24 DISEASES OF CROP-PLANTS 



ing the vessels produce wilt disease usually destructive of the 

 whole plant. The most notorious of these is the Panama disease 

 of bananas. Others attack cotton, okra, tomato, cow pea, 

 pigeon pea, etc., etc. 



Other species of Fusarium cause rots of the lower stem 

 (foot-rot) of seedlings or young plants, as in cow pea, beans 

 (Phaseolus), and cotton in these islands. 



Others again cause tuber rots (potato), fruit rots (tomato, 

 water melon, cacao), or stem cankers, while numerous mis- 

 cellaneous diseases have been described with which Fusarium 

 species have been associated. 



The mycelium of Fusarium species is made up of septate, 

 variously branched, hyaline or light-coloured hyphse. It ramifies 

 through the vascular system (in the wilt diseases) or the tissues 

 in general (in the rots) and in some species develops at the surface 

 into a more or less compact layer forming the basis for the 

 production of conidia. The conidia are typically of two forms : 

 the microconidia, often called the Cephalosporium stage, which 

 are rounded in shape, produced one by one from the tip of a 

 short hyphal branch, and often adhering in roundish heads ; 

 and the macroconidia (Fusarium stage), which are elongated, 

 fusoid, usually more or less curved, and when mature generally 

 several-septate. The latter are produced on simple or branched 

 conidiophores, and accumulate in loose and or sometimes slimy 

 masses on the surface of the medium, when they may appear 

 either white or light-coloured, frequently with some shade of 

 pink. The microconidia are sometimes produced within the 

 infested tissues, and in Panama disease, according to Drost, 

 follow the course of the sap and form new centres of infection 

 in the vessels. 



Chlamydospores, single or double, and occasionally in chains, 

 are capable of being formed by the mycelium of most of the 

 species. They are thick-walled and resistant. 



Attempts to control the Fusarium wilts by treatment are 

 of no avail ; entry is commonly made from the soil, and the 

 injury is deep-seated. With annual crops relief from serious 

 infestation may be obtained by rotation, but, these fungi being 

 usually indefinitely persistent in the soil, a long period is some- 

 times necessary to reduce the fungus sufficiently for the sus- 

 ceptible crop to be repeated. In the Panama disease of bananas 

 infection is also conveyed in the suckers used for planting, so 

 that an uninfected source of planting material is required as 

 well as clean land. It does not appear that vigour in the 

 host, as affected by cultivation, manuring, soil and climate has 

 much influence in regard to wilt resistance. 



The line on which the most notable successes in wilt control 

 have been developed is that of breeding resistant varieties or 

 strains, which is rendered possible by the narrow specialisation of 

 the fungi concerned. There are wilt-resistant cottons, cow peas, 



