EFFECTS OF TEMPERATURE ON GERMINATION 221 



The tolerance of fungus spores to low temperatures should be 

 subjected to study by methods patterned after those dealing with 

 thermal death points. Such studies appear not to have been ac- 

 complished, except for relatively few species. This topic is sum- 

 marized by Luvet and Gehenio (1941) and is briefly discussed 

 in Chapter 5. Investigations of the effects of cold on fungi, espe- 

 cially rusts, have been largely concerned with overwintering, as 

 related to the source of inoculum for the development of disease 

 outbreaks. Christman (1905) and Horner (1921) are among 



Fig. 41. Effect of temperature upon survival of Botrytis cinerea. Percent- 

 age surviving plotted against time in minutes, except for the 37° C curve, 

 the intervals of which are 30 minutes. (After J. H. Smith.) 



those who have investigated survival of urediniospores of cereal 

 rusts. Ewert (1910) noted that a small proportion of conidia of 

 Psendopeziza ribls survived the winter when exposed to outdoor 

 temperatures as low as — 22° C. Several exposures of Fusicladhim 

 dendriticum and F. pirinum to freezing greatly reduced their 

 percentage of germination. The conidia of Mycosphacrella sen- 

 tinel, however, artificially subjected to alternate freezing at tem- 

 peratures as low as -16° C and thawing, retained germinability 

 as well as did untreated ones. 



Many Ascomycetes known to possess a conidial stage can over- 

 winter in this stage. The ascospores of others are mature in fall 



