456 PLANT DISEASES 



anything between 80 and 90 F., where the ' soft ' foliage 

 with ever-turgid cells forms a very suitable medium, on 

 which the fungus develops so rapidly and luxuriantly that 

 the whole of the plants in a house 200 feet long are not 

 unfrequently completely destroyed within a week of the 

 first appearance of the pest. 



The leaves are attacked first ; minute pale-green spots, 

 looking almost transparent when held between the eye 

 and the light, are first observable ; these soon increase in 

 size and run into each other, at the same time becoming 

 brown, and the whole leaf dies almost at once. This 

 destruction of the foliage takes place so rapidly that the 

 plants are simply killed by the over-activity of the roots. 



When the diseased leaves turn brown, myriads of conidia 

 appear on the surface, each supported on a long, slender 

 stem. These conidia are quickly carried from diseased to 

 healthy plants by insects, mites, spraying, etc. 



Those conidia that fall to the ground germinate at once, 

 and form a mycelium which spreads rapidly in the soil, 

 and eventually produces conidia, which also become dis- 

 persed and aid in spreading the disease. The mycelium 

 can live in soil until the following season, when conidia 

 are again formed, and the infection of a new crop follows. 



PREVENTIVE MEANS. Spraying with dilute Bordeaux 

 mixture containing an insecticide will check the fungus 

 and also insect-carriers of the spores. The soil should 

 also be sprayed. 



The soil should be thoroughly disinfected after the crop 

 is removed. 



As much air should be admitted into the houses as 

 circumstances will permit. 



Massee, Leaflet No. 76, Board of Agric. (1902). 



