REPORT OF COMMITTEE ON PUBLICATION 147 



leaves, being susceptible when young, gaining immunity with maturity, and 

 again becoming susceptible in the latter part of the season. The writer has 

 also made a similar observation. This change is most strikingly exhibited 

 in the more susceptible varieties. It does not occur, however, in young vines 

 in a nursery nor is it so apparent in cold rainy seasons. He also claims that, 

 unless the vine is in a receptive state, infection will not occur regardless of 

 how favorable the other conditions may be. It is not clear, however, exactly 

 what is meant by the condition of receptivity. 



Istvanffi and Palinkas (1913) are of the opinion that susceptibility de- 

 pends on the vapor tension in the sub-stomatal cavity and the other inter- 

 cellular spaces, upon the turgor of the cells and to a certain extent upon the 

 chemical composition of the cell sap, since chlorotic leaves are relatively 

 resistant. 



On the other hand, Averna-Sacca (1910) is of the opinion that the compo- 

 sition of the cell sap may be the important factor. To be more specific, he 

 claims that the acidity plays an important part in rendering a vine resistant. 

 In support of this contention he points out that the acidity of resistant forms 

 varied from 4.3 to 10.3 per cent, whereas the acidity of the more susceptible 

 varieties is only .5 to 2.6 per cent. 



Resistance to the mildew does not seem to be due to any morphological 

 characters of the leaf. The densely matted hairs on the lower surface of the 

 leaf of certain varieties cannot be a factor, since it has been determined that 

 small drops of water readily permeate them to the surface of the leaf. 

 Furthermore many of our most resistant forms are smooth leaved while 

 certain of those which are most susceptible are densely clothed with hairs. 

 The writer has determined that there are several thousand more stomates 

 per square centimeter on the leaves of certain susceptible varieties than on 

 those of resistant forms but in any case there are amply sufficient to permit 

 ready infection. 



Infection experiments may offer some explanation of resistance. In the 

 case of the Delawares the germ tube very readily penetrates the stomate 

 and makes a rapid and prolific growth within the leaf, whereas after the tube 

 has penetrated the stomate of the leaf on a Clinton it seems to encounter 

 some influence which checks its growth, the substomatal swelling is smaller, 

 the secondary mycelium is more attenuated and the growth during a given 

 period is much less. 



A consideration of the difference in the abundance of fruiting on the 

 leaves of susceptible and resistant varieties is also significant. It is evident 

 that in order to produce an abundance of spores the fungus must absorb 

 sufficient food material and an insufficient or unavailable supply will reduce 

 the number of spores. Granting this we may say that the food in the resist- 

 ant forms is not available or that some influence is present which resists the 

 assimilation of the available food, because the fructification on the resistant 

 varieties is strikingly less than on the more susceptible forms. 



The apparent immunity of the berries is due to the waxy covering which 

 hinders the adhesion of water to their surface and to the lack of stomates. 

 That this resistance is only apparent immediately becomes evident if the 

 fungus obtains entrance in some other way. The growth of the mycelium is 

 very rapid and the berry is quickly rotted. 



