

438 ROBERT F. PATTON 



tubes to continue growth and form a vesicle in the substomatal chamber 

 has not yet been explained. This could be the result of interaction of 

 inherent spore capability with an influence exerted by the guard cells of 

 the host. 



DEVELOPMENT IN THE HOST 



Development of the fungus in needle tissue subsequent to penetration 

 has been described for susceptible trees largely from the standpoint of 

 the intimate relationship of fungus and host cells (Clinton and McCormick, 

 1919; Colley, 1918; Boyer, 1962; Hirt, 1964). Following penetration, a 

 loose mat of much-branched hyphal tissue develops. Haustoria are formed 

 in mesophyll cells but as the fungus proliferates, much of the mesophyll 

 tissue disintegrates, and finally a dense sclerotium-like mass of com- 

 pacted mycelium is formed. Eventually the fungus breaks through the 

 endodermis, enters the vascular tissue, and grows down the needle from 

 which it invades the cortical parenchyma of young stems (Boyer, 1962), or 

 the phelloderm and adjacent phloem cells in older bark and subsequently 

 deeper tissues of the phloem (Hirt; 1964) . Hirt (1964) also followed 

 in great detail further developments through the production of pycnia 

 and aecia. 



The most detailed observations of the cellular response to infection 

 are those of Boyer (1962, 1964a) and Boyer and Isaac (1964). Cells con- 

 taining haustoria show an increase in granularity of cytoplasm. As the 

 diseased cell ages, there is a marked reduction in size and number of 

 chloroplasts , the cytoplasm appears homogeneous, and finally the nucleus 

 dissolves. At the same time osmotic and permeability changes occur (Boyer, 

 1962). Primary emphasis in these investigations has been given to a histo- 

 chemical study of phenols in white pine. One of the characteristic features 

 of infection in foliage cells up to one season old was fragmentation of 

 the vacuoles in which the phenols are localized. Observations with the 

 electron microscope indicated that fragmentation of the vacuoles involves 

 synthesis of new membranes, and this was considered in relation to the 

 possible role of phenols in resistance (Boyer and Isaac, 1964). There 

 was no evidence, however, that phenols released by vacuolar fragmentation 

 were toxic to the haustoria or the intercellular mycelium (Boyer, 1964a) . 



RESISTANCE TO INFECTION AND DEVELOPMENT 



TYPES OF RESISTANCE 



Experience in testing selections and progenies of white pines under 

 conditions of both natural and artificial inoculation has indicated there 

 is more than a single type of resistance to C. ribiaola. For example, 

 there may be resistance of the needles to initial infection and resistance 

 of the phloem tissue of the stem to establishment and continued growth of 

 the fungus (Boyer, 1967b; Hoff, 1966; Patton and Riker, 1966). 



The appearance of needle spots that failed to develop further or to 

 form stem infections was reported by Riker et at. (1943) as one indication 

 of resistance. Trees with such spots were clearly inoculated and were 

 thus not merely disease escapes. Such foliar resistance is believed to 

 be of major importance in rust resistance in both eastern and western 



white pines (Hoff, 1966; Patton and Riker, 1966; Patton, 1967). 



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