ONION SPRAYING AND DUSTING EXPERIMENTS! 



By William L. Doran, Research Professor of Botany, and 

 Arthur I. Bourne, Research Professor of Entomology 



Introduction 



The field fxperiinnits here reported were conducted in 1929, 1930, and 

 1931, with the olijert of adding to the i<nowIedge of th(> effects of Bordeaux 

 mixture (4:4:50 and 8:4:50), milk of lime (4:50), and copper-lime dust on downy 

 mildew of onion (caused by Peronospora schleideni Fng.), on the blast disease 

 of onions, and on growth and yields of onions in the absence of these diseases. 



Downy Mildew 



Down.v mildew of onion was general and severe here in 1924, 1925, and 

 1926. This disease was also reported as present in 1927 and 1928, although it 

 caused little if any loss in those two years. 



It has been recommended (2, 14, 6)^ that onions be spraj'ed with Bordeaux 

 mixture for protection against this disease. Such a recommendation is some- 

 times made with reservations, however, and these seem to be justified by the 

 observation of Knott (10). Writing in 1930 of conditions in New York State, 

 he made the statement that, although a few growers try spraying or dusting 

 onions with a copper fungicide each year, nf) satisfactory control of downy 

 mildew has thus been obtained. 



The disease was, however, found Xo he preventable or controllable by a 

 copper fungicide in experiments in other States and, presumably, under other 

 conditions. Milbrath (13) has reported that Bordeaux mixture (with a sticker) 

 and also copper-lime dust, showed some promise as tested against downy mildew 

 of onion in California. Similarly, Bordeaux mixture (with a sticker) is reported^ 

 to have given good results in the control of the disease in Colorado; and Dykstra'' 

 has informed us that Bordeaux mixture (5:5:50), applied at intervals of 10 to 14 

 days, satisfactorih- controlled downy mildew of onion in small-scale tests in 

 Oregon. 



Both Jones (9) and \Mietzel (15) observed injury to onion leaves sprayed with 

 Bordeaux mixture. That aspect of the matter is considered subsequently in the 

 present paper; for, in the case of plant diseases such as downy mildew and blast 

 of onion, which do not occur or do not measurably reduce yields (in Massachu- 

 setts) every year, the effects of fungicides on the crop in the absence of disea.se. 

 as well as the cost of spraying, are of especial importance, .since there is at present 

 no way of forecasting the ])robay)le pres(Mice or absence of these diseases, and that 

 would involve spraying every year if at all. 



1 Tho onion industry in M;is.«acluisott.s in confined prinoipally to that part of the Connectioiit 

 Valley locatod in Hampshire and Franklin Conntio.s, and the obsorv-ations and results of experi- 

 ments recorded in tliis paper are similarly limited geographically. 



^ Reference is made by number to "Literature Citeil," p. 1S4. 



^ Plant Disease Keporter, Supplement 01. Diseases of vegetal)le and field crops in the Iriitcd 

 States in 1927. 



* Letter of September 11, 1!)29, from T. P. Dykstra, Assistant Pathologist, Oregon Agr. 

 Kxpt. Sta. 



