50 PROTECTION OF PLANTS, 1918-19 



ditions in this region which would leave the potato practically free from all three 

 of these troubles are so rare as to be the exception. In other words, a given 

 season is either favorable in varying degrees to the development of early blight, 

 and tip-burn or is favorable to late blight. That is the reason why spraying 

 properly done will always pay for itself. 



Early blight, caused by the fungus, Alternaria solani, is a disease of the 

 foliage only. New infections apparently originate in the spring from spores 

 which over-winter in the soil. For the destructive development of this disease, 

 there must prevail frequent wet and cloudy weather during the month of July, 

 followed then by a period of generally high temperatures in August. The moist 

 periods of high humidity and abundant rains in July are necessary to promote a 

 rapid succession of infection cycles; this results in general infection throughout 

 the fields. High temperatures following such general infections are favorable to 

 the rapid growth of the mycelium in the leaves and to the consequent killing of 

 the large circular areas of tissue. It is interesting to observe that although the 

 plants may be generally infected, hot weather is necessary for maximum des- 

 tructiveness. On the other hand if no prolonged rainy weather precedes the hot 

 weather but little early blight appears.^ 



Tip-burn, a physiological disease, as yet not fully explained, is probably due 

 to over-transpiration which causes the wilting and death of small areas of leaf- 

 tissue, usually near the tip of the leaf. High temperatures either in July or 

 August may bring a destructive development of this trouble. Tip-burn may 

 appear in the absence of early blight and, therefore, is probably little affected by 

 the presence or absence of previous rainy periods. 



Late blight is very definitely associated with certain sequences in weather 

 conditions. The early seasonal development of the late blight fungus, Phytoph- 

 thora infestans, is still largely shrouded in uncertainties. The fungus may affect 

 the tubers in late autumn, but the prevalence of tuber-rot is not as directly propor- 

 tional to the amount of foliage blight as might be expected. Considerable 

 foliage blight may be accompanied by a relative absence of tuber-rot and vice 

 versa. The role that affected tubers play in the primary foliage infections the 

 following season is problematical. Affected tubers rarely produce many sprouts, 

 which reach the surface of the ground and those that do are weak and soon die. 

 Close observation and experiments show that but few such sprouts are capable of 

 producing spores above ground, therefore it is doubtful if they are responsible 

 for primary infection cycles in the late spring.* It must not be overlooked, 

 however, that even weak and short lived sprouts from slightly affected tubers may 

 produce conidia below ground. These may then account for zoospores which 

 may cause infection by splashing onto the low rain-beaten foliage of nearby vines. 

 Either by this possible means or by over-wintering oospores in the dead leaves or 



