ONION DISEASES 



1473 



Fiisariuin Blight 



This is often serious on young onions 

 in old soil and is the forerunner of heavy 

 losses from soft rot in storage. 



Heai-t Rot 



Bacterial 

 This disease appears to come in all va- 

 rieties of onions, following the topping, 

 by its rapid invasion of the center of the 

 bulb through bacterial infection. It 

 should be controlled by attention to dis- 

 infection of the topping machine or to 

 similar treatment to that recommended 

 for dry rot. This disease ends in the 

 complete destruction of the bulbs through 

 a soft rot different from that described 



under soft rot. 



A. D. Selby, 



Wooster, Ohio. 



Mildew 



Peronospora scMeideniana 

 F. D. Bailey 



This disease is one that has long been 

 known as a serious disease in England 

 and Europe and other parts of the world 

 as well as in various sections of the 

 United States. It is, without doubt, the 

 most serious and destructive disease of 

 the onion known. It has been reported 

 from various sections of Western Oregon 

 and has been particularly serious during 

 the unusually moist season of 1912. It 

 is found most commonly on beaverdam 

 soils. 



Symptoms 



The disease attacks the leaves, causing 

 them to collapse (Fig. 1). It usually ap- 

 pears first on a few plants in the field, 

 but spreads rapidly in warm, damp weath- 

 er. In the first stages of attacks the 

 leaves of an infected plant will show a 

 peculiar violet tint. If these leaves are 

 examined closely this color is found to 

 be due to the presence of a downy growth 

 on the affected surfaces. In a day or two 

 the leaves become weak and gradually col- 

 lapse. The collapsed leaves dry up and 

 may become covered with black mold, 

 which the grow^er frequently mistakes for 

 the cause of the trouble. It takes only 

 three to Ave days from the first appear- 

 ance of the trouble for the plants to be 

 completely collapsed. 



Cause 



The disease is caused by a fungus be- 

 longing to a group of disease-producing 

 forms commonly referred to as the 

 "downy mildews" on account of the ap- 

 pearance of the fungus on the surface of 

 the diseased parts. It is known technical- 

 ly as Peronospora scMeideniana. This 

 fungus, like many others, has two spore 

 stages. In the summer stage the spores 

 are formed on branches of the fungus 

 body which appear in great numbers on 

 the surface of affected leaves. It is these 

 spore-bearing structures which cause the 

 downy appearance mentioned above. The 

 spores are spread by the wind and cause 

 new infections. Since the time required 

 for the fungus to kill the leaves is very 

 short, it is readily seen why the disease 

 spreads so rapidly over a field once it be- 

 comes started. 



Treatment 



In those sections where onion mildew is 

 troublesome, the foliage should be kept 

 covered with a fungicide during the grow- 

 ing season to prevent the germination of 

 the spores on the surface of the plant 

 and the consequent infection. The best 

 fungicide for this purpose is Bordeau-K 

 mixture. It should be used in the 5-5-50 

 strength and should be applied at inter- 

 vals of from ten days to two weeks be- 

 ginning when the plants show three 

 leaves. Onion foliage is so smooth that 

 Bordeaux prepared in the usual way does 

 not adhere readily. It is necessary, there- 

 fore, to add some sticker to the mixture. 

 This sticker is prepared as follows: 



Mix together in an iron kettle two 

 pounds of resin, one pound of sal soda 

 crystals, and one gallon of water. Boil 

 in the open air until the mixture is of a 

 clear brown color. This will • require 

 about one to two hours. 



The above amount is sufficient for 50 

 gallons of spray. 



Literatnre 



1904. New York (Cornell) Experiment 

 Station Bulletin No. 218. 



Smudge 



Vermicularia circinans Berk. 

 This fungus develops as a superficial 

 spotting upon the exterior, especially at- 



.^— 10 



