Cucumber and Melon Leaf Blotch. 197 



and forms an exceedingly delicate cobweb-like mycelium which 

 runs on the surface of the soil and produces myriads of very 

 minute conidia which are dispersed by currents of air, and infect 

 the leaves in a manner similar to that of the larger conidia borne 

 on the leaves. 



The mycelium in the soil originating from diseased fallen 

 leaves continues to extend and produce conidia so long as the 

 requisite conditions as to moisture and temperature are present. 

 When these conditions fail, the mycelium passes into a resting 

 condition, but readily assumes renewed activity when stimulated 

 by returning moisture and heat. By this means the fungus 

 survives from one season to another in the soil, and the disease 

 is almost certain to recur year after year in a house that is once 

 infected, unless the soil is thoroughly sterilised. 



It is important to remember that the disease under considera- 

 tion can only assume the proportions of a destructive epidemic 

 when attacking plants grown under glass, and where a high 

 temperature and an excess of moisture are present. Such 

 conditions, accompanied by a deficiency of light, result in the 

 production of " soft " foliage, and it is only such soft foliage 

 that the fungus can attack. Experiments carried out at Kew 

 prove that the fungus cannot inoculate leaves that have 

 developed under " lights," or in the open air. Plants that are 

 badly diseased, if removed to the open air produce new 

 foliage, which remains perfectly healthy. 



The disease is entirely an artificial creation, rendered possible 

 by the rushing mode of cultivation followed. 



The seed remains perfectly free from disease, hence there is 

 no fear of its introduction from this source, and its sudden 

 appearance in a new locality remained inexplicable until 

 indicated by the following incident : — An establishment in 

 Hertfordshire sending consignments of cucumbers to Covent 

 Garden market, remained free from the disease until the 

 commencement of the present season, when on one occasion 

 some empty " flats " or packing boxes that had contained 

 cucumbers, sent from a place where the disease was known to 

 be rampant, were by mistake returned from Covent Garden to 

 the Herts establishment, where from that date the disease 

 appeared and is now practically beyond control. 



