The Role of Inoculum in the Primary and Secondary Infection of Certain Plant Pathogens

Bailey, Douglas John (1997). The Role of Inoculum in the Primary and Secondary Infection of Certain Plant Pathogens. PhD thesis The Open University.

Abstract

The infection cycle of some economically important soil-borne plant pathogens involves a combination of primary infection from particulate inoculum residing in the soil and secondary infection as disease is spread from infected to susceptible hosts. This study examines the role of the two types of inoculum in the primary and secondary infection of soil-home plant disease using a combination of experimentation and mathematical modelling.

The infectivity of inoculum can be quantified by the pathozone profile which measures changes in the probability of infection when inoculum occurs at different distances from the host. The study showed that the germinability of inoculum, the growth of the mycelial colony and the infectivity of mycelium at the surface of the host combine to dictate the shape of the pathozone profile for Rhizoctonia solani on radish. The ultimate shape of the pathozone depended on inoculum type and was particularly sensitive to changes in the density and distribution of the mycelium in the fungal colony. Mycelium from an infected radish plant grew much further and at a higher density than that from particulate inoculum (mycelial discs). This resulted in pathozone profiles that differed in shape. For particulate inoculum the profile rose and fell with distance whilst for an infected plant the decay was sigmoidal.

The characteristic shape of the pathozone for different inoculum types of Rhizoctonia solani was summarised using simple, non-linear models in which certain parameters were allowed to vary with time. Thus, the pathozone behaviour of single plants could be used to predict the progress of disease at the population level both in an unprotected crop and in a crop protected with the biological control agent Trichoderma viride. Predictions were particularly accurate for an epidemic restricted to primary infection or for an epidemic dominated by secondary infection in the absence of biological control. The model underestimated the extent of secondary infection in the protected crop.

The contribution of inoculum type to the spread of disease was examined in a contrasting host-pathogen system involving the infection of wheat roots by the take-all fungus, Gaeumannomyces graminis var tritici. Two phases of disease progress were identified, an initial phase dominated by primary infection during which particulate inoculum decayed, and a subsequent phase involving an increase in secondary infection driven by the availability of susceptible host tissue. From a low density of initial inoculum this resulted in a disease progress curve which rose monotonically to an initial plateau and was sigmoidal thereafter. These results were consistent with observations from field data. The biological control agent. Pseudomonas corrugata, reduced the probability of infection on the seminal roots of a wheat plant from particulate inoculum. However, suppression of primary infection alone is not considered adequate for control of take-all over an entire season.

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