196 



CUCUMBER AND MELON LEAF BLOTCH. 



Cercospora Meloms, Cke. 



This fungus, although first observed and described as a new- 

 species so recently as 1896, has spread with remarkable rapidity, 

 and at the present moment is the most destructive parasite 

 with which the cultivator of cucumbers and melons has to 

 contend. In several instances growers report an annual loss 

 of ,£1,000, whereas others have had to abandon the cultivation 

 of these plants owing to the repeated destruction of their 

 entire stock, in places where the fungus has secured a firm 

 foothold. 



The foliage is the part attacked. The first indication of the 

 presence of the disease is the appearance of a few small,, 

 scattered, pale green spots on the upper surface of the leaf. 

 The spots gradually increase in size and also in number, and 

 often run together, gradually passing through grey to a 

 brownish or ochreous colour. If at this stage the upper 

 surface of a diseased spot be examined with a pocket-lens, it 

 will be seen to be covered with delicate upright brown threads, 

 each bearing a conidium at its tip. This represents the fruiting 

 portion of the fungus, the mycelium or hyphae being buried in 

 the substance of the leaf. 



The minute conidia or reproductive bodies are carried from 

 diseased to healthy leaves by currents of air, insects, clothing, 

 &c, or by spraying, and if the leaf surface is moist such conidia 

 germinate and the germ-tubes enter the tissues of the leaf 

 directly. 



Very frequently a leat becomes quite dry and crumbles to the 

 ground within twenty-four hours of the first infection. Such 

 dead fallen leaves are much more responsible for the rapid 

 spread of the epidemic than are the conidia which pass directly 

 from one leaf to another. 



When the dry fragments of a diseased leaf fall on damp earth, 

 the mycelium present in the tissues quickly commences growth 



