INTRODUCTION +3678Theoretical and practical studies of sporulation in vivo have been neglected for years. Consequently, although factors influencing sporulation are as important in disease development as those influencing infection or dispersal, they have seldom been accounted for quantitatively in the theory of epidem ics. This situation evolved probably from (a) the tedious experiments re quired, with endless counting of spores, and frustrations due to variations between replicates of similar treatments, (b) an incorrect view that the number of spores trapped over an infected fi eld is simply related to the number of spores produced during the previous night, and (c) a misconcep tion that sporulation in petri dishes refl ects the situation in nature. Conse quently, the literature contains thousands of publications on processes like infection, dispersal, and sporulation in cultures, but fewer than one hundred studies in which sporulation in vivo has been evaluated quantitatively. 83 0066-4286/78/090 1-0083$0 1.00 Annu. Rev. Phytopathol. 1978.16:83-101. Downloaded from www.annualreviews.org Access provided by McGill University on 02/04/15. For personal use only. Quick links to online content Further ANNUAL REVIEWS 84 ROTEM, COHEN & BASHISporulation in vitro is apparently a fascinating subject. The relatively simple system of a fungus reproducing on a defi ned medium has provided an insight into physiological, biochemical, and genetic phenomena which are difficult to study in the complicated host-parasite system. However, the studies in vitro are often irrelevant to the sporulation patterns of the same fungus in vivo. First, they do not account for the infl uence of the host on reproduction. Second, they deal only with facultative parasites. Third, they fail to evaluate some environmental factors, such as humidity, which is constantly high in petri dishes, or temperature, which may affect differently sporulation of the same fungus in vivo (81). Space limitation prevents us from citing studies of sporulation in vitro; the reader is referred to reviews by Cochrane (11), Hawker (29), Smith & Galbraith (70), and Turian (76), and to the literature cited in these reviews. We deal only with sporulation on the host plant and are fully aware of the danger in discussing phenomena supported by a limited number of published studies and some unpublished data of our own and of our co-workers. We hope to induce future research and are encouraged by the changing approach to our subject. We follow Johnson & Taylor, who concluded that the number of spores produced by a pathogen on its host refl ects the pathogenicity of the pathogen and is the sum of the effects of all components of resistance mechanisms in the host (38). We are inspired by Waggoner & Horsfall (80), Waggoner et al (81), and Kranz et al (43), who consider numerical values of sporulation a basic ingredient of simulation programs of epidemics. We believe that epidemi ology, control, breeding, and other research programs could be assisted by improved knowledge on sporu...
