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ALLAN KLINGSTROM 



of aecia. It should be pointed out again that 2 years elapse between 

 inoculation and the development of aecia. Further, it has not been 

 established which climatic factors play a decisive part during this 

 period. 



Table 1. Production of P. pini aeciospores from 50 inocula- 

 tions per week performed on 25 6-year-old pines for 7 

 consecutive weeks 



Temperature at 

 Inoculation time of 

 date inoculation 



Weather at 



time of 

 inoculation 



No. of inoculated 

 branches producing 

 aecia 2 yr. later 



June 15 

 June 22 



June 29 

 July 6 



28 C 

 25 C 



18 C 

 20 C 



clear, breezy 



partly cloudy 

 rising wind 



overcast, windy 



light showers 

 calm 



10 

 1 



July 13 



July 20 

 July 27 



18 C 



27 C 

 21 C 



partly cloudy 

 breezy 



clear, breezy 



clear, windy 



Aeciospores can be kept viable for a long time when held at low 

 temperatures. Spores stored at 4°C and -25°C have infected pines after 

 1 year. Condensation cannot be allowed to form in the test tube in 

 which the spores are stored. Spores collected in damp weather do not 

 store as well as those gathered in dry weather. Their reduced germina- 

 tion, in addition to that caused by the spores' metabolic activity, can 

 also be due to degeneration caused by contaminating Penicillia and 

 similar fungi. Under storage at 20°C, the spores' ability to germinate 

 successively declined during a period of about 4 weeks. At the same time 

 the color of the spores changed from yellow to white. 



A generally held opinion about P. pini on pine is that the pines 

 show juvenile resistance but are susceptible in the higher age classes. 

 This reflects the common occurrence of the rust on older trees long 

 exposed under natural conditions (Mtilder, 1953). Inoculation apparently 

 breaks through whatever barries (real or fancied) there might be. 



Even very small plants can be inoculated (Fig. 1), and tests have 

 been successful on year-old plants. A serious obstacle, however, is that 

 many plants die before a definite diagnosis for Peridermiwn can be made. 

 Other fungi, e.g., needle casts, confound the diagnosis. Further, such 

 small plants have only one leader shoot that can be inoculated. Waiting 

 1 year more makes it possible to utilize both the terminal leader and 

 surrounding shoots. And by then it is unusual for the plants to die 

 before the Pevidermium attack can be diagnosed with certainty. 



