Morphological Characteristics and Susceptibility of Basil Species and Cultivars to Peronospora belbahrii

Abstract

<jats:p>Sweet basil (<jats:italic>Ocimum basilicum</jats:italic>) is the most economically important culinary herb in the United States. In 2007, a new disease, basil downy mildew (BDM), caused by the oomycete pathogen <jats:italic>Peronospora belbahrii</jats:italic>, was introduced into the United States and has since caused significant losses in commercial basil production. Although no commercial sweet basils available are resistant to <jats:italic>P. belbahrii</jats:italic>, other species of <jats:italic>Ocimum</jats:italic> have exhibited potential tolerance, resistance, or both. The objectives of this work were to determine if leaf morphological characteristics including stomata density and leaf curvature correlated with infection of plants by <jats:italic>P. belbahrii</jats:italic>, and thus could be used as selected characters in plant breeding. In 2011, 20 <jats:italic>Ocimum</jats:italic> cultivars including sweet (<jats:italic>O. basilicum</jats:italic>), cinnamon (<jats:italic>O. basilicum</jats:italic>), clove (<jats:italic>O. basilicum</jats:italic>), citrus (<jats:italic>Ocimum</jats:italic> ×<jats:italic>africanum</jats:italic> syn. <jats:italic>Ocimum citriodorum</jats:italic>), spice (<jats:italic>Ocimum americanum</jats:italic> syn. <jats:italic>Ocimum canum</jats:italic>), and holy basils (<jats:italic>Ocimum tenuiflorum</jats:italic> syn. <jats:italic>Ocimum sanctum</jats:italic>) were evaluated for susceptibility to downy mildew. 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 downy mildew. Using those same 20 <jats:italic>Ocimum</jats:italic> species and cultivars, stomata length and density and leaf curvature were measured and correlated with downy mildew incidence and severity. In general, basil species with higher stomatal densities had higher downy mildew incidence and severity. High stomatal densities were mainly found in the sweet, cinnamon, and clove basils. Citrus and spice species with longer stomatal lengths generally exhibited lower downy mildew incidence. Holy basil, the least susceptible of all <jats:italic>Ocimum</jats:italic> sp. to <jats:italic>P. belbahrii</jats:italic> evaluated in this study, had the greatest stomatal density and shortest stomatal length. Some sweet basil cultivars with the highest downy mildew incidence also had the greatest downward leaf curvature, whereas other sweet basil cultivars with moderate downy mildew incidence had leaves that were nearly flat or curved upward. Holy, citrus, and spice basils with low downy mildew incidence had leaves that were nearly flat or curved upward. This study suggests that leaf curvature and stomatal density and length affect downy mildew development and sporulation. Considerations of these leaf morphological characteristics may be useful phenotypic traits in breeding for downy mildew resistance in <jats:italic>Ocimum</jats:italic>.</jats:p>

Characterization of the infection process by Peronospora belbahrii on basil by scanning electron microscopy

“…(2013) or Cohen et al. (2017), additional studies will be needed to ascertain if environmental conditions dictate the mode of host penetration utilized by P. belbahrii and, concomitantly, to determine if reduced stomatal number is a phenotype to pursue in breeding for basil lines resistant to downy mildew (Homa et al., 2016).…”

CRISPR-Editing of Sweet Basil (Ocimum basilicum L.) Homoserine Kinase Gene for Improved Downy Mildew Disease Resistance

“…Sweet basil (Ocimum basilicum L.) is an important culinary herb and source of aromatic essential oils. Of all the Ocimum species, O. basilicum is the most susceptible to downy mildew (DM) disease, which is caused by the obligate Oomycete pathogen Peronospora belbahrii (Wyenandt et al, 2010 ; Homa et al, 2016 ). P. belbahrii , first reported in Uganda in the 1930s, was distinguished from other Peronospora species in 2005, and has become a global problem wherever sweet basil is grown (Belbahri et al, 2005 ; Thines et al, 2009 ; Wyenandt et al, 2010 ; Cohen et al, 2017 ).…”

Comparative Morpho-micrometric Investigations in Six Indigenous Ocimum Species of India with DOE Based HPTLC Method for Multi-class Component Analysis

“…4 a – f . Stomata length, density, and leaf curvature were recorded in all these species except O. kilimandscharicum and O. filamentosum 31 , 35 . The petiole of the species also shows similar tissue arrangement in general, having a single-layered epidermis followed by 1 – 2 layers of collenchymatous hypodermis.…”