EPIDEMICS OF DIAPORTHE-ADUNCA IN EXPERIMENTAL AND IN NATURAL-POPULATIONS OF PLANTAGO-LANCEOLATA AND THE EFFECT OF PARTIAL RESISTANCE ON DISEASE DEVELOPMENT

Epidemics of the splash-dispersed pathogenic fungus Diaporthe adunca o n its host, the perennial herb Plantago lanceolata, were followed duri ng two consecutive years in transects at roadsides in the Netherlands. Epidemics of D. adunca were also studied on clones of a susceptible a nd a partially resistant genotype of P. lanceolata grown either in a p ure stand or in a 1:1 mixture in small plots in the garden. The epidem ics in the natural and experimental populations could be adequately de scribed by logistic and Gompertz models, but large differences were fo und in final disease levels and relative growth rates. The effect of p artial resistance on the epidemic in the mixture was less than in a pu re stand, probably due to the provision of inoculum from the highly di seased susceptible genotype to the partially resistant genotype. In th e garden focal and wind-direction effects were seen. In the natural po pulations the epidemics developed from numerous primary infected scape s making foci and wind-directions effects less conspicuous.