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United States Department of Agriculture

Agricultural Research Service

Research Project: GENOMICS, GERMPLASM DEVELOPMENT AND IPM OF HOP Title: PCR detection of Pseudoperonospora humuli in air samples from hop yards

Authors
item Gent, David
item Nelson, M -
item Farnsworth, J -
item Grove, G -

Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 7, 2009
Publication Date: August 6, 2009
Citation: Gent, D.H., Nelson, M.E., Farnsworth, J.L., Grove, G.G. 2009. PCR detection of Pseudoperonospora humuli in air samples from hop yards. Plant Pathology. p. 1365-1369.

Interpretive Summary: Downy mildew, caused by Pseudoperonospora humuli, is one of the most destructive diseases of hop. A molecular assay was developed to detect airborne spores of the pathogen to optimize timing of control measures. During extensive field testing, the assay detected the pathogen in air no later than 8 days after the appearance of trace levels of disease signs and/or detection of airborne spores. Inoculum was detected on average 4.5 days before (range -8 to 14 days) the first appearance of disease in six commercial hop yards, or 1.3 days after (range -5 to 1 days) spores were detected in experiment plots. In commercial yards, use of the assay to initiate the first fungicide application led to enhanced disease control or a reduction in fungicide use in four of six yards compared to growers’ standard practices. These results indicate that the efficiency and effectiveness of disease management can be improved when control measures are timed according to first detection of inoculum using the tools developed in this research.

Technical Abstract: Downy mildew of hop, caused by Pseudoperonospora humuli, is an important disease in most regions of hop production and is managed largely with regular fungicide applications. A PCR assay specific to P. humuli and the related organism P. cubensis was developed and used to monitor airborne inoculum in hop yards to initiate fungicide applications. The PCR amplified as little as 1fg of genomic DNA of P. humuli, and yielded an amplicon in 70% of reactions when DNA was extracted from single sporangium. In the presence of 25 mg of soil, an amplicon was amplified in 90% of reactions when DNA was extracted from 10 or more sporangia. During nine location-years of validation, PCR detection of the pathogen in air samples occurred no later than 8 days after the appearance of trace levels of disease signs and/or detection of airborne spores in a volumetric spore sampler. Inoculum was detected on average 4.5 days before (range -8 to 14 days) the first appearance of basal spikes in six commercial yards, or 1.3 days after (range -5 to 1 days) sporangia were detected in a volumetric spore sampler in experiment plots. In commercial yards, use of the PCR to initiate the first fungicide application led to enhanced disease control or a reduction in fungicide use in four of six yards compared to growers’ standard practices. These results indicate that the efficiency and efficacy of downy mildew management can be improved when control measures are timed according to first detection of inoculum.

Last Modified: 12/24/2014
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