PATHOLOGY AND GENETICS OF WHITE PINE BLISTER RUST 433 



GERMINATION OF BASIDIOSPORES 



Germination of basidiospores requires 100% relative humidty or direct 

 contact with water and may occur within the range of above to 20°C 

 (Hirt, 1935) , essentially the same conditions required for germination of 

 teliospores. Germination per se , however, is not an accurate measure of 

 response of basidiospores to external conditions, as indicated by Bega 

 (1960) . He noted that other factors were involved, including type of 

 germination, vigor (length of germ tubes), the period involved, and the 

 type and condition of the substrate. Most conclusions about basidiospore 

 germination capabilities have been based on spore behavior, under rela- 

 tively constant temperature and moisture conditions, on a water film or 

 an agar surface. Deviations from such results might well be expected on 

 pine needles under fluctuating temperature and moisture conditions in the 

 field. 



Three different types of germination of basidiospores have been seen 

 on white pine needles: indirect germination with formation of a secondary 

 basidiospore; direct germination with formation of one and up to seven 

 thin, sometimes branched germ tubes; or direct germination by means of a 

 single thick, somewhat irregular germ tube that did not branch or did so 

 only rarely (Patton and Nicholls, 1966). In our experience all three 

 types usually have occurred on the same needle along with ungerminated 

 spores, although in some experiments one type occasionally was more preva- 

 lent than others. In fact, the type of germination has been extremely 

 variable in inoculation experiments and spore germination trials. Such 

 variation was not related to the type, age, or source of needle. Certainly, 

 variation in germination on needles from different sources was not the 

 reason for differences in amount of infection observed with increasing 

 age of the pine host or with inherent resistance of a selection (Patton, 

 1967). 



The role of indirect germination in nature is still undetermined. 

 Hirt (1935) reported that germination of secondary basidiospores usually 

 resulted in formation of germ tubes. Bega (1960) , however, was able to 

 carry secondary basidiospore formation on water agar through six genera- 

 tions, but indicated that on pine needles only very few (0 to 1%) 

 secondary basidiospores were formed. This was taken as an argument that 

 secondary basidiospore formation was a response to an unsuitable host or 

 substrate. In some of our tests, however, up to 47% of spores cast on 

 needles (in counts of 200 to 900 spores per test) have produced secondary 

 basidiospores (Patton, 1967) . 



The relative significance of thick and thin germ tubes in terms of 

 infection capability still is unknown, although observations so far indi- 

 cate that penetration into stomata, formation of substomatal vesicles, and 

 subsequent infection of the needle mesophyll result from growth of "normal" 

 thin germ tubes (Patton, 1967). 



Finally, equating conditions for basidiospore germination with infec- 

 tion is still not valid. It seems possible that individual spores have 

 some inherited tendency to germinate in one way or another, but perhaps 

 microenvironmental influences, such as minute variations in a water film 

 on a needle surface, have the greatest effect in determining what type 

 of germination occurs. One example of variation under experimental 

 conditions may illustrate our lack of knowledge concerning spore behavior. 

 Basidiospores were cast on a collodion membrane floating on water in a 

 closed petri dish with diurnal fluctuation of temperature between 10 



