DISEASES OF BANANA 243 



Causative Fungus. 



A fungus is associated with the disease from its initial stages, 

 the colourless intracellular mycelium developing primarily on 

 the inner sides of the walls of the vessels in the affected parts. 

 Micro-conidia are formed in this situation and in pure cultures 

 (the Cephalosporium stage) ; apparently they are carried for- 

 ward with the ascending sap and give rise to new points of 

 infection within the plant. On the external parts of the plant, 

 especially the failing leaves, and also in pure cultures of the 

 fungus, sickle-shaped, hyaline, mostly 3-celled macro-conidia 

 are developed (the Fusarium stage). Chlamydospores of more 

 or less spherical shape also occur in cultures, borne singly or 

 in pairs terminally on short branches, or singly and in rows 

 (intercalary) on the hyphse. Irregularly spherical sclerotia 

 about I mm. in diameter, yellow to deep indigo, also occur in 

 old cultures. The spore measurements are : Cephalosporium 

 form, 5-8x2.5-3 microns; Fusarium form, mature, 3-septate, 

 20-40x3.8-4.5 microns. Chlamydospores: oval 9x7; spheri- 

 cal 7-7.5 ; paired 13 X 7 microns. These are Ashby's particulars 

 of the fungus in Jamaica, and are in close agreement with those 

 more recently given by Brandes of cultures derived from various 

 sources in the West Indies and Central America. The latter 

 author has adopted for the fungus the name Fusarium cubense, 

 on the ground of its presumed identity with the fungus obtained 

 from banana in Cuba by E. F. Smith in 1900. 



Nearly all pure cultures on sterilised media, especially on 

 potato and rice, develop a marked odour (of lilac ? ) which, as 

 Lathrop has shown, is most probably due to the production of 

 the volatile toxic propylic aldehyde ; odourless strains have 

 been isolated also from characteristic cases of the disease. Similar 

 odour-producing and odourless strains of the very closely related 

 Fusarium vasinfectum Atk., the cause of the vascular wilt disease 

 of cotton and okra, have proved to be equally pathogenic to 

 those crops so that in so far as the symptoms indicate a toxic 

 action of the parasite they are apparently not caused mainly by 

 a volatile aldehyde. Like most of the species of Fusarium 

 parasitic in the vascular systems of plants, F. cubense develops 

 a characteristic pink or crimson pigment when growing on 

 sterilised rice. 



Drost in addition to the two forms above specified describes 

 hemispherical pycnidia with one-celled spores, 8-10 x 2-3 microns, 

 and perithecia with 2 -celled ascospores. On the basis of the last- 

 named form he proposed the name Leptospora Muses, but its 

 connection with the parasite lacks confirmation. 



Successful infection experiments are recorded by Drost, 

 Prescott, and Brandes ; those of the last named carried out on 

 an elaborate scale, and conclusive in their results, 



