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

Authors: Zhang, Guirong; Thompson, Arthur - Art; Schisler, David; Johnson, Eric

Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/2/2019
Publication Date: 1/31/2019
Citation: Zhang, G., Thompson, A.R., Schisler, D.A., Johnson, E.T. 2019. Characterization of infection through sporulation by Peronospora belbahrii on basil using light and scanning electron microscopy. Plant Pathology. 5:e01117. https://doi.org/10.1016/j.heliyon.2019.e01117.
DOI: https://doi.org/10.1016/j.heliyon.2019.e01117

Interpretive Summary: Downy mildew caused by Peronospora belbahrii is a major yield-limiting disease of sweet basil (O. basilicum) production worldwide. Traditional methods for reducing plant disease such as the application of fungicides and breeding for host resistance have had limited success in reducing downy mildew on basil. Documenting how the pathogen infects basil plants and then produces spores would help scientists to know when in the pathogen’s life cycle it is most vulnerable to control measures. ARS scientists in Peoria, Illinois found that spores of the pathogen on basil leaves germinated in 3 days and hyphae from germinated spores directly penetrated leaves soon after, rather than growing through natural openings in leaves. After 7 more days, spore producing reproductive structures of the pathogen formed on both the bottom and top of young leaves which helps explain how the pathogen spreads rapidly from plant to plant. These results contribute to a better understanding of the infection process and spread of P. belbahrii and should contribute to the development of more effective measures for reducing the severity of basil downy mildew.

Technical 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 e'ective measures for reducing basil downy mildew.