Evaluating fungicide efficacy, plant varietal resistance and leaf morphology in developing control strategies for basil downy mildew ( Peronospora belbahrii)

Sweet basil (Ocimum basilicum) is the most economically-important culinary herb in the U.S. Recently, a new disease, basil downy mildew (BDM) caused by the oomycete pathogen Peronospora belbahrii, was introduced into the U.S. and has caused significant losses in commercial basil production. BDM was first reported in October, 2007 in Florida. Since then, the pathogen has spread across the country, most likely via infested seed and weather patterns.;In 2010 and 2011, biological and conventional fungicides were evaluated for the control of BDM using cv. 'Italian Large Leaf' basil. In each year, only K-Phite (phosphorous acid, FRAC code 33) provided the best level of disease control.;In 2010 and 2011, Ocimum species and varieties including sweet, cinnamon, Thai, citrus, spice and holy basils were evaluated for susceptibility to BDM. In both years, sweet basils were determined to be the most susceptible; cinnamon, clove and Thai types were moderately susceptible; and citrus, spice and holy types were least susceptible to BDM.;Stomata length and density were measured on 20 basil species and varieties in 2011. In general, basil species with higher stomatal densities had higher downy mildew incidence. High densities were mainly found in the sweet, cinnamon and clove basils. Species with longer stomatal lengths (the citrus and spice types) generally exhibited lower downy mildew incidence. Holy basil, the least susceptible of all Ocimum spp. evaluated in this study, had the greatest stomatal density and shortest stomatal length.;Leaf curvature was measured on 20 basil species and varieties. Some sweet basil varieties with the highest mean BDM incidence also had the greatest downward leaf curvature, while other sweet basil varieties with moderate mean downy mildew incidence had leaves that were nearly flat or curved upward. Holy, citrus and spice basils with low mean BDM incidence had leaves that were nearly flat or curved upward. This suggests that leaf curvature may have a greater impact on sweet basil susceptibility and that other factors contribute to the plant's relative susceptibility. These leaf characteristics contributing to reduced incidence may be useful in breeding for BDM resistance.