CHALLENGE OF BIOASSAY PLANTS IN A MONITORED OUTDOOR ENVIRONMENT

Mobile potted plants placed in a field can discretize episodes conducive 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 healthy wheat plants (Triticum aestivum) were placed in a wheat field 32 times for 24 h periods and then subjected to a wet period of 24 h or returned directly to a growth chamber. Pyrenophora tritici-repentis, Phaeosphaeria nodorum, and Cochliobolus sativus were the most common pathogens. Pyrenophora tritici-repentis caused the greatest number of lesions and exhibited the highest infection efficiency of the three pathogens. Amount of infection was positively correlated among the pathogens but correlations of tan spot and spot blotch with septoria nodorum leaf blotch were lower when there was an added wet period. Weather was monitored to find correlates with infection period. Leaf wetness duration of 6-7 h was minimal for infection by P. tritici-repentis, confirming results obtained at constant temperatures. Infection periods could also be related to unconventional variables such as wind direction, indicative of environmental associations that can be revealed by this bioassay technique. Current analytical techniques promise a better understanding and predictability of plant disease epidemics.