86 



only to begin active growth with the production of spores, 

 when the conditions are once more favourable. In this 

 way the disease survives in greenhouses from one season 

 to the next, and once a house is infected the disease is 

 almost certain to recur year after year unless precautions 

 are taken to thoroughly disinfect the soil and all parts of 

 the house. 



The practice of disinfecting greenhouses with burning 

 sulphur is largely employed in some districts. Whilst 

 this is an excellent preventative of insect pests and 

 of certain mildews, it is useless against the Leaf -Blotch 

 of cucumbers. The writer recently established this with 

 certainty. A large house which had been badly infected 

 with the disease was cleared out and thoroughly disin- 

 fected with burning sulphur. A few fragments of 

 diseased leaves and fruits were then collected from the 

 soil of this greenhouse and brought into the laboratory. 

 Spores of the Cercospora, taken from this material, ger- 

 minated in water in a few hours and cultures of the fungus 

 were readily obtained on suitable media. From this 

 experiment it is clear that the spores and resting filaments 

 of Cercospora melonis remain uninjured in houses dis- 

 infected by burning sulphur alone. Probably more certain 

 results would be obtained by spraying the house 

 thoroughly, and also drenching the soil with Formalin 

 (i pint per 20 galls, of water). The Board of Agriculture * 

 recommends the use of Jeyes fluid (i oz. per gall, of water) 

 for this purpose, but the writer has no experience of this 

 as a disinfectant against parasitic fungi. Needless to say 

 all diseased material should be destroyed by burning. It 

 is possible to control this disease to some extent by spray- 

 ing with Liver of Sulphur (potassium sulphide) two ounces 

 in three gallons of water, to which two ounces of soft soap 

 is added to facilitate the sticking of the solution to the 

 leaves, which should be thoroughly wetted by the spray. 

 The disease is only prevalent where cucumbers are forced 

 under glass, but if the ventilation is carefully regulated 

 much can be done to reduce the possibility of an epidemic. 

 A most effective way of combating it is by growing 

 disease-resisting varieties of the cucumber, of which there 

 are a number on the market. The most reliable of these 

 have rather coarse, hard foliage, but unfortunately the 



* Leaflet No. 76. 



