Effects of Inoculation Timing on Symptom Development in Ulmus Americana L

—Field inoculation trials are an important component of screening American elms (Ulmus americana) for levels of resistance to Dutch elm disease. A major concern in screening is variability in disease ratings from year to year. Previous studies have demonstrated that timing of inoculation can have a significant impact on disease susceptibility. In this study, trees were inoculated in the main stem using a drill method of inoculation. A recently collected isolate of Ophiostoma novo-ulmi with known pathogenicity was used for inoculations. Three different inoculation times were examined: early (May 26), mid ( June 23), and late (August 4) season. Trees were assessed for wilt symptoms at 2, 4, 6, and 8 weeks post inoculation using a disease severity scale of 1-6. The trees in the early season inoculation group had the highest mean disease severity ratings at 4, 6, and 8 weeks post inoculation (WPI), while the late season inoculation group had the lowest disease rating at every time point as well as the smallest area under the disease progress curve. Scientists evaluating American elms for resistance to Dutch elm disease should avoid late season inoculations due to reduced disease susceptibility.