85 



attacked is also produced. In Ihe latter case, BaciUus 

 solajiacearnin is the cause. The Septoria disease of the 

 leaves produced by Seftona lycoijersici is the only other 

 malady likely to be confused with the leaf-rust, but m 

 this case the spots are always small and concentric, and 

 the spores are produced in minute black bodies scattered 

 over the patches. 



The Leaf-Blotch of the Cucumber caused by Cercosfora 

 jnelonis is another destructive disease of leaves. It was 

 first described in this country by Dr. C. Cooke m 1896, 

 and since then it has become so wide-spread that many 

 horticulturists have been compelled to cease growing- 

 cucumbers. Once the fungus appears in greenhouses it is 

 extremely difficult to eradicate. The leaves are most often 

 attacked, but the fungus frequently spreads to the fruit. 

 An outbreak of the disease is usually first indicated by 

 the appearance of pale, scattered spots on the leaves. 

 These spots gradually increase m size, become brown, 

 and the leaves are so rapidly killed that death of the 

 plant may soon result. 



}>Iicroscopic examination shows that, m the region of 

 the spots, the tissues of the leaf are occupied by filaments 

 of the fungus, that the chlorophyll bodies are pale 

 111 colour and many of the cells of the leaf are shrivelling 

 and dying. From the fungus within the leaf stiff branched 

 filaments grow out and stand more or less erect from the 

 surface. These aerial threads are dark m colour, and 

 bear numbers of large conidia which fall free as they 

 mature. Each somewhat spindle-shaped conidium is 

 divided into about seven or eight cells, and may germi- 

 nate in a warm, moist atmosphere by sending out fila- 

 mentous germ-tubes from any of the cells. The germ- 

 tubes may then produce new infections by growing through 

 the stomata into the healthy tissues of the leaf. This 

 spreading by means of conidia that germinate imme- 

 diately, takes place very rapidh* under favourable con- 

 ditions. In addition to producing large conidia on the 

 leaves the fungus is said to grow as a saprophyte on decay- 

 ing leaves and damp soil producing myriads of smaller 

 spores which also rapidly spread the disease. If the con- 

 ditions are unfavourable to the germination of the spores, 

 and especially at the end of the season, the large conidia 

 persist alive as resting spores. Filaments of the fungus 

 are also able to pass into a resting condition m the soil, 



