Root Pathogens

Abstract

AbstractRoot pathogens affecting root systems of living plants are represented by a taxonomically wide array of organisms, including viruses, protozoa, chromists, fungi, nematodes, and plants. A number of root pathogens function also as vectors of viruses. In addition to the well-known, widely spread species from agriculture, probably many are still hidden in unmanaged vegetations given their limited ability for dispersal and problems associated with identifying functions of pathogens in plant species mixtures. Key ecological traits of root pathogens include host range, mode of dispersal and survival, saprotrophic ability, and temperature and moisture requirements for growth and survival. Several evolutionary convergent traits occur for root pathogens, because they share general properties of soil life, including the relative difficulty to move within the soil and, from this, the need to survive for prolonged periods of time under adverse conditions. Saprotrophic activity by the pathogen may occur, often on dead organic material originating from its own host. There are various survival structures (single- and multicelled) that combine a very low maintenance respiration with inaccessibility for infection by other organisms. Also soil biostasis, the phenomenon of non-germination of viable propagules in a living soil, contributes to the longevity of soil-borne pathogens. Generally dispersal in soil is in the order of magnitude of meters per year; long-range transport is primary caused by transport of infected plant material or infested soil. Some root pathogens fruit aboveground and then spores will become airborne, and thus transported over considerably larger distances.Infection incidence and severity by root pathogens depends on the abiotic environmental conditions (mainly temperature, soil moisture content and pH), inoculum density, amount of susceptible host tissue and activity of antagonists. Under optimal conditions, root pathogen populations accumulate, creating an environment that is unfavourable for susceptible successors. This general phenomenon forces farmers to introduce crop rotations, and causes young plants to grow less well near mature trees, or at places where mature trees have been removed. On the other hand, lack of root pathogens explains the success of bioinvasive plants in a number of cases. In unmanaged, natural vegetations, root pathogens may survive in perennial root systems. Their effects are much less clear than in agriculture, since they can be replaced by resistant or less susceptible plant species before they die. The role of root pathogens in unmanaged vegetations has been addressed only marginally, but their role is potentially large. A clear indication that pathogens play a role in unmanaged vegetations is that overyielding of root biomass in species mixtures as compared to the sum of root biomass production in monocultures of the respective species has been observed repeatedly. Overyielding could be removed by application of fungicides to the soil. Where pathogens limit the development of certain plant species, they leave space for other plant species, thus promoting vegetation diversity in space or in time. Ecological concepts on root pathogens have been developed mainly on the basis of agronomic problems but these may differ for unmanaged vegetation. Especially the uncultivable pathogens require more attention.KeywordsFusarium OxysporumPowdery ScabPotato Cyst NematodeVerticillium DahliaeSoilborne PathogenThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.