22 CRANBERRY DISEASES. 



fungus, such as sufficient heat and moisture. If a similar condition of 

 affairs should prove to obtain rather generally among similar para- 

 sitic fungi, as seems to be possible from observations and experiments 

 made with other cranberry diseases and several anthracnoses on 

 different hosts, it will have an important bearing upon the results 

 and conclusions derived from ordinary infection experiments in 

 which plants have been used which were not grown from uninfected 

 seed under conditions which would preclude possible infection at 

 any time previous to their use. It would also have an important 

 bearing upon the value of inferences regarding the time of infection 

 based upon the time of the outbreak of the disease. 



It has been supposed that in the case of diseases such as the black- 

 rot of the grape the time of its external destructive appearance fol- 

 lowed within a short period after the germs of the disease had entered 

 the tissues of the plant. In other words, it has been taken for 

 granted that as soon as a germ tube gains entrance to the tissues of 

 its host it proceeds to develop under normal conditions and soon pro- 

 duces its characteristic injuries. On the contrary, however, it seems 

 much more probable that there is no regular period of incubation, 

 but that the development of the parasite depends largely upon the 

 conditions surrounding it and its host. If the host plant becomes 

 weakened in any way or if the conditions of heat and moisture are 

 especially favorable for the fungus its development may be rapid and 

 continuous, but if these conditions do not obtain the fungus may 

 remain in an inactive or dormant condition, or its development may 

 be very slow or intermittent and in some cases perhaps entirely sup- 

 pressed. Many illustrations of this condition can apparently be 

 found among foliicolous pyrenomycetes which develop their fruits 

 so abundantly during the winter on old fallen leaves. There is no 

 evidence, so far as we know, that infection of these leaves takes place 

 during the winter. The scanty observations which have been made 

 indicate rather that the mycelium is present in the leaves when they 

 fall, though there is no outward indication of its presence. 



TIME AND MANNER OF INFECTION. 



We have been unable thus far to discover exactly when and in what 

 manner infection of the leaves and fruits takes place. The pycnidial 

 form of the fungus may be found within from ten to fifteen days 

 after the water with which the vines are usually flooded during the 

 w r inter has been removed. This appears to be about the normal 

 period required for the development of the pycnidia when the growth 

 of the fungus is regular and continuous, as shown by its growth and 

 development in pure cultures. The pycnidia appear first upon old 

 leaves of vines which have apparently been weakened or killed by the 

 no 



