Measurements of aerial spore load by qPCR facilitates lettuce downy mildew risk advisement

Authors: DHAR, NIKHILESH - University Of California; MAMO, BULLO - University Of California; SUBBARAO, KRISHNA - University Of California; KOIKE, STEVEN - University Of California - Cooperative Extension Service; FOX, ALAN - Fox Weather, Llc; Anchieta, Amy; Klosterman, Steven

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/29/2019
Publication Date: 8/1/2019
Citation: Dhar, N., Mamo, B.E., Subbarao, K.V., Koike, S.T., Fox, A., Anchieta, A.G., Klosterman, S.J. 2019. Measurements of aerial spore load by qPCR facilitates lettuce downy mildew risk advisement. Plant Disease. 104(1):82-93. https://doi.org/10.1094/PDIS-03-19-0441-RE.
DOI: https://doi.org/10.1094/PDIS-03-19-0441-RE

Interpretive Summary: Downy mildew is one of the most destructive diseases of lettuce and is caused by the microbe Bremia lactucae. The diseased leaves show chlorotic symptoms that render them unmarketable and also white, downy spores under the leaves. These spores become airborne and disseminate the pathogen. A very effective approach to control lettuce downy mildew in conventional production relies on the use of fungicides. However, fungicides are often applied in weekly to ten-day intervals without knowing whether the pathogen is present or at what levels. Such applications can lead to fungicide insensitivity by the pathogen. In this study we deployed a spore trap system to measure airborne B. lactucae spore loads indirectly by DNA levels of the pathogen within three commercial fields that each contained experimental plots. Subsequently, we advised whether or not to apply fungicides on a weekly basis within the three experimental plots. This approach allowed the savings of approximately 1.7 sprays over the course of these field experiments. Reduction of fungicide applications for management of B. lactucae may decrease lettuce production costs for the growers while slowing down the development of fungicide resistance in the pathogen.

Technical Abstract: The lettuce downy mildew pathogen, Bremia lactucae, is an obligate oomycete that causes extensive produce losses in lettuce. Initial chlorotic symptoms that severely reduce the market value of the produce are followed by the appearance of white, downy sporulation on the abaxial side of the leaves. These spores become airborne and disseminate the pathogen. Due to the heterogeneity and quick adaptation of this pathogen in the field, an effective approach to contain lettuce downy mildew is through repeated application of fungicides to prevent outbreaks. However, in addition to direct economic costs, repeated application of fungicides leads to the development of fungicide-insensitivity in the pathogen. We therefore deployed a quantitative PCR assay-based detection method using a species-specific DNA target for B. lactucae coupled with a spore trap system to measure airborne B. lactucae spore loads within three commercial fields that each contained experimental plots. Based upon these measurements, we advised whether or not to apply fungicides on a weekly basis within the three experimental plots. This approach allowed the savings of approximately 1.7 sprays over the course of these field experiments. The reduction of fungicide applications to manage B. lactucae can decrease lettuce production costs for the growers while slowing down the development of fungicide resistance in the pathogen.