236 OOLOCASTA ELTfiTTT. 



The corms may be entirely lacking or, if formed, may be small and 

 shrunken. Where only isolated spots occur there is little apparent 

 injury to the plant and the conn matures normally. This is the 

 condition which has been most frequently found. Nothing is 

 clearer than the close connection between the intensity of the 

 disease and the atmospheric conditions prevalent during the growth 

 period of the host. Much the worst attack noticed at Pusa during 

 the last six years occurred in 1909, a year when the rainfall was 

 nearly double the normal, there was almost continuous cloud or 

 rain during August and September, the total precipitation for 

 these two months was 28'44 inches and the 8 a.m. relative humidity 

 exceeded 80 per cent, on fifty-three out of sixty-one days. In the 

 present year (1912) the disease had not appeared up to the end of 

 September, the rainfall during the previous two months having 

 been only 12*96 inches and the 8 a.m. relative humidity exceeding 

 80 per cent, on forty days only. Sawada notes that the disease is 

 worse in shady than in sunny localities in Formosa. 



Infection of the conn was first noticed in 1909. It has been 

 observed each year since, though not to the same degree. The 

 corm rot is sometimes apparent even in the field but more usually 

 develops subsequently in storage. As in the tuber rot caused by 

 Phytophthora infestans it appears to be, when not complicated by 

 secondary organisms, a dry rot, the tissues being dry but rather 

 soft to the touch. Wet rot may develop in stored corms in which 

 Phytophthora Colocasiw can be detected but this is probably due 

 to secondary invasion by various soil organisms. 



II. Microscopic characters of the disease and of its cause. 

 The mycelium of the parasite is present in all the parts which 

 show external discoloration. It has been found in the leaf (includ- 

 ing the petiole), the inflorescence and the corm. The hyphaB are 

 large, usually 5 or 6m, but varying from 4 to 9/* in diameter, 

 unseptate and branch copiously. In the leaf-mesophyll they are 

 purely intercellular but epidermal cells are often traversed by 

 hyphse both entering and emerging from the leaf, Haustoria 



