The effect of some biological, physical, and chemical factors on the formation of pis at in was studied by the chemical analysis aftha extracellular fluids (diffusates) from the endocarp tissues of pea pods. The following points were demonstrated

Chemical reagents were tested for their effectiveness to stimulate pisatin production in the endocarp of pea pods. The following compounds were found most active:

SummaryIn an investigation into the effects of water relations on sporulation of Peronospora tabacina Adam in tobacco leaf disks the following results were obtained:(i) Both diffusion pressure deficit (D.P.D.) and relative humidity (R.H.) were shown to be of critical importance to sporulation. The threshold values for maximum sporulation were 2·6 atm and 97 per cent. respectively. Minor deviations from these values towards either a higher D.P.D. or a lower R.H. very significantly reduced sporulation intensity.(ii) Some aspects of the effect of R.H. level X time were analysed.(iii) It was shown that in addition to the requirement of a minimum period of optimum conditions at a specific time of day for sporulation, there exists also a stage of conidiophore development which is completely dependent on favourable humidity conditions.(iv) Subminimum periods of optimum humidity on consecutive days were shown to be non-cumulative, and to have no positive effect on sporulation.(v) Both change in R.H. and mechanical shock were shown to be capable of causing conidial discharge.These results are briefly discussed in relation to epidemiology of disease development. INTRODUOTIONYarwood (1956) has recently reviewed published work concerning the relationship between humidity and sporulation in vivo of plant pathogenic fungi. Little quantitative data on this subject has been published and qualitative results in most publications have been limited to field observations under uncontrolled conditions.Studies of the sporulation of Peronospora tabacina Adam have been reported by Clayton and Gaines (1933), Armstrong andSumner (1935), andDixon, McLean, andWolf (1936). These authors have shown that under field, seed-bed, and laboratory conditions sporulation was limited to the range of humidity from atmospheric saturation to slightly below dew-point and that it occurred most abundantly when these conditions occurred at night or over prolonged periods when the sky was overcast.The mechanism of conidial discharge in P. tabacina has been previously studied by Pinkard (1942) who claimed that it was entirely a response to change in humidity.In a preliminary note (Cruickshank and Muller 1957), a technique was described for the quantitative study of sporulation as a function of individual environmental factors under controlled conditions. This paper reports in greater detail the interaction between water relations and sporulation in vivo. An attempt is also made to evaluate the relative importance of humidity changes and mechanical shock in the discharge of conidia of this fungus.

Representative groups of microorganisms were assayed for sensitivity to pisatin in agar media. Detailed growth studies were carried out on a small number of filamentous fungi of special interest to the problem of disease resistance in plants. The following points were demonstrated.

SummaryIn a study of the effect of some further biological, physical, and chemical factors on the formation of pisatin, the following points were demonstrated:(1) Pisatin formation following heat treatment (45°C) or anaerobic storage was dependent on the duration of the exposure period. Delayed inoculation, following some treatments, resulted in an increased capacity of the host to form pisatin. Pisatin concentration and host susceptibility were inversely related. (2) Pisatin formation was stimulated by several saprophytic fungal species. (3) Detached pea pod tissues retained their capacity to form or release pisatin for 20 days following inoculation. (4) Pisatin was stable in the tissues of Pisum sativum and mycelium of Ascochyta pisi. (5) Pisatin was localized in inoculated tissues and did not diffuse to neighbouring healthy tissues. The formation of pisatin in relation to alterations in host metabolism is briefly discussed.