DISEASES CAUSED BY FUNGI 7 



Many fungi produce either ascospores or basidiospores in 

 their more elaborate fructifications, and also conidia in some more 

 direct and simple manner. In a vast number of species the 

 latter method is the only one yet recognised, in which case the 

 fungus is classed in a heterogeneous group known as the Fungi 

 Imperfecti, on the assumption that a more definite form of 

 fructification exists or has existed. 



The longevity of spores is extremely variable in different species. 

 Many have been shown to be viable after months or years of 

 desiccation, others quickly die. Stahel has found, for example, 

 that the basidiospores of Marasmius perniciosus, the witch- 

 broom fungus of cacao, will not germinate after desiccation, 

 exposed in a thin layer, for an hour. In many species, of which 

 Colletotrichum falcatum, the cause of red-rot in sugar-cane, is 

 an example, special thick-walled resistant spores (chlamydospores) 

 are formed directly on the mycelium. 



The dispersal of spores is effected most commonly by wind or 

 minor air-currents. They are frequently set free in vast numbers 

 visible like puffs of smoke, and their lightness makes it con- 

 ceivable that they may be borne considerable distances through 

 the air. They are carried by birds and insects, washed from one 

 plant to another by rains or transported by streams, and dis- 

 tributed by man, adhering to clothing, implements or articles 

 of merchandise. The chance of any individual spore finding an 

 appropriate resting place is very small, but this is offset by the 

 immensity of their numbers. 



For germination to take place the appropriate conditions in 

 respect of moisture, heat and light must prevail, and these vary 

 with different parasites Humid conditions provided by duU 

 wet weather, by soil moisture, or by the transpiration of the host 

 itself, may suffice, or a definite drop or film of water may be 

 required. Heat is presumably never deficient in the tropics, 

 indeed there is reason to believe that the prevalent temperatures 

 are above the optimum for many species, a limiting factor 

 which may be held to account for the relative infrequency of 

 members of some of the parasitic orders largely developed in 

 temperate countries. 



Infection of plants by a fungus parasite may take place by 

 the entry of mycelium nourished on some adjoining material, 

 such as the soil, the roots of another plant, or dead adhering 

 leaves or branches, or may be effected through the germination 

 of spores directly on the appropriate living organs. It implies 

 first the penetration of the outer defences of the plant : the cuticle 

 and cell-wall or the corky integument, as the case may be. 

 Advantage may be taken of wounds of various kinds for this 

 purpose, including as such the pathways to living tissue provided 

 by the attachment of dead twigs or branches. Entry may be 

 made through the stomata, or the fungus may be equipped to 

 force or to dissolve a passage, or to provide one by poisoning the 



