SPORULATWN 



309 



mum, or degrees of fluctuation, IMathur et al. (1950) reported that 15 

 to 20°C. favors conidium formation by Colletotrichum lindemuthianum in 

 culture. Sporulation was less at 25°C. and ceased at 30°C. Mrak and 

 Bonar (1938) found that temperature influenced the relative size of asci 

 and spores of Debaryomyces. The ascus was much larger than the spore 

 cluster at 4°C., but the spores nearly filled the ascus at 25°C. 



13 



15 



7 9 11 



Incubation period (days) 



Fig. 58. The effects of temperature and time of incubation on sporulation of Piri- 

 rularia oryzae on rice-polish agar. (Courtesy of Henry and Andersen, Phytopathology 

 38 : 272, 1948.) 



An interesting selective effect of temperature upon type of asexual 

 sporulation is found in Choanephora cucurbitarum (Barnett and Lilly, 

 1950). This fungus produces two types of asexual spores, those produced 

 in typical sporangia and conidia borne in heads. Only the conidia are 

 found commonly in nature, while both types are abundant in culture. 

 When the fungus was grown in Petri dishes at 25°C., 87 per cent of the 

 reproductive structures were conidial heads, while 13 per cent were 

 sporangia (Table 54). When the temperature was increased to 30°C., 

 this proportion was nearly reversed. At 31°C. many sporangia but no 

 conidia were formed. No sporulation occurred at 34°C., but mycelial 

 growth was abundant. Temperature also affected, either directly or 

 indirectly, the size of the sporangia. Those produced at 25°C. averaged 

 60 to 90 n in diameter, while those formed at 30 or 31°C. were much larger, 

 averaging approximately 145 fi. It seems likely that this effect is indirect, 

 being a reflection of the relative number of conidia, which are formed first 

 under favorable conditions. We may assume that the production of 

 abundant conidia uses much of the food materials which might also go 



