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314 ALLAN KLINGSTRO'M 



coincidence of rainy weather with the brief period of time when the 

 shoots are elongating and receptive to the fungus. The ability of the 

 teliospores to germinate increases with rising spring temperatures, after 

 a chilling requirement is satisfied. Although germination is inhibited 

 very effectively to start with, the inhibition disappears in conjunction 

 with warm weather and rain (Klingstro'm, 1965a) . A similar development 

 has been demonstrated under .American conditions (Ziller, 1965) . The 

 frequency of pine twisting rust changes from year to year under natural 

 conditions and within very wide limits. 



In large parts of the southern European range, the fungus is able to 

 overwinter in living aspen twigs. Consequently it is less dependent on 

 host alternation with pine. This has been emphasized in several works 

 (Klebahn, 1958; Regler, 195"; Moriondo, 1956, 1961), but it has not been 

 determined whether the fungus can overwinter in each of the various 

 Populus species or hybrids. Overwintering has not been proved in 

 Populus tvemula L., the most common alternate host in northern Europe, 

 even though Kujala (1950) suggests that overwintering on P. tvemula 

 occurs. And the observations of Roll-Hansen (196") seem to make the 

 overwintering on ?. tvemula certain. 



!-hz~j-!?s:?j. ?::::::: : „: - inoculation .an? inoculum 



Only a few test reports regarding the inoculation of pine with .V. 

 pinitovqua are available. Regler (195") worked with 10 1 -year-old and 

 10 2-year-old pine plants in a greenhouse. Aspen leaves with germinating 

 teliospores were held in close contact with the annual shoots of pine. 

 To some extent Schtltt (1964) used the same method, although he also relied 

 on spontaneous infection. Moriondo (1961) tried to isolate individual 

 pine shoots in test tubes together with telia-bearing aspen leaves. 



Successful inoculations can be obtained on a small scale, but it is 

 difficult to judge the usefulness of these methods, particularly if a 

 large quantity of pine material is to be treated. Tests have been made 

 with larger numbers of pine plants under nursery conditions (Klingstro'm, 

 1965a). The host plant material used in the tests was put in shallow 

 beds topped with wire netting. Moist aspen leaves were placed on the 

 netting so that the basidiospores were free to fall on the host material. 

 The leaves were held in position under a cloth which was placed over 

 the wire netting and kept damp for one night. Massive infection of 

 year-old plants was obtained. A similar method has been used in American 

 tests with Melampsova albevtensis Arth., y.elampsova medusae Thtfm. and 

 Melampsova oocidentalis Jacks, where individual plants enclosed in plastic 

 bags were exposed to free-falling spores (Ziller, 1965). 



Success of inoculation tests with .V. pinitovqua on pine is affected 

 by the difficulty of working with germinating teliospores and by the 

 extreme sensitivity of basidia and basidiospores. Basidiospores on a 

 moist aspen leaf in a dry atmosphere perish in a minute or so. Further, 

 both teliospores and basidiospores are sensitive to elevated temperatures 

 (Klingstrom, 1969). Even brief exposure to 25 to 50°C, common in labora- . I 

 tories in the summer, is sufficient to kill the basidiospores. On the 

 other hand, dry aspen leaves can be kept for a long time at low temperatures ^ 

 without any apparent effect on the ability of the teliospores to germinate 

 (KlingstrOm, 1969). The storing of dry inoculum therefore presents no ; 

 problem, a fact that has been established by other workers (Molnar and 

 Sivak, 1964; Ziller, 1965). 



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