DISEASES CAUSED BY FUNGI 25 



cabbages, and melons, and some varieties of bananas are not 

 subject to attack by Panama disease. 



The Diplodias. 



Nomenclature in the genus Diplodia is particularly confused 

 as the result of attempts to divide up its species amongst smaller 

 genera : Lasiodiplodia, Botryodiplodia, Chatodiplodia, etc., 

 based on characters that have proved to be unstable. The 

 present tendency is to revert to the use of Diplodia pure and 

 simple, and with this policy, in the present unsatisfactory con- 

 dition of knowledge concerning the delimitation of the species, 

 the writer heartily agrees. 



There are many saprophytic Diplodias described, but the 

 species concerned in plant diseases are essentially weak para- 

 sites, which (as E. J. Butler has pointed out in relation to D. 

 TheohromcB), though capable of living as saprophytes, are more 

 often found on dying or recently dead plants than on old dead 

 plant tissue. It would appear that the advantage they secure 

 from their ability to infest weakly but still living tissue is neces- 

 sary for their success in competition with the pure saprophytes. 

 The limitations of the parasitic powers of a typical Diplodia are 

 well illustrated on lime trees. In young and vigorous trees the 

 infestation of the stub of a broken branch stops short at its base ; 

 in older weaker trees the fungus infests a proportional amount 

 of the wood and bark connected with the stub, giving rise to a 

 sharply defined dead segment, eventually running out to a 

 point, on the older branch or stem. In very weakly trees the 

 extension of this sort of damage gradually kiUs back the whole 

 tree. 



Diplodias of the semi-parasitic type have a general and 

 abundant distribution in the West Indies, but how many species 

 are involved is not known. They have a range of feeding habit 

 wide enough to include fruit pulp and close-grained wood. The 

 diseases with which they are typically associated may be grouped 

 as follows : 



[a) Diebacks of twigs and branches when these are in a 

 weakened condition, entry being made from dead tips, broken 

 branches, or other wounds productive of dead material on which 

 the fungus can get a footing. This type has attracted most 

 attention in the tropics on cacao, Citrus, and Hevea, but is 

 common on other trees of lesser importance. 



[b) Root diseases of similar nature, occurring on plants 

 debilitated by age, poor or badly drained soil, or other causes. 

 These are met with occasionally on cacao, more frequently on 

 limes in the West Indies ; and on tea in the Eastern tropics. 



[c) Rots of fruits, which very frequently start from wounds, 

 but do not always seem to be dependent on them, especially if 

 the fruit has been checked in its development. Examples are the 

 Diplodia rots of cacao pods, cotton bolls, citrus fruits and melons 



