470 Anthracnose of the Cucumber Under Glass. [Aug., 



the hollows of the leaf surface by overhead damping. The 

 lesions commence as pale green water-soaked spots barely dis- 

 tinguishable by the untrained eye, but quickly assume a charac- 

 teristic appearance, becoming dry and reddish brown in the 

 centre with a yellowish water-soaked surrounding zone. The 

 spots vary in shape, being almost circular in areas untouched 

 by any large vein, but irregular patches where they form over 

 a vein. They frequently crack in the centre and the desiccated 

 tissue may not infrequently be beaten out by the daily over- 

 head damping. The spots increase rapidly in size, become more 

 circular and blotch-like, finally coalesce, and the leaf dies At 

 the final stage the leaves have a scorched appearance and are 

 covered with spots. 



As the disease advances, lesions develop on the leaf stalks 

 and stems, showing as sunken water-soaked areas which rapidly 

 become dry and powdery. They are usually at first linear in 

 shape but may spread round the stem. Under glasshouse con- 

 ditions it is not uncommon to see the soft tissues of the stem 

 completely destroyed, leaving the fibrous part exposed, and 

 causing the death of the plant above the point of attack. On 

 stem and stalk lesions abundant spores are produced, giving rise 

 to a pinkish colour which turns black with age. 



The lesions on the fruits appear as pale-green water-soaked, 

 sunken areas, the surface of which, owing to abundant spore 

 production becomes pink in colour and finally black. The tissues 

 under the lesion are destroyed and a cavity is produced, which 

 is exposed by the cracking of the surface above. When the 

 leaves are attacked, the health of the plant is impaired only by 

 the serious reduction of leaf area, but lesions on the stem are 

 more serious and may cause the rapid death of the plant by 

 destroying the tissues. 



The Organism Causing the Disease. — Colletotrichum 

 oligochaetum, Cav., was readily isolated from infections on 

 leaves, stems and fruits. Proof that it causes the disease was 

 obtained by spraying young cucumber plants with a water sus- 

 pension of spores, and keeping them under suitable condit.ons 



The fungus has been described by Cavara, and later workers. 

 In the present study it has been cultivated on a wide range of 

 artificial media. 



Sources of Infection. — The fact that the fungus will thrive 

 upon such substances as new and rotten wood, straw, and cotton 

 wool, provided that these are kept sufficiently moist ; led to a 

 systematic examination of glasshouse structures, boxes, wooden 

 tanks, " flats " and heaps of manure. 



