Family Chenopodiacese 135 



of the season. Leaf spot does not develop to any- 

 appreciable extent under dry conditions. In dry 

 winters it is of no economic importance. It is best 

 favored by poor soil conditions, that is by an excess 

 of acidity or a lack of humus. 



The Organism. It is caused by Heterosporium 

 variabile, a semi-parasitic fungus which causes circu- 

 lar, or subcircular, sooty brown spots, having a 

 definite outline (fig. 21 f) on both sides of the leaf, 

 surrounded by a brown area of dead leaf tissue. 

 The spots at first have a light color, becoming darker 

 with age, finally turning velvety to olive black. This 

 color is largely due to the appearance of the spores 

 on the surface of the spots. These occur singly, 

 although they may involve the entire leaf when a 

 great number coalesce. The mycelium grows in the 

 host cells and consists of septate branches, the cells of 

 which are olive green in color, irregular in shape, 

 granular and oily in content. Conidiophores pro- 

 ceed vertically from cracks in the epidermis of the 

 spot, and at the tip end of each a spore is borne on a 

 small short pedicel. As the conidiophore continues in 

 growth, new spores are formed at the tip end, thus 

 giving the fruiting stalk a twisted appearance. The 

 mature spore is three celled (fig. 21 g), cylindrical, 

 with round or slightly pointed ends, spiny, and sooty 

 or olive green in color. On pure culture the fungus 

 may produce only one-celled spores. The latter 

 seem to retain their vitality for at least six months. 



The very serious spinach blight of the Norfolk, 

 Va., trucking region should be mentioned. It was 



