Development of bean rust epidemics in a field planted with alternate rows of a resistant and a susceptible snap bean cultivar
1988
Aylor, D.E.
In 1986 and 1987, half of a 0.4-ha field was planted with a cultivar of bean plants susceptible to Race 38 of the bean rust pathogen Uromyces appendiculatus. In the remaining half of the field, every other row was planted with the same susceptible cultivar alternating with a resistant cultivar on which no pustules were produced. The middle row of the susceptible cultivar in each half of the field was inoculated approximately 15 days after planting. On subsequent dates, counts of pustules on plants from rows equidistant from the inoculated row in the two halves of the field were compared. Generally, there were fewer pustules per plant in the half of the field with the two cultivars in alternating rows than in the half with only the susceptible cultivar. In 1986, the cumulative number of pustule days per plant in the alternating planting was about 30% less than in the half planted entirely to the susceptible cultivar. In 1987, this difference was about 6% less. The relatively small effect of the alternate row planting in reducing bean rust was probably a result of high rates of self-infection of plants. Dispersal gradients for urediniospores of U. appendiculatus are initially very steep and include a long, relatively flat tail, characteristic of a power law. Such a dispersal gradient not only allows plants far from a disease focus to become infected, but also strongly favors self-infection of susceptible plants. As bean plants grew and filled the space between rows, spores produced on lower leaves probably were even more likely to be deposited on the plant on which they were produced than to be carried to other plants. As a consequence, a small initial benefit of the alternate planting tended to decline with increasing number of generations of the pathogen.
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