FACTORS AFFECTING INFECTION GRADIENTS FROM A POINT SOURCE OF PUCCINIA POLYSORA IN A PLOT OF ZEA MAYS

Two plots of Zea mays were planted in April and September, respectively, at Ibadan, Nigeria during 1956. A point source of inoculum consisting of potted maize plants heavily infected with the American corn rust, Puccinia polysora Underw., was introduced at pre-arranged times before the plots became naturally infected. Disease gradients from these point sources of infection were assessed by the number of pustules subsequently appearing on plants along the circumferences of concentric circles around the source of infection. 'Control' plots were planted at the same time as the main trial and the pattern of natural infection was observed in an attempt to assess the degree of contamination in the main experiment. In the centre of the main experiment a volumetric spore trap gave continuous records of the uredospore content of the atmosphere immediately above the crop during both plantings for periods of 2 weeks before planting until 2 weeks after the last assessment of infection. The forms of the disease gradients were considerably altered by several factors -principally contamination by inoculum from outside sources, the method of assessment and the time at which the assessments were made.