^Family Chenopodiaceae 131 



tioned above, clean culture is an important consid- 

 eration. Diseased leaves should be collected and 

 destroyed by fire; the diseased refuse should never 

 find a place in the manure pile. Where spinach has 

 been grown too long on the same land a rest must be 

 given by rotating with other crops. 



DOWNY MILDEW 

 Caused by Peronospora effusa Rabenh. 



The downy mildew of spinach is of widespread 

 occurrence. The disease causes great damage during 

 seasons of heavy rainfall or during dry weather ac- 

 companied by heavy dews at night. It is rare in dry 

 weather and absence of dews. 



Symptoms. The trouble is characterized by yellow 

 spots of conspicuous size on the upper part of the 

 leaves (fig. 21 a). On the under side of the leaves 

 and corresponding to the spots above, is seen a 

 mat of dirty white to violet gray fruiting bodies of 

 the fungus. 



The Organism. Downy mildew is caused by the fun- 

 gus Peronospora effusa. The spores of the parasite 

 are borne on branches which generally emerge through 

 the breathing pores or stomata of the lower part 

 of the leaf (fig. 21 b) and germinate by sending out 

 a slender germ tube. Infection takes place when the 

 germ tube penetrates the upper side of the leaf, gen- 

 erally through the stomata. The winter stage or 

 oospores may be found in the affected leaves. The 



