382 Soil Factors on Disease Resistance 



a high soil-teinperature or increased humidity. All these cases require 

 more detailed investigation which it is the object of this paper to provoke. 



In connection with the investigation of the wilt disease of indigo 

 and of Hibiscus cannabinus, some observations have been made on the 

 conditions which appear to precede the attacks of two insects Psylla 

 isitis Buckt. on indigo and the red cotton bug (Dysdercus eingulatus 

 Fabr.) on Hibiscus cannabinus. 



Psylla frequently attacks the stems and leaves of indigo leading to 

 malformation and twisting of the growing points. Sometimes the attack 

 stops at the end of the rains and the new leaves then become quite 

 normal. Stems showing Psylla attack below and healthy foliage above 

 are quite common and the attack does not spread to the new growth. 

 It has been frequently observed at Pusa that applications of fresh un- 

 decayed organic matter (such as green manure) applied shortly before 

 sowing as well as dressings of oil cake after sowing are always followed 

 by severe attacks of Psylla. Examination of the root-system shows 

 restricted and abnormal development and much discolouration of the 

 active roots. The sequence of events is so well marked that the matter 

 deserves to be studied in much greater detail. The fermentation of fresh 

 organic matter in the soil seems to lead to changes in the sap and in 

 the cells of the leaf which predispose the indigo plant to attack. In all 

 cases we have examined, root discolouration precedes and accompanies 

 the insect attack. 



Equally interesting are the attacks by the red cotton bug on Hibiscus 

 cannabinus at Pusa. These always follow the destruction of the fine roots 

 and the onset of wilt. Year after year patwa grows normally during the 

 early rains but when the wilt appears in September and October the 

 plants attract swarms of the red bug. The wilt-free plots of Roselle in 

 the neighbourhood are not attacked. Here again we appear to be con- 

 fronted with a change in the cell-sap arising from root damage which 

 prepares the way for the parasite. 



The rusts of wheat and linseed. During the progress of the wheat 

 experiments at Pusa, many hundreds of pure lines have been grown and 

 in many cases the same cultures have been repeated year after year. 

 In some instances, these pure lines have been grown in the field and also 

 in flower-pots. In others, they have been grown immediately after the 

 rains with or without a deep cultivation before sowing. Interesting 

 differences between the amount of damage done by black rust (Puccinia 

 grattiinis Pers.) to the same unit species in the same year have often 

 been observed according to the way the plants were grown. In all cases, 



