CHALLENGE OF BIOASSAY PLANTS IN A MONITORED OUTDOOR ENVIRONMENT

Mobile potted plants placed in a field can discretize episodes conduci ve or nonconducive to disease processes in a fluctuating environment. Models of infection and other disease components derived in controlled environments can be confirmed with this bioassay method. In 1992 heal thy wheat plants (Triticum aestivum) were placed in a wheat field 32 t imes for 24 h periods and then subjected to a wet period of 24 h or re turned directly to a growth chamber. Pyrenophora tritici-repentis, Pha eosphaeria nodorum, and Cochliobolus sativus were the most common path ogens. Pyrenophora tritici-repentis caused the greatest number of lesi ons and exhibited the highest infection efficiency of the three pathog ens. Amount of infection was positively correlated among the pathogens but correlations of tan spot and spot blotch with septoria nodorum le af blotch were lower when there was an added wet period. Weather was m onitored to find correlates with infection period. Leaf wetness durati on of 6-7 h was minimal for infection by P. tritici-repentis, confirmi ng results obtained at constant temperatures. Infection periods could also be related to unconventional variables such as wind direction, in dicative of environmental associations that can be revealed by this bi oassay technique. Current analytical techniques promise a better under standing and predictability of plant disease epidemics.