BUTLER AND KULKARNI. 235 



however, much less prominent than in, for instance, the allied 

 potato blight, and is sometimes difficult to detect by the naked 

 eye. As the spot widens sporangia continue to be produced at 

 progressively increasing distances from the centre. 



The above may, in mild cases, constitute the whole extent 

 of the attack. Frequently, however, the petiole becomes infected. 

 The course of the disease here is similar to that on the leaf-blade 

 except that the spots soon become elongated in the direction of 

 the axis of the stalk. The damage caused is, however, greater, 

 for if the spot is of any considerable size, it leads to such shrinking 

 and disorganisation of the tissues that collapse of the structure 

 above is brought about. The leaf hangs on the stalk and rapidly 

 decomposes. Sometimes the petiole breaks at the point of worst 

 attack, the part below drying up so rapidly as to prevent further 

 progress. If this does not happen extension down the petiole 

 may continue as far as its sheathing base ; infection of the stein 

 at this point was naturally looked for and though no definite case 

 was seen, the ready extension down the petiole renders it not 

 improbable. In Pliytophthora infestans it was at one time believed 

 that infection of the potato tubers, which is well known to occur, 

 results from extension of the mycelium from the leaves and stalks 

 down within the tissues. It is now more generally held to be due 

 to direct infection from sporangia and zoospores fallen from the 

 above-ground parts and germinating in contact with the tubers. 

 Similarly, in Colocasia direct infection of the conn must occur and 

 the more readily that it is not uncommon to find conns partly 

 exposed at the surface of the soil. Though internal infection 

 through the stem may occur, the fact that no actual case could be 

 found suggests that external infection, from reproductive bodies 

 formed on the leaves and petioles, is the more usual course. 



In a few cases infection of the inflorescence was observed. 



In a severe attack all the leaves may be lost and the plant 

 killed. In milder cases the young leaves may be free from disease 

 or only slightly involved, while the older ones are covered with 

 patches of varying extent or are hanging rotted on their stalks. 



