140 DISEASES OF TROPICAL PLANTS 



CH. 



at all stages in their development. At first, the tissue sur- 

 rounding a fruit scar turns slightly darker in colour and shrinks. 

 Frequently at this time these cankers or lesions will show a 

 number of small pink spots on their surface, which are in 

 reality the fruiting pustules of the fungus which causes the 

 disease. While this diseased portion is small at first, it 

 gradually spreads out in all directions until the branch may 

 be from one -half to two -thirds girdled. Barely does the 

 disease entirely girdle the branch, because the latter generally 

 dies before the disease has progressed that far. However, if the 

 twig is not more than half girdled, it generally is not killed, 

 and so the scars of cankers may be seen on branches of all sizes. 



In the healthy tissue surrounding the canker, generally 

 there is an increased growth, so that in many cases from a 

 short distance away the canker has the appearance of a knot 

 on the branch. The tree tries to heal over the wound caused 

 by the canker, but is never very successful in doing it. Scavenger 

 beetles and a number of saprophytic fungi and bacteria get 

 into the dead wood of the canker and prevent the healing over 

 of the wound. 



A longitudinal section of a cankered branch shows that the 

 fungus which causes the disease has penetrated and killed the 

 bark, cambium, and part of the wood. 



The fungus seems to gain an entrance to the fig branch in 

 the fruit scar alone, and this infection takes place within less 

 than a year after the development of the fruit. However, as 

 the fungus is not a rapid grower, the canker does not appear 

 for several months after it has really gained entrance to the 

 host tissue. 



The fungus forms cushions of pseudoparenchymatous tissue 

 on the surface of the cankers, these sometimes beginning their 

 development underneath some of the layers of cells of the host, 

 and finally breaking through and sometimes forming directly 

 on the surface. These cushions are very irregular in shape and 

 size. Sometimes the surface is rounded and smooth, and some- 

 times it is very irregular. On these cushions on small narrow 

 conidiophores, the small, elliptical, hyaline spores are developed, 

 these being cut off singly from the ends of the conidiophores. 

 At the time the spores are developing, the pustules are a light 

 pink in colour. 



It is believed that the disease can be controlled by 

 frequently cutting and burning the .diseased twigs so as 

 to destroy them before the fungus comes to fruit. 



Limb Blight. This disease (Fig. 59) is caused by 



