INTRODUCTION To understand how a disease is produced by a systemic, intracellular submi croscopic (below ca 0.3,.. ) agent, it is necessary to study the complete se quence of pathological aberrations and host responses that occur following infection. In such studies of pathogenesis or pathogeny (113) one gathers in formation about: (a) how and where the causal agent enters the host; (b) which are infected and how they are affected by the initial infection; (c) the effect of infected on adjacent noninfected cells in producing dis eased tissues; (d) the host response to the diseased tissues such as formation of wound periderm or tyloses; and (e) the maleffects of the tissues on other parts and functions of the plant. A standard set of steps in pathogenesis cannot be formulated for all diseases because the steps vary with the disease, the host, and the vector. Most plant pathologists do not consider all of these steps but rather spe cialize in certain well defined areas of experimentation such as: (a) infection processes (93, 100) ; (b) synthesis of virus particles (5, 72, 82); (c) translo cation of the agent (93); etc. This paper will consider cytological and ana tomical aspects of pathogeny beginning with the first appearances of the agent and! or with the first pathologic changes in the host that are detectable with the light or electron microscope. Until Doi et a1 (17) discovered mycoplasma associated with several plant diseases, and Goheen et al (40) discovered Rickettsia-like organisms associ ated with Pierce's disease, all graft-transmissible diseases with submicro-
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H. Schneider (1973) studied this question.