary. 



Even though pine materials of particular interest in the study of 

 Melampsora should become available, the question of how to score attack 

 by the twist rust in an acceptable way is still to be solved. This is 

 particularly the case with pines 3 years or more of age. Scoring number 

 of lesions per pine does not take into consideration host variation in 

 size or the number of first whorl shoots. Neither does number of lesions 

 per shoot take size into consideration, but classes terminal leaders and 



