COMPONENTS OF PATHOZONE BEHAVIOR

Pathozone behaviour of soil-borne plant pathogenic fungi is characteri zed by curves for the change in probability of infection with distance of inoculum from a host. We identify three major components that give rise to the pathozone profile for infection efficiency: the germinabi lity of inoculum, given that it occurs in the pathozone (P-1), the gro wth of the resulting fungal colony outwards to make contact with the h ost (P-2), and the infectivity of mycelium once contact is made (P-3). The probability of infection (P) is then given by the product, P(1)xP (2)xP(3). Using Raphanus sativus and two contrasting types of inoculum of Rhizoctonia solani as a model experimental system, we measured eac h of the three components by quantifying germinability, colony archite cture (to determine the chance of contact by one or more hyphae) and t he change in susceptibility over time as a measure of infectivity as m ycelium arrives from different distances away. The germinability of in oculum is uniform across the pathozone whereas the probability of cont act declines with distance, and the net effect of host susceptibility and infectivity increases. The characteristics of these components res ult in pathozone profiles that vary in shape. Infection efficiency can decline in an exponential or sigmoidal fashion with increasing distan ce from the host or follow a curve that rises close to the host and th en falls asymptotically to zero with increasing distance. Colony archi tecture was summarized by the distribution of the furthest extent of h yphal growth amongst colonies growing out from single inoculum units ( described, because of its asymmetrical pattern, by a gamma distributio n) and by the radial density of hyphae on concentric circles at differ ent distances from the centre of the colony (described by a negative b inomial distribution with parameters changing with distance). The mech anistic model for the components of pathozone behaviour is tested by c omparing the magnitude and shape of pathozone profiles predicted by th e model with independent estimates for P obtained by placing inoculum at fixed distances from the host, and measuring the proportion of succ essful infections. The shape depends on the relative magnitudes and ch ange with distance of the three components, P-1, P-2 and P-3, which di ffered between the two types of inoculum. The profiles could be reprod uced accurately when the probability of contact was based on the distr ibution and density of hyphae reaching the host rather than on the fur thest extent of hyphal growth amongst colonies. The model is used to a nalyse the effects of replication and stochastic variability in the co mponents of pathozone behaviour on the comparison of treatments for di sease control.