22 DISEASES OF CROP-PLANTS 



growth of heavy foliage by nitrogenous manuring, all favour 

 their development. The cotton boll rot is frequently confined 

 to the branches close to the moist soil. 



The potato blight is perhaps the most amenable of all impor- 

 tant diseases to Bordeaux or Burgundy sprays, and cacao pod- 

 rot has proved to be similarly controllable. These fungi have no 

 importance as saprophytes, as, although they can be artificially 

 grown in pure culture on vegetable media, in nature their exist- 

 ence on dead material is quickly inhibited by the growth of other 

 organisms. The removal of dead material is desirable however 

 on account of the resting spores it may contain. 



The Anthracnoses. 



The term anthracnose is not strictly specific, but is usually 

 and conveniently applied to diseases caused by fungi of the 

 genera Colletotrichum and Gloeosporium. These are conidial 

 forms, not strictly separable from each other, since the presence 

 or absence of dark-coloured setae (bristles), on which the division 

 is based, is not a constant character. Certain of the species have 

 been shown to be the conidial forms of Ascomycetes of the genera 

 Glomerella and Gnomonia, but the perithecia are rarely met 

 with in the active stages of disease. 



Many species of Colletotrichum and Gloeosporium are pure 

 saprophytes, others have weak powers of parasitism which 

 enable them to attack senescent leaves or over-ripe fruit, and 

 many are virulent parasites capable of attacking perfectly healthy 

 tissue. All are able to exist as saprophytes on dead material. 



The hyaline septate mycelium permeates the infested part 

 causing the death of the cells. The conidia are borne in acervuli 

 or pustules, for the production of which a matted or compacted 

 layer of h3^ha2 (the stroma) forms near the surface of the medium. 

 From this bed arise crowded short conidiophores, which bear 

 apical non-septate conidia, twice to several times as long as 

 broad. These are hyaline, but may appear coloured in mass. 

 The rupture of the overlying tissue exposes the acervulus or 

 fruiting layer, which continues to produce conidia. These 

 accumulate in little heaps, adhering in a gelatinous medium until 

 they are washed away by rain. The disease produced may be 

 one of leaves, flowers or fruit or any combination of these, or 

 it may extend to herbaceous stems or even woody twigs. 



Characteristic anthracnoses in the West Indies are those of 

 mango {Gl. mangifercB )and of lime {Gl. limetticolum) . Both cause 

 small canker spots or tip-wither on the young shoots, dead patches 

 on the leaves, blight of the buds and open flowers, shedding of 

 newly set fruit, and spots or cankers on the fruit that remains. 

 Other local examples are the anthracnoses of Agaves, beans 

 (Phaseolus), cotton, papaw, and cassava. An example of a 

 ripe-rot Gloeosporium is everywhere familiar in the fungus 

 which causes spotting on ripe bananas {Gl. musarum). Col. 



