PHYSICAL CONDITIONS INFLUENCING THE DISEASE. 23 



loss of foliauo.. Notwithstiiiuliiig the nunibor of knowii fucts to the 

 contniry, there arc even now man}' grower.s who retain thi.s idea to the 

 utter and needless loss of their crops. The writer lias met men who so 

 flrml}' believe that leaf curl is due to uncontrolla])U' climatic influences 

 that they would not consider other explanations, Ix'ing- unwillino- to 

 visit the orchard, thoutih the crop was bcino- lost through curl and 

 by so doing future crops might have been saved. 



To gather the ex])erience of peach growers in general respectijig the 

 conditions under which leaf curl devidojis most severely, a circular of 

 inquiry was addressed to several hundred orchardists in November, 

 1893. The replies to some of the questions are presented. Among 

 the inquiries the growers were requested to state if they had observed 

 the disease to be more prevalent after a cold spell in the spring. To 

 this question 97 replies were received, 89 affirmative, 6 negative, and 

 2 growers said they had observed no diti'ercMice, which shows that the 

 orchardists are almost unanimous in holding that a cold spell in the 

 spring favors the developnuMit of curl. 



To the second (juestion, as to whether the trees were most all'ected by 

 curl in a wet or drj^ season, there were 104 replies. Of these, 78 

 stated that peach trees were most aftected in wet seasons, 8 that they 

 were most affected in dr}^ seasons, and 18 that there was no difference. 

 Here again is seen a marked agreement in the replies, a great majority 

 of the growers recognizing that wet vears favor the disease. 



The above-considered conditions — a cold spell in the spring and wet 

 weather — maj^ be explained by stating that such conditions favor, on 

 the one hand, the serious development of the fungus causing the dis- 

 ease, and, on the other, they result in a much greater susceptibility of 

 the tissues of the peach leaves to the attacks of the parasite. Where 

 both cold and rain occur together in the spring, about the time the 

 leaves are pushing, the disease is liable to develop seriously and few 

 varieties can then resist it. The action of wet, cold weather upon the 

 tissues of the peach, making them much more subject to curl than they 

 otherwise would be, has been considered in relation to other plants in 

 a paper b}^ Prof. H.. Marshall Ward,^ who says that whe?i the combined 

 ejects of tlie physical environment are unfavorahle to the host, hut 7iotso 

 or are eveoifavorahle to the parasite, wejind the disease assuming a more 

 or less pi'onoimced epidemic character. He is not here speaking of curl, 

 but the statement holds perfectly true for that disease. A cold, wet 

 spell succeeding warm spring weather, has a tendency to saturate and 

 soften the tissues of the peach, as in the case of other plants. The 

 sudden checking of active transpiration, due to lowered temperature 

 and saturated atmosphere, soon results in the tissues of the plant lacing 

 suffused with water. "The stomata," as Ward puts it, "are nearly 



^Ward, Prof. H. Marshall, The Relations between Host and Parasite in Certain 

 Diseases of Plants, Croonian Lecture, Proc. Roy. Soc, Vol. XLVII, No. 290. 



