|Abstract or Summary
- Podosphaera macularis causes one of the most important diseases, powdery mildew, of Humulus lupulus (hop). If left unmanaged, hop powdery mildew can cause total crop loss due to disease or browning of hop cones rendering the cones unmarketable to buyers. The Hop Powdery Mildew Infection Risk Index (HOPS) is heavily relied on for management of the disease. However, HOPS assumes constant inoculum presence, which is not likely to be true. Evidence suggests that temperature fluctuations influence inoculum availability. As such, knowledge of inoculum presence and density and their relationship to environmental conditions may increase management efficacy by reducing pesticide use and cost of crop production.
Molecular methods for detecting and quantifying P. macularis were developed and used to analyze air samples taken from hop fields for two seasons. A protocol for DNA extraction from field samples was modified in order to use PCR to detect and quantify P. macularis DNA from the field. Primers and a probe designed to the internal
transcribed spacer (ITS) region of P. macularis ribosomal DNA have made inoculum detection possible using both conventional polymerase chain reaction (PCR) and real-time quantitative PCR (qPCR). DNA extracted from conidia impinged on glass rods coated in silicon grease has been used as template for qPCR, and we have successfully detected as DNA extracted from 10 spores Specific and sensitive detection of P. macularis DNA with conventional PCR was possible under conditions typically used to evaluate qPCR; however results from field samples indicated that sensitivity was an issue, in that spores were not detected in samples from fields with high disease pressure.
The effects of constant exposure (5, 10, 15, 20, 25, 30, and 35°C) and 6 hr exposure (18, 22, 26, 30, 34, and 38°C) on sporulation of P. macularis were tested. Exposure to constant low and high temperatures decreases sporulation, which indicates that inoculum may not always be available once the epidemic has started. Sporulation is also decreased during brief exposures to temperatures above 30°C. These data indicate that inoculum availability is reduced when the temperature exceeds 30°C in the field. A modification of HOPS should be examined to incorporate an algorithm which takes the findings in this study into account.