Dynamics of Pseudoperonospora cubensis Sporangia in Commercial Cucurbit Fields in Michigan

Abstract

Granke, L. L.. and Hausbeck, M. K, 2011, Dynamics of Pseudoperonospora cubensis sporangia in commercial cucurbit fíelds in Michigan. Plant Dis. 95:1392-1400.Major outbreaks of cucurbit downy mildew have been observed since 200.5 in Michigan, the top state for pickling cucumber production in the United States. Airborne concentrations of Pseudoperotwspora ctibensis sporangia were monitored from 2006 to 2009 in a commercial cucurbit field in each of five Michigan counties. Temperature, relative humidity, leaf wetness, and rainfall were recorded on site in 2008 and 2(X)9. Seasonal and daily trends for sporangial concentrations were examined using loess regression to determine when sporangia were present throughout the growing season. Airborne sporangia were detected from June through September, and concentrations increased during this period. Peak airborne sporangial concentrations occurred between 8:00 and 13:00 h. Airborne sporangial concentrations were positively correlated with temperature (/• = 0,17 to 0.38, P < O.O(X)1) and negatively correlated with relative humidity (r = -0.16 to -0.45, P < 0.0046) and leaf wetness (r = -0,14 to -0,39, P < (),(X)OI) in 2008 and 2009 at all sites. In autoregressive moving average models, sporangial concentrations were negatively related to relative humidity (slope = -0.03 to -1.34, P < 0.0496) at three of five sites in 2(H)8 and four of five sites in 2009. Hence, sporangia are likely to be airborne above the crop canopy during periods with high temperatures and low relative humidity and leaf wetness.

Relationships Between Airborne Pseudoperonospora cubensis Sporangia, Environmental Conditions, and Cucumber Downy Mildew Severity

“…Spores were detected on 70 to 80% of days sampled in our study, which was within the range of days (42 to 100%) during which spores were detected in the previous Michigan study in commercial, managed fields (9). Spores were detected on 70 to 80% of days sampled in our study, which was within the range of days (42 to 100%) during which spores were detected in the previous Michigan study in commercial, managed fields (9).…”

“…A previous spore trapping study in commercial cucurbit fields in Michigan also found that spores were trapped within 4 days of spore sampling initiation when sampling started in June (9). In this study, airborne P. cubensis sporangia were detected in mid-May, prior to cucumber being planted in the field and within 5 days of spore trapping initiation.…”

“…P. cubensis is spread field-to-field via windbome sporangia, which may be transported long distances (up to 1,000 km; 19). The number of airborne sporangia observed above a field varies based on time of day, distance from the canopy, environmental conditions, and disease severity within the field (9,16,22). Airborne sporangium concentrations are thought to be the most important biofic factor determining disease onset (3).…”

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

“…Typical of most downy mildew organisms, aerial concentration of P. cubensis spores above the crop canopy exhibits a diurnal periodicity (Figure 3) with peak concentrations occurring between 0800 and 1000 h (36,70). The pathogen is a prolific producer of spores and can produce up to 2 × 10 8 spores/ha in a day (70).…”

“…The structure and information content of disease hazard warnings issued by the CDM ipmPIPE are characteristic of predictive systems that typically have sustained use in disease management (32). At the field scale, direct detection and estimation of airborne inoculum have also found a role in disease hazard warnings and scheduling of fungicide applications (33,36,37). In Michigan, sporangia characteristic of P. cubensis have been detected within days of field plantings (37).…”