286 Diseases of Truck Crops 



under proper conditions of ventilation and tempera- 

 ture, the rot progresses very slowly. However, in 

 poorly constructed houses the bulbs rot very fast and 

 the disease then spreads by contact. 

 Damping Off, see Pythium, p. 43. 



Blight, Dow>fY Mildew 



Caused by Peronospora schleideni, Ung. 



Of all the onion diseases, bHght (also known as 

 downy mildew) is perhaps the most important from 

 an economic standpoint. It may often wipe out 

 from seventy per cent, of the stand to the entire crop. 

 The disease usually accompanies muggy, damp, or 

 rainy weather. 



Symptoms. The disease is best diagnosed early 

 in the morning when the dew is still present on the 

 foUage. Diseased parts have a peculiar violet tint. 

 This is due to the downy cover of the fruit by the 

 ftingus. Soon the affected leaves lose their green 

 color, becoming yeUow in spots, and by the second 

 or third day they have all collapsed, and are entirely 

 covered by the downy fruiting stalks of the causative 

 fungus (fig. 54 a). If the weather is vmfavorable the 

 disease wiU be seen to work in restricted spots in the 

 field with the tops of the affected plants collapsed. 

 However, after several days the diseased onions begin 

 to recover by sending out new top growths. The 

 previously diseased leaves now dry and break away. 



