i66 DISEASES OF CROP-PLANTS 



discoloration of the surface, made up of slender stalks bearing 

 the minute oval conidia. These when produced in mass form a 

 white deposit. Fresh conidia continue to form on the stalks 

 if the air is humid, and a felted white covering may be formed 

 over the whole or part of the pod. The thick-walled resistant 

 spores are formed in this layer and in the underlying tissues. 



Infection. 



The most abundant, but least resistant, means of dispersal 

 is afforded by the conidia, which may be washed by water or 

 blown by wind on to other pods. If they come to lie in a film or 

 drop of water they function as sporangia, producing lo to 30 

 zoospores, which swim about by means of the two cilia with which 

 they are provided. The zoospore settles down and produces a 

 germ tube which penetrates the epidermis either directly or by 

 way of a stoma. The whole process occupies only an hour or two. 

 If the surface is merely damp with no free water present the 

 conidium produces one or more germ-tubes without an inter- 

 mediate zoospore stage. 



The thick-waUed spores are set free by the decay of the pod 

 and are capable of remaining dormant in the soil or elsewhere 

 until conditions are favourable for their germination. This takes 

 place by the production of a short germ-tube bearing a conidium 

 at its top from which infection may ensue as described above. 



By these means pods on a healthy tree may become infected. 

 How far or under what circumstances direct infections of the 

 bark, producing canker, may occur does not seem to be known. 

 It is, however, well-established that diseased pods commonly 

 infect the stem on which they are borne, giving rise to canker, 

 and, vice versa, that pods borne on cankered trees may become 

 infected from the stem. 



According to Rorer's experiments, " when the tissue of the 

 pod at the stem end becomes well invaded by the fungus, the 

 mycelium runs back into the cushion either through the bark 

 or the central woody cylinder of the stem of the pod, or through 

 both. If it has gained entrance to the tree through the thin 

 bark layer of the pod stem it generally spreads out in all direc- 

 tions so that the whole cushion and surrounding area of bark 

 becomes cankered ; but, on the other hand, if the mycelium 

 grows into the tree through the woody part of the pod stem the 

 cushion may not become cankered. The mycelium will extend 

 in narrow lines for some distance up or down the stem in the 

 cambium layer but sooner or later wiU grow outward into the 

 bark and then spread out rapidly, forming larger cankered areas." 



The canker spots are quickly invaded by secondary fungi 

 which occupy the tissues and complete their decay. 



Influence of External Conditions 



The parasite under discussion belongs to a group of fungi 



