PCR detection of Pseudoperonospora humuli in air samples from hop yards

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 1 fg of genomic DNA of P. humuli, and yielded an amplicon in 70% of reactions when DNA was extracted from single sporangia. 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 4AE5 days before (range )8 to 14 days) the first appearance of basal spikes in six commercial yards, or 1AE3 days after (range )5 to 1 days) sporangia were detected in a volumetric spore sampler in experimental plots. In commercial yards, use of 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 hop downy mildew management can be improved when control measures are timed according to first detection of inoculum.

Phylogenetic investigations in the genus Pseudoperonospora reveal overlooked species and cryptic diversity in the P. cubensis species cluster

“…In 2002 the disease re-emerged in Argentina and the reduction in hop cone yield of infested fields varied between 20% and 34% (Pérez et al 2009). But due to advances in forecasting and disease management, hop downy mildew control seems feasible (Mozny et al 1993;Dolinar and Žolnir 1994;Gent et al 2009). …”

Epidemiology and Population Biology of Pseudoperonospora cubensis: A Model System for Management of Downy Mildews

“…Efforts to sequence more strains from diverse cucurbit hosts and the sister species P. humuli are underway (120) and will provide a rich resource to address questions about host specificity, adaptation, and breeding for durable resistance. An immediate outcome of this work has been the identification of genetic markers conserved in the species that may be useful in diagnostic assays and inoculum detection systems (33,120). Preliminary analyses indicate bifurcation of P. cubensis and P. humuli based on mitochondrial DNA and transcriptome data from their primary hosts, providing further evidence of population differentiation by host (87,88,107).…”

Detection of Fungal Pathogens in the Environment

“…First fungicide application based on the PCR detection of P. humuli enhanced the effectiveness of disease control or reduced the fungicide use in four of the 6 yards compared to growers' standard practices. Diagnosis of P. humuli using the PCR assay may enhance the efficiency and efficacy of hop downy mildew disease management (Gent et al 2009). …”