Peach Li:ai- Cikl. 2S5 



This phase of the hfe history of the fungus offers a most interesting 

 problem for further investigation. 



Ecology. — The conditions favoring the development of the disease 

 have been carefully studied. Pierce ('00) collected much data in regard 

 to the relation of the weather to the development of the curl. Selby 

 ('99) also made a careftil study of the same phase of this subject. Many 

 growers have noticed that Curl is favored by cold, wet weather while 

 the leaves are opening. Naturally this has led many to consider such 

 weather conditions to be the cause of the disease. The cause, however, 

 is not the " weather " but the fungus. The growth of the fungus and 

 infection of the host by it are favored by the conditions above stated, 

 for several reasons. The low temperature and moist atmosphere incline 

 to check transpiration from the leaf out of all proportion to the absorp- 

 tion of water by the roots frotn a relatively warm soil, consequently 

 the leaf tissue becomes abnormall}^ gorged with water, causing a suc- 

 culent, watery growth less able to resist the attack of the fungus. It is 

 true also that the same conditions which render the host most sus- 

 ceptible are most favorable to growth of the fungus causing the disease. 

 In fact, any condition favoring a superabundance of moisture is, in 

 general, most favorable to the spread and growth of this as well as most 

 fungi. For this reason peach orchards situated near large bodies of water 

 are more likely to be severely attacked by Curl. The direction of pre- 

 vailing winds also has an influence. When blowing from dry regions 

 they bring much less moisture than when coming over a large lake or 

 ocean. Professor Pierce believes that heavy dews have little or no effect 

 in inducing Curl. They do not weaken the vitality of the leaf, since they 

 occur at night while transpiration is normally low and the stomates 

 almost closed. He also asserts that something more than dew is neces- 

 sary to cause germination of the spores of Exoasciis deformans. 



CONTROL OF THE DISEASE 



No one would suspect from what we actually know of the life history 

 of this fungus that spraying during the dormant period would be suc- 

 cessful in preventing the disease. J*erhaps we would still be ignorant 

 on this point if the California growers had not long ago noticed that 

 Curl was almost entirely controlled on trees sprayed with lime-sulfur 



'00. Pierce, N. B., Peach Leaf Curl; Its Nature and Treatment, U. S. D. A., 

 Div. Veg. Phys. and F*ath., Hul. 2o:22-;50, 1900. 



'99. Selby, A. I).. Variations in the Amount of Leaf Curl of tlic I\vith (Exjuscus 

 deformans) in the Light of Weather Conrlitions, Proc. Soc. Prom. Agr. vSci., Ann. 

 Meeting 20:9^-104, 1.S99. 



