F, J. F. SHAW 51 
the growth of the parasite or to a condition directly favouring the development 
of the parasite ; or both these factors may operate at once. 
In dealing with a fungal disease one of the first factors to be considered is the 
variation between the climates of different years. It has already been mentioned 
that the disease develops most severely upon the mature plant, the crucial months 
being usually August and September. The principal features of the climate in these 
months in 1917, 1918, and 1919 are shown on Plates VII, VIII, and LX from which 
it appears that these months in 1917 in Bihar were generally more humid and 
cooler than the corresponding periods in 1918 and 1919. These two factors of 
temperature and humidity probably affected both parasite and host ; the weather 
of 1917 being in respect of its higher humidity more favourable to the fungus and 
delaying the ripening of the crop, thus giving the parasite more time to act. Sucha 
correlation between humidity and disease is by no means uncommon. Thus the 
determining factor in the incidence of wheat rust in certain parts of India appears 
to be the atmospheric humidity during the early months of the year.! 
The influence of climate on the incidence of ** black band ” disease was further 
emphasized by a study of the weather conditions during the inoculation experi- 
ments of 1919. In certain cases (Experiments XVa and XVIB, c, d) the percentage 
of successful infections was high, while in others (Experiment X VIa) the inocula- 
tions were a failure. It is suggested, by a comparison of the dates on which 
infections were made with the conditions of temperature and humidity prevailing at 
the time (Pl. LX), that the successful inoculations were those which coincided with 
a relatively high humidity, and that inoculations which failed were those carried 
out during a period of lower humidity and higher temperature. All these infec- 
tions were carried out on “ kakya bombai ” in the field, and for a complete mvesti- 
gation of the influence of climate on infection a detailed record of humidity and 
temperature actually recorded in the jute field during a series of infections is 
required. It is not possible to give these data at present but Plates X and XI 
show a complete record for these conditions during Experiments XVIa, 6 and 
XVIIle, d, obtained from a hygrometer working in a laboratory withm a short 
distance of the site of the experiments. A more numerous series of observations 
is needed to establish the relations between humidity and temperature, and the 
success or failure of inoculations. It may be recalled, however, that in certain 
cases the limits of humidity within which infections can occur have been proved 
to be relatively narrow. Thus it is stated? that infections of wheat with 
Puceima graminis tritici do not succeed below a humidity of 95 per cent. ata 
1 Butler, E. J. ‘Fungi and Disease in Plants.” Thacker, Spink & Co., Calcutta, 191>. 
p. 110. 
2 Lauritzen, J. L. “ Relations of Temperature and Humidity to Infection by certain Funci.” 
Phytopathology, Vol. IX, Jan., 1919. 
