A theory relating focal epidemics to crop-weed interactions.

Phytopathology is concerned with the prevention of crop losses due to pathogens in agricultural plant populations and with plant population biology regarding the understanding of the causes of changes in numerical abundance and gene frequency in wild plant populations (1). In line with this separation of the two research fields, pathosystems were divided by Robinson (12) into artificial (crops) and natural (wild plants) pathosystems. Pathogens with weeds as hosts can neither be classified as artificial nor as natural and, therefore, a weed pathosystem may be distinguished and defined as any subsystem of an ecosystem that involves parasitism causing diseases of weeds (4). In general, characteristics of a weed pathosystem involve the occurrence of host and pathogen in a relatively uniform physical environment strongly influenced by humans and spatial variability due to both spacing and developmental stage of the host. Knowledge of weed pathosystems is of interest for developing biological weed control using indigenous pathogens (7). Three approaches of biological control were distinguished by Charudattan (2): (i) the inoculative approach, (ii) the augmentative approach, and (iii) the inundative approach. The inoculative approach or classical approach is based on the innate ability of a pathogen to build up epidemics after introduction of a small amount of inoculum in an area where the target weed is present, but the pathogen was absent until the time of introduction. In contrast, the inundative or bioherbicide approach is based on the use of native pathogens that are applied onto the whole weed population. This knockdown approach requires relatively large amounts of infectious units of a pathogen. The augmentative approach may be viewed as the middle between the inoculative and inundative approach in terms of the amount of inoculum used and the management actions necessary to create epidemics. The contrasts between the augmentative approach at one side and the inoculative and the inundative approach at the other side remained a bit vague. The augmentative approach was, therefore, redefined recently by Müller-Schärer and Frantzen (8) as the system management approach. The approach excludes the use of exotic organisms (classical approach) and the use of mass amounts of inoculum applied like a herbicide to the whole weed population (bioherbicide approach). The system management approach is intended not only for extensive agroecosystems, but also for intensive agroecosystems. The aim of the system management approach is less to eradicate a weed as to manage the weed pathosystem and so reducing competition exerted by the weed on a crop. Stimulating epidemics and reducing competitiveness of the target weed are the key factors of the system management approach. In the following, we will present first a theory to dovetail these two key factors and, subsequently, the development of theory into practice will be illustrated by an epidemiological study directed to the model weed pathosystem Senecio vulgaris L.-Puccinia lagenophorae Cooke.