140 Diseases of Greenhouse Crops 



Timber Rot 



Caused by Sclerotinia lihertiana Fckl. 



Symptoms. The disease seems to attack the stem 

 end of the plant nearest to the ground line. Af- 

 fected stems at first water-soaked, then become 

 invaded with a cushion of white mycelial growth. 

 Rapid wilting, with no recovery, immediately fol- 

 lows. As the affected plant dies, the shriveled stem 

 becomes covered with black masses of fungous 

 bodies, sclerotia. The same fungus also causes 

 lettuce drop. For a description of the causal or- 

 ganism and methods of control, see p. 151. 



Powdery Mildew 



Caused by Erysiphe cichoracearum D. C. 



Powdery mildew of cucumbers is not a serious 

 greenhouse trouble. Like all powdery mildews, the 

 causative fungus grows on the surface of the leaf, 

 giving it a white mealy appearance. From the 

 mycelium are produced erect threads which bear the 

 summer spores of the fungus. The ascus or winter 

 stage appears as minute dark-brown, rounded cap- 

 sules enclosing a group of spore sacs within which are 

 formed the ascospores. 



Control. The conditions which favor mildew are 

 overwatering, lack of ventilation, lack of light, and 

 too high a temperature. Proper attention to these 

 factors will help to remove the cause and to effect 

 a cure. 



