66 BOTANICAL GAZETTE [july 



NOTES FOR STUDENTS 



Infection as related to humidity and temperature. — The importance of 

 humidity and temperature in relation to infection has long been known to 

 plant pathologists. The problem, however, has been taken up once more by 

 Lauritzen^ in a study of diseases caused by Puccinia graminis on wheat, 

 Ascochyta fagopyrum on buckwheat, and Colletotrichum lindemuthiamim on 

 red kidney bean. 



The apparatus used consisted of two double-walled chambers, each pro- 

 vided with heating coils, thermostats, and thermoelectric connections to 

 furnish heat and power to run the fans. The outer waU of each chamber was 

 made of glass, the inner of wood except the lids (which were of glass), the wood 

 portions being coated with paraffin of a high melting point to prevent absorp- 

 tion of water. Evaporation surface for the control of humidity was furnished 

 by pans containing either water or saturated salt solutions. No attempt was 

 made to control light, after experiments designed to test its relation to infec- 

 tion had given negative results. Carbon dioxide is thought to have been of 

 little or no importance in the experiments, since the experimental chambers 

 were not so tight but that gaseous exchange was possible between inside and 

 outside whenever temperature variations occurred. Pure line strains were 

 used in the case of all 3 of the fungi, but only of wheat in the case of the 

 hosts. Plants of the same age and spores of the same age were used as far as 

 possible, although there was some unavoidable variation in both cases. 



The author uses as a criterion by which to judge the effect of temperature 

 and humidity the percentage of plants infected out of the total number exposed 

 to infection. He thus avoids most of the objections which have been made 

 to the criteria used by other workers. Where they have measured the 

 effect of temperature and humidity (on the fungus alone) by the rate 

 of extension of the germ tube and development of the mycelium, by the time 

 required for germination of spores, or by the percentage of germination, he 

 measures it by a criterion which takes account of fungus and host as well. 



The lower temperature limit for infection of wheat by Puccinia was found 

 to be about 42° F.; of buckwheat by Ascochyta about 45° F.; and of bean by 

 Colletotrichum about 57° F. The upper temperature limits in the 3 cases were 

 respectively 80, 100, and 80° F. The range of humidity (relative) for infec- 

 tion of wheat varied between 92 and 100 per cent; of bean, between 92 and 

 100 per cent; of buckwheat, between 90 and 100 per cent. In no case did he 

 find the temperature ranges for host and parasite identical. The temperature 

 range for growth of bean and wheat is wider than that for the germination of 

 spores of an infection by C. lindcmuthianum and P. graminis. The upper 

 temperature limit for the growth of bean is higher than that for germination 

 of spores of and infection by C. lindemuthianum, a fact which has made possible 



' Laxiritzen, J. T., The relation of temperature and humidity to infection by 

 certain fungi. Phytopathology 9:1-35. 1919. 



