Resurgence of Cucurbit Downy Mildew in the United States: A Watershed Event for Research and Extension

Abstract

In 2004, an outbreak of cucurbit downy mildew (CDM) caused by the oomycete Pseudoperonospora cubensis (Berk. & M. A. Curtis) Rostovzev resulted in an epidemic that stunned the cucumber (Cucumis sativus L.) industry in the eastern United States. The disease affects all major cucurbit crops, including cucumber, muskmelon, squashes, and watermelon. Although the 2004 epidemic began in North Carolina, the cucumber crop from Florida to the northern growing regions in the United States was devastated, resulting in complete crop loss in several areas. Many cucumber fields were abandoned prior to harvest. The rapid spread of the disease coupled with the failure of fungicide control programs surprised growers, crop consultants, and extension specialists. The epidemic raised several fundamental questions about the potential causes for the resurgence of the disease. Some of these questions revolved around whether the epidemic would recur in subsequent years and the possible roles that changes in the host, pathogen, and environment may have played in the epidemic.

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

“…Second, disease outbreaks in the Great Lakes and mid-Atlantic regions may be due to the spread of inoculum from disease outbreaks around North Carolina rather than from overwintering sites in the south. It is also possible that disease outbreaks in the Great Lakes region could be due to other inoculum sources, such as greenhouses within the region (47). Spatiotemporal studies also indicate that a risk window is associated with CDM outbreaks in the eastern United States.…”

“…cubensis possesses a high degree of pathogenic and genetic plasticity. Reported hosts of the pathogen span 20 genera, with the genera Cucumis, Cucurbita, Citrullus, Lagenaria, Benincasa, and Luffa being among the most economically important (47,89,95,116). Specificity among host species varies within P. cubensis, and pathotypes have been defined to distinguish host preference (59,107,108).…”

“…This suggests that host matrix is an important factor in oospore formation (19 (19,107) in which the A1 mating type was observed to be exclusively associated with cucumber, whereas the A2 with other cucurbits support the role of host matrix in oospore formation. It is plausible that the A1 mating type may also have been responsible for the resurgence of CDM in the United States in 2004 given that epidemics during the resurgence were primarily associated with the breakdown of resistance in cucumber (46,47). The A2 mating type has been reported in China (21) and the United States (106).…”

“…Late season plantings can be exposed to inoculum at the cotyledon stage and a delay in fungicide application can result in complete crop loss (47). Growers must continuously weigh the threat of sporangia influx into their region, assess weather conditions suitable for disease development, and estimate the cost of fungicide sprays.…”

“…Consequently, much has been learned about the ecology and biology of P. cubensis and the epidemiology of CDM. This chapter builds on recent reviews (47,57,97) and describes our current understanding of the pathogen and the disease as well as directions for future research efforts. More broadly, we describe aspects of the population biology and ecology of the pathogen and dynamics of CDM at different temporal and spatial scales that can serve as a framework to advance concepts and management for other downy mildew diseases.…”