The effect of clumped planting patterns on epidemics of damping-off disease in cress seedlings

Environmentally controlled experiments on damping-off disease of garden cress (Lepidium sativum) caused by Pythium irregulare were used to study the effect of clumped planting patterns upon epidemic rates. In clumped stands of seedlings the rate of multiplication of disease was, in general, slower than in unclumped, evenly spaced stands of the same overall density. When the number of clumps per unit area was varied, but the proportion of the total area occupied by clumps kept constant, there was no significant change in the rate of linear advance of the disease between different treatments, but the multiplication rate in randomly inoculated plots was higher at 100 clumps/m² than 200 clumps/m² due to an unexpected interaction between the multiplication rate and the distribution of primary disease foci. When the area occupied by clumps was varied, but the number of clumps per unit area and the overall plant density were kept constant, both the multiplication rate and the rate of linear advance of the disease were significantly reduced in the stands with the most condensed clumps.