PARASITISM OF GLOMERELLA. 95 



which the appressoria, or chlamydospores, assist the fungus in gaining 

 entrance to the tissues of the host. In the majority of cases the most 

 probable explanation of the dormant infections which have been 

 shown to be present in so many instances in leaves and fruit showing 

 no external evidence of disease, is that the conidia or ascospores ger- 

 minate whenever they come in contact with the plant surface under 

 favorable conditions of temperature and moisture and produce 

 appressoria, which are able to endure more unfavorable conditions 

 than the spores and which in turn send a germ tube through the 

 epidermis. This tube apparently penetrates at first but a very short 

 distance and does little harm to the host cells, remaining in a dormant 

 or inactive condition until the host becomes weakened or injured or 

 the organ infected dies a natural death. Bodies resembling appres- 

 soria have been found on the surface of normal apple leaves upon 

 which the fungus developed in a moist chamber and they are some- 

 times found in abundance on the surface of lemons and other citrus 

 fruits. It is difficult positively to identify these bodies on a leaf sur- 

 face and trace the germ tube in the tissue, and the writers have as 

 yet been unable to devote the necessary time to this feature of the 

 investigation to verify conclusively the suggested explanation of the 

 facts observed. 



Large series of microtome sections of presumably infected leaves, 

 the unused portions of which developed the fungus when placed in a 

 moist chamber, have been studied, but the presence of fungus hyphse 

 has not been demonstrated with certainty. This would be quite nat- 

 ural if the supposition that the dormant infection is restricted to a 

 short hypha or germ tube just penetrating the surface is correct. 

 The experiments with leaves in moist chamber, especially deciduous 

 leaves, show that the discoloration of the tissue and the development 

 of the fungus start at rather definite points on the leaves and spread 

 from these in a more or less circular manner, as shown in Plate V. 

 In the case of the citrus fruits there appears to be quite clear evidence, 

 from the work of Rolfs (67), Bessey (12, 13), as well as from that of 

 the writers with leaves and shoots in moist chambers that the fungus 

 in some cases enters the stem by way of the flowers and works back 

 through the tissue. The development of the fungus first in the petiole 

 and along the midrib, as frequently happens in the case of orange 

 and rubber plant leaves in moist chamber, would suggest this possi- 

 bility, though in these latter cases the infection may have been local- 

 ized on the petiole or midrib. 



These organisms generally develop most rapidly and do most dam- 

 age to nearly or quite mature fruit and have developed special meth- 

 ods of insuring their survival and distribution from season to season, 

 not only by means of appressoria but by hibernating hyphae in old 

 fruits, leaves, and branches and by ascospores. 



252 



