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

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.