TEMPERATURE AND REPRODUCTION 111 



ature is above that lethal to the smut fungus during the period 

 when the seedlings are being grown in seed beds and are being 

 transplanted. The prevalence of peach-leaf curl and apple scab is 

 correlated with cold, wet spring weather. Late blight of potatoes 

 is entirely absent, or at least never epiphvtotic, in the Coastal 

 Plains area of the southeastern United States if the crop matures 

 in late May or in June, when summer temperatures prevail. The 

 fundus which causes downy mildew of tobacco disappears rather 

 quickly after a few warm days with temperatures in excess of 85° 

 F [Dixon, McLean, and Wolf (1936)]. The observations of 

 Stevens (1917) led him to conclude that temperature is the chief 

 climatic influence in the growth of the chestnut-blight fungus, 

 Endothia parasitica. Sclerotium rolfsii is limited to warm regions 

 and becomes of importance only during hot weather. 



TEMPERATURE AND REPRODUCTION 



There is abundant evidence that temperatures favorable for 

 germination or for growth of fungi may be slightly lower than 

 those favorable for reproduction. In some instances mycelial 

 growth occurs at high temperatures that are inhibitory to repro- 

 duction. Ames (1915) determined that the spores of Thielaviop- 

 sis paradoxa germinate at 5° to 6° C, and, although there is slight 

 growth at 10° C, this organism must be provided with tempera- 

 tures in excess of 10° C to induce fruiting. If the temperature 

 is elevated to 36° C, however, the mycelium develops, but conidia 

 are not produced. Similar differences were noted at both the 

 upper and lower limits for Glomerella rufomacirians, which ger- 

 minates at 4° C, but requires a minimum of 12° C to produce 

 spores. Pemcillhnn digitatinn is able to germinate and grow at 

 30° C, but no conidia are formed at this temperature. 



Sweet (1941) recorded that the formation of cleistothecia by 

 Magnnsia nitida and M. brachytrichia occurs throughout the 

 range 16° to 38° C, although conidial germination is secured 

 throughout the range 1.5° to 43° C. Production of conidia, 

 however, is limited to the range 10° to 38° C in M . nitida, and 16° 

 to 40.5° C in M. brachytrichia. 



Sporulation by Peronospora tabacina occurs within a range of 

 temperature from 42° to 63° F and is most abundant at 56° F 



