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

Abstract

Basil downy mildew caused by Peronospora belbahrii is a disease of sweet basil (Ocimum basilicum) production worldwide. In this study, sweet basil was grown in plant growth chambers and inoculated with sporangia of P. belbahrii harvested from previously infected plants. Plants were placed in closed, clear plastic bags and leaves harvested over time and observed using scanning electron microscopy. In most cases, sporangia germinated myceliogenically on abaxial and adaxial leaf surfaces as early as three days after inoculation. Germ tubes and the tips of hyphae ramifying on leaf surfaces directly penetrated basil leaves to initiate the infection process. Hyphal growth was not observed to gain entrance to the interior of leaves through stomata, though growth over these openings was observed. Most frequently, seven days after inoculation, one or more sporangiophores grew through stomata to produce new sporangia on both the abaxial and adaxial surfaces of leaves. Macroscopic signs of infection were visible on both sides of leaves approximately ten days after inoculation under the conditions of this study. These results contribute to a better understanding of the infection process and disease onset of P. belbahrii and should help in the development of more effective measures for reducing basil downy mildew.

CRISPR/Cas9-mediated mutagenesis of sweet basil candidate susceptibility gene ObDMR6 enhances downy mildew resistance

“…Basil downy mildew (BDM) caused by the obligate biotrophic oomycete Peronospora belbahrii threatens the global production of sweet basil [ 5 ]. This devastating foliar disease can cause damage at nearly all growth stages of sweet basil [ 6 ]. BDM is characterized as a dark-hued mat observed on the abaxial surface of infected leaves caused by the emergence of sporangiophores and sporangia [ 5 , 7 ].…”

Dual transcriptional analysis of Peronospora belbahrii and Ocimum basilicum in susceptible interactions

“…Seeds of Genovese basil (Ocimum basilicum, Johnny's Selected Seeds, Winslow, Maine) were sown into seven punnets (13 x 13 cm) containing pasteurized soil (Sunshine Propagation Mix, Sungrow Horticulture, Canada) on a weekly basis. The punnets were placed into flat trays that were filled with water as needed and incubated in a growth chamber as previously described (ZHANG et al 2019). Liquid fertilizer (around 500 ml of Peter's fertilizer at 1 g/l) was added to a plant tray five-seven days prior to harvest of leaves.…”

“…P. belbahrii requires several hours of leaf wetness for infection (GARIBALDI et al 2007; COHEN AND BEN-NAIM 2016). The sporangia produce a single germ tube on the leaf surface that forms an appressorium which penetrates the epidermis (ZHANG et al 2019) but other laboratories have noted appressorium penetration in the stomatal opening (KOROCH et al 2013;COHEN et al 2017). It is unknown if P. belbahrii germ tubes can absorb nutrients; if not, then growth would be limited to the energy reserves in the sporangium.…”

Downy Mildew of Basil: A New Destructive Disease Worldwide

“…Discovery of oospores suggests the potential for sexual reproduction, but little is known on P. belbahrii oospore formation or is it homothallic or heterothallic. Currently, only one mating type has been found [22], although it is already presumed that it is heterothallic [26,36,37]. The pathogenicity of oospores is investigated, but without positive infections [30,38], and their role in the basil downy mildew epidemiology is not known.…”

“…Conidia germinate in 3-5 days into one or two germ tubes and infect plant tissues via a germ-tube which penetrates through leaf stomata [28,36] and it takes 3 h [35]. Germ tubes rarely form an appressoria-like structures prior infection.…”