340 DISEASES OF CROP-PLANTS 



fungus Sclerotium Rolfsii, discussed in another section as a general 

 parasite. 



Its attacks on ground-nut have caused very considerable 

 losses during periods of wet weather. 



Kidney Beans. 



Anthracnose. 

 Kidney, Haricot, or bush beans {Phaseohis vulgaris) are grown 

 to some extent as a garden crop in these islands, usually from 

 American seed. The bean anthracnose is of fairly frequent 

 occurrence, being probably in most if not all cases introduced 

 on the seed, and sometimes causes considerable loss of crop. 

 It is much less severe on local forms of Phaseolus. 



Symptoms. 



The disease makes its appearance on the young seedUngs, 

 and ultimately infects the leaves, stems and pods of the developed 

 plants. The fungus penetrates the pods to the seeds within, 

 causing spots to be developed on the latter which form a starting 

 point for a new cycle of disease. The spots appear on the seeds 

 as sunken black or brown specks in the case of light attack, 

 or as brownish or blackened spots, usually in the form of a ring 

 with a dark or rifted centre, where the attack is severe. On the 

 seed-leaves, circular or kidney-shaped blotches appear, which 

 may enlarge until the whole leaf is involved. Elongated cankers 

 are produced on the stems, and many young plants are lost 

 from the direct or indirect consequences of stem and root in- 

 fection. Young leaves may be crinkled owing to the effect 

 of the fungus on the margins ; older leaves are affected mostly 

 on the veins, and the portion of the leaf -blade beyond the attacked 

 vein turns yellow and dies for want of water. On the pods the 

 lesions start as small reddish pin-point spots, which enlarge 

 into circular or kidney-shaped sunken areas with a raised rim. 

 Yellow or pink spore-masses are produced in the centres of these 

 spots. Severe infestation occurs as a consequence of continued 

 wet weather, and a general yellow appearance, due to the dying 

 of the leaves, is then recognizable in the fields. 



The Fungus. 



The causative fungus is CoUetotrichum lindemuthianum (Sacc. 

 et Magn.) Br. et Cav. The hyaline spores are at first one-celled, 

 but later are divided by a single septum. They are surrounded 

 by a mucilaginous envelope. In mass they are at first pink, 

 later becoming more or less creamy. They are usually kidney- 

 shaped, 2.7-5.3x10.6-20 microns. 



Control. 



Spraying with Bordeaux mixture usually gives disappointing 



