360 Diseases of Greenhouse Crops 



leaf. These upon enlarging meet and coalesce, giv- 

 ing the appearance of large, white blotches. The 

 spores of the causal fungus develop on the spot un- 

 der moist conditions only. 



The Organism. The fungus grows well on agar 

 media, usually in concentric rings. The color of 

 the mycelium is at first grayish white, but as the 

 spores are formed the entire growth takes on an 

 olivaceous tint due to the color of the spores. The 

 conidiophores of the fungus are borne in clusters and 

 are erect, pale olivaceous, and septate. The conidia 

 are borne in chains on the tips of the conidiophores, 

 and are flask shaped, muriform, and olivaceous (fig. 

 75, b and c). 



ControL To control this disease it is necessary to 

 have a clear idea of the factors which favor leaf 

 spot under greenhouse conditions. Dorsett * enu- 

 merates the following : 



"1. Not keeping the houses or frames clean, fresh, 

 and sweet by frequently repairing and painting them, 

 and by removing and destroying rubbish of all kinds 

 as soon as it appears. 



"2. Not keeping the plants clean and in the best 

 possible growing condition at all times. 



"3. Not selecting stock from strong, vigorous 

 plants that have been entirely free from disease. 



"4. Not being careful to select only strong, vigor- 

 ous, healthy stock from the cutting bed for planting 

 in the spring. 



* Dorsett, P. H., U. S. Dept. of Agr. Div. Veg. Phys. and Path., 

 Bui. 23: 7-16, 1900. 



