522 SPECIAL PLANT PATHOLOGY 



Webber prepares the mixture as follows: Place the resin, caustic 

 soda and fish oil in a large kettle, pour over them 13 gallons of water, 

 and boil until the resin is thoroughly dissolved, which requires from 

 three to ten minutes after boiling has commenced. While hot, add 

 enough water just to make 15 gallons. It is advised to make about 

 two sprayings when the white fly (Aleyrodes) is in the larval stage. 

 In Florida winter sprayings are important, but a spraying in May is 

 also often desirable. In all cases dilute the stock solution with 9 

 parts of water. 



Lettuce (Lactuca saliva, L.) 



Drop {Sclerotinia lihertiana Fckl.). — This is one of the most disas- 

 trous of the sclerotium-producing fungi to garden and greenhouse 

 plants, being widely distributed and difficult to control. It attacks 

 greenhouse lettuces, causing at first flagging, then indications of 

 water-soaked areas over the stem and basal part of leaves, finally fol- 

 lowed by the collapse of the whole plant into a formless mass. The 

 mycehum may grow on the surface of the lettuce leaves and black 

 sderotia may be formed there commencing as white condensations 

 which finally turn black. Conidiospore formation is not certainly 

 known in the lettuce-drop fungus. Sclerotia, however, are commonly 

 formed which measure 3 cm. in length and these are formed even on 

 artificial culture media. The apothecia are wineglass-shaped with 

 long black stalks. The asci formed on the upper depressed side of the 

 apothecia are cylindric and measure 130 to 135/i by 8 to lo/i, while the 

 ascospores are small, 9 to 13/1 by 4 to 6.5^- 



All dead and diseased lettuce plants should be destroyed by fire 

 and the ground where they grew soaked with some suitable fungicide 

 so as to confine, or practically exterminate the disease. The soil 

 should be sterilized with steam before planting. 



Lilac {Syringa vulgaris, L.) 



Powdery Mildew {MicrosptuBra alni (Wallr.) Wint.). — During the 

 summer months and late in the autumn, the upper surface of the leaves 

 of the lilac will be found covered with a whitish mildew which consists 

 of interlacing hyphae, which form a cobwebby, superficial growth. 

 Short haustoria are produced which grow into the epidermal cells. 



