CONNECTICUT VALLEY ONION INDUSTRY 47 



Downy mildew attacks tlie onion tops, starting witii a few onions in 

 different parts of a field, and spreading from these centers of infection. 

 In its early stages, even before the tissues are killed by its attack, the 

 mildew may be detected in early morning when the leaves are wet with 

 dew, or during a rain, as small violet colored patches. In the sun these 

 patches dry out and collapse, leaving yellowish or white spots. The mil- 

 dew quickly spreads over the entire leaf which becomes water-soaked in 

 appearance, collapses and breaks over. The shriveled leaves are soon 

 blackened by mold. L'sually, new leaves develop following the first at- 

 tack, and if weather conditions continue to favor development of the dis- 

 ease, these may also succumb. However, even with the new crop of 

 leaves the bulb seldom attains full size, and the crop is thus reduced. 



Downy mildews characteristically thrive best under moist, cool condi- 

 tions, and downy mildew of onion is no exception. Periods of rain or 

 high humidity with low night temperatures occurring between early July 

 and late September are likely to bring on the disease, and at such times 

 the fields should be closely watched for the first symptoms. With weather 

 conditions favoring the disease its spread is very rapid, often considerable 

 areas becoming affected within a few days and leaves dying within a short 

 time after infection. It was not uncommon last season to find entire 

 fields laid low by the disease. 



Control measures for onion blight have been worked out. The various 

 phases of control are based on (I) the ability of the mildew to live over 

 winter in old onion refuse, (2) influence of environmental conditions and 

 (S) susceptibility of the fungus to toxic fungicides. 



(1) In order to reduce the possibilities of infection from previous 

 diseased crops, onion refuse should be burned. A three or four year 

 crop rotation also may be helpful in this connection. 



(2) As excessive moisture favors development of the mildew, every 

 practicable means should be taken to promote the drying oft' of the onion 

 tops following periods of high humidity. L'^nder-drainage, clean culture, 

 exposure to prevailing winds, all help in drying oft' a wet field. 



(3) Spraying with 4-4-50 Bordeaux mixture with three pounds of resin 

 fish-oil soap added as a sticker is effective if the applications are suffi- 

 ciently prompt, thorough, and frequent. If spraying is to be the pro- 

 gram, the weather conditions should be closely watched, as the first ap- 

 jilication must anticipate appearance of the mildew. All tops must be 

 tlioroughly covered by the fungicide, and spraying should be repeated at 

 least once a week or every tliree or four days if weather conditions favor- 

 ing s])read of the disease are prolonged. 



Onion growers are not equipped with adequate spraying machinery, if 

 indeed such machinery is in existence. Yet there can be no doubt that 

 downy mildew will continue to attack the onion whenever moisture and 

 temperature conditions suited to its development occur. Growers must 

 face this possibility each season and must decide for themselves whether 

 it will be more profitable to take the loss from possible reduction of yield, 

 or ai>ply the control measures above outlined. 



