The dispersal of plant pathogens

Abstract

Dispersal is what happens between take-off of a spore and its deposition — it does not include its germination or infection of the plant — whereas spread implies that the pathogen reaches and infects plants (Van der Plank, 1967). These and other terms, (dissemination, transmission) are, however, rather loosely used. Spread can be used to denote progressive colonization of the infected organ or plant, passage of the pathogen from infected plants to others in the same field or crop area, or long distance spread of pathogens between plants which are widely separated, as in intercontinental spread. Many pathogens spread from plant to plant by means of infective propagules, which may be spores, bacteria or viruses, but there are other pathogens which do not spread in this way or which do so only to a very limited extent. Most of the pathogens restricted to roots have little power of independent movement through the soil (except sometimes by rhizomorphs or similar structures) and their spread is likely to be by water borne or insect borne propagules or by redistribution of contaminated soil or diseased roots. Plasmodiophora brassicae, generally tends to occur in patches indicating areas of contaminated soil although this pattern is commonly upset by ploughing or other measures which scatter the spores or diseased root tissue. Such patches, if undisturbed, enlarge slowly from year to year provided susceptible plants are sown.