434 ROBERT F. PATTON 



and 21°C. Germination patterns of different 4-hour spore casts varied. 

 In one instance spores on one-half of the microscope field observed with 

 a 6.5X objective had germinated with thin, long, branched germ tubes. On 

 the other half of the field, and separated as if by an invisible straight- 

 line barrier, almost all the spores had germinated to form secondary 

 basidiospores . Presumably some subtle difference between these two por- 

 tions of the membrane substrate or its immediate environment was enough 

 to produce this distinctive difference in the type of germination among 

 basidiospores in a relatively uniform spore cast. Thus, it is easy to 

 conceive that the interaction of an inherited tendency plus a given 

 microclimate on the surface of a needle or within a stomatal pit may 

 determine the infection capability of a spore on an individual needle. 

 This may also involve the ability of a spore to form infection structures, 

 such as the vesicle and infection hyphae. 



NUCLEAR CONDITION OF BASIDIOSPORES 



Along with studies of germination morphology, investigation of the 

 nuclear condition of basidiospores may help clarify the reasons for 

 variability in germination behavior and possibly also the infection 

 capability. Most basidiospores are uninucleate. Sometimes, but not 

 always, during germination the spores are activated so that nuclear divi- 

 sion occurs. Colley (1918) reported that division of the single nucleus 

 of the basidiospore to form a binucleate basidiospore was quite common. 

 In some preliminary work in Wisconsin we found that the spore may contain 

 even 3 or 4 nuclei, after which one or more germ tubes are formed. Germ 

 tubes and vesicles with at least 2 nuclei have been observed in germinating 

 basidiospores on collodion membranes. The binucleate condition of basidio- 

 spores results not only from nuclear division of an originally uninucleate 

 basidiospore, however. It may also result from binucleate cells of a 

 promycelium, where perhaps a wall has failed to form after division of the 

 fusion nucleus of the teliospore. Such spores have been seen still 

 attached to the sterigmata. Whether germination behavior or infection 

 capability of such binucleate spores differs from that of the usual uni- 

 nucleate basidiospore is still unknown. 



THE INFECTION PROCESS 



An old question asked by Spaulding and Rathbun-Gravatt (1925) is still 

 pertinent today, particularly in view of our efforts in breeding programs 

 to separate resistant from susceptible individuals: why does not infection 

 always occur when external conditions are apparently favorable to it? It 

 seems to me that the more detailed knowledge we have of the infection 

 process the closer we can come to answering this question, and the more 

 certain we shall be that stock released as resistant will stand the test 

 of time . 



MODE OF PENETRATION 



It seems clear now that, in most if not all cases, the fungus enters 

 the pine needle through a stoma, and subsequently produces a substomatal 

 vesicle and infection hypha (Clinton and McCormick, 1919; Patton and 

 Johnson, 1966; Patton, 1967). Hirt (1938) gave a brief account, without 

 illustration, of direct penetration of epidermal cells, but at the same 

 time reported that substomatal vesicles were seen. Boyer (1962) had 



