Effect of Postsymptom Application of Fungicides on Urediniospore Production by Puccinia triticina on Wheat and P. hemerocallidis on Daylily.

Greenhouse experiments were conducted to evaluate the effect of post-symptomatic fungicide applications on urediniospore production of leaf rust of wheat, Puccinia triticina, and daylily rust, P. hemerocallidis. Fungicides from five chemical classes were evaluated for their effect on urediniospore production at 24-h intervals post application for 96 h. The quinone outside inhibitor (QoI) fungicide (azoxystrobin) significantly reduced cumulative urediniospore production by P. triticina (40.1 and 11.3% of control) and P. hemerocallidis (37.9 and 35.6% of control) in both trials. The demethylation inhibitor, myclobutanil, significantly reduced urediniospore production by P. hemerocallidis 24 h post treatment in both trials and cumulative production in one trial. Myclobutanil did not affect urediniospore production by P. triticina. Four QoI fungicides significantly reduced urediniospore production by P. triticina at all collection times (24, 48, and 72 h) and cumulative production in both trials. No differences were observed between the QoI fungicides with P. triticina. The four QoI fungicides significantly reduced urediniospore production by P. hemerocallidis compared with the nonfungicide control, with significantly fewer urediniospores produced in the pyraclostrobin-treated lesions compared with azoxystrobin, fluoxastrobin, and trifloxystrobin. Differences in leaf morphology between daylily and wheat such as leaf thickness and waxiness may contribute to the differences observed between the two pathosystems. The significant reductions in urediniospore production by postsymptom applications of QoI fungicides, combined with their known fungistatic properties, suggest that these chemistries would be useful for reducing the spread of viable inoculum to noninfested plants.