550 INSECTS AFFECTING VEGETATION 



furry covering of the fruiting parts of the fungus. These will stand 

 out very abundantly and plainly on account of the moisture. Later 

 in the day the parts are more or less collapsed by the drying of sun 

 and wind, and the diseased leaf is more difficult to see. Aside from 

 this peculiar furry violet appearance, the affected leaf is at first like 

 the healthy ones about it. By the second day it has begun to lose 

 its bright color, being paler or even yellowish in spots, where the 

 disease first started. It will now be moldy or furry throughout 

 most of its length and by the third or fourth day collapsed and 

 broken over. The fungus usually appears first on one side of the 

 leaf about midway between its tip and base, from which it spreads 

 rapidly through the entire leaf. Unless the weather is especially 

 favorable the disease will not spread over the field rapidly, but areas 

 several feet in extent will first be evident, here and there through 

 the patch about the places which were first attacked. After several 

 days the affected onions begin slowly to recover. New leaves appear. 

 These replace the affected ones which are drying up. If weather 

 conditions are favorable to the onions this new growth is rapid and 

 often in a week the field appears almost as thrifty as ever. The af- 

 fected leaves have quite dried up and disappeared while the new 

 growth is erect and green. 



Treatment will be found to be most successfully accomplished by 

 systematic spraying, beginning the latter part of June or the first 

 of July, and continuing throughout the season. This has been used 

 very successfully by onion growers in Europe. 



Two things are always to be insisted upon in applying the spray : 

 1st, thoroughness ; every side of the leaf must be covered. 2d, force ; 

 the mixture must be applied with sufficient force to make it stick to 

 the smooth surface of the onion leaves. A nozzle that gives a fine 

 spray should be used. The Bordeaux used was mixed in the propor- 

 tion of four pounds of lime and six pounds of blue vitriol (copper 

 sulfate) to a barrel of water. It was found that in some cases this 

 seemed to burn the leaves. A larger proportion of lime would very 

 likely obviate this trouble. A reduction of the strength of the solu- 

 tion has also been recommended. 



Location and drainage have much to do with controlling the 

 blight. Properly drained lands are less liable to the disease than wet 

 ones. All barriers, such as tall weeds along ditches or the sides of 

 the field should be removed so as to admit the air freely to the grow- 

 ing onions, thus assuring the rapid evaporation of dews and rains. 

 Lands for onion fields should be chosen in places as much exposed 

 as possible. Swamps surrounded by high hills are especially unfav- 

 orable. 



We have now considered the means of attacking the fungus in 

 the summer stage. There remains to be considered the winter stage. 

 Since the summer stage which is the destructive one, perishes with 

 the approach of winter, it is evident that if the resting spores by 

 which the parasite manages to pass the winter could be destroyed 

 the disease could not reappear the following season. These spores, 

 as we shall see, pass the winter in the dead leaves of the onion. 



