Ii8 Diseases of Greenhouse Crops 



Leaf Spot 



Caused by Cercospora heticola Sacc. 



There is perhaps no beet disease that is of greater 

 economic importance than leaf spot. The trouble 

 is well known to truckers and it seems to be found 

 wherever beets thrive. 



Symptoms. The disease first makes its appear- 

 ance on the leaves as tiny circular whitish spots. 

 These gradually increase in size and assume a brown- 

 ish color. The spots soon multiply and involve the 

 entire leaf area (fig. 16, c), which becomes dry and 

 brittle. Leaf spot attacks the outer and older leaves 

 first. As the inner foliage advances in age, they too 

 become infected in turn. Serious though the dis- 

 ease may appear, it never kills the plant. The re- 

 sult, however, is noticeable on the roots, which are 

 undersized and elongated instead of round. Leaf 

 spot generally appears in overwatered and poorly 

 ventilated houses. The disease increases in severity 

 as the plants are weakened by heat. 



The Organism. The fungus, Cercospora heticola^ 

 Sacc, like most fungi, is composed of a vegetative 

 part of mycelium and of spores. The latter are 

 microscopic in size, somewhat needle-shaped, and di- 

 vided by means of a cross wall into cells numbering 

 from two to seven (fig. 16, d.). Each of these cells 

 may germinate by sending out a thread-like tube, 

 which penetrates the leaves through the stomata. 

 The spores are borne on a cluster of stalks or conidio- 

 phores, at the base of which is formed a small 



