Graduate Thesis Or Dissertation
 

Distribution of Phytophthora rubi and Pratylenchus penetrans in Pacific Northwest Red Raspberry (Rubus idaeus) Fields

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  • The Pacific Northwest produces over 95% of the nation’s processed red raspberries, valued at over $65 million. Two of the major soilborne pathogens effecting the production of raspberry in this region are the oomycete Phytophthora rubi, which causes root rot in red raspberry plants, and the plant-parasitic nematode Pratylenchus penetrans, also known as root lesion nematode. Current management of these two pathogens relies heavily on pre-plant soil fumigation. However, current regulations have made this practice more difficult and expensive. Additional issue with soil fumigation include lack of efficacy at deeper soil depths and an inability to penetrate old raspberry root material which remains in a field at the time of fumigation and may harbor soilborne pathogens. Phytophthora rubi can be effectively managed with application of post-plant fungicides, but few options exist for the post-plant management of P. penetrans. Knowledge of when or where these soilborne pathogens are present in the raspberry production system could provide valuable information for growers to target management practices. The objectives of this study were to 1) determine the residency time of P. penetrans in red raspberry roots, 2) determine the vertical distribution of P. penetrans in the soil at three different times within the re-planting process, and 3) determine the horizontal distribution within red raspberry fields of P. rubi and any factors that may be correlated with P. rubi incidence. For the first objective, P. penetrans infected raspberry roots were buried in the ground and nematode populations within the roots were monitored over time. To achieve the second objective, soil cores down to 90 cm were collected at three different times in two commercial fields: pre-fumigation, post-fumigation and at planting. Pratylenchus penetrans soil populations were then monitored at each time. For the final objective, an intensive sampling strategy was employed in four commercial red raspberry fields in the Pacific Northwest with P. rubi DNA concentrations and visual disease rating, P. penetrans root and soil populations, soil texture and GPS data, including elevation data being collected. The results for P. penetrans root residency time demonstrated that P. penetrans can reside in roots from 6 to 8 months after plant removal. This indicates that P. penetrans has the ability to survive in root material up to and after the time when fumigation typically occurs. The results for the vertical distribution showed that P. penetrans was present at all sampling dates in both fields and nematodes survived in different areas of the soil profile depending upon soil type. Finally, the results from the horizontal distribution of P. rubi indicated that the pathogen is distributed throughout a field. While there were factors (elevations, soil texture, visual disease rating, or nematode population densities) that were related to P. rubi concentrations within a field, no universal relationship appeared between P. rubi DNA concentrations and measured factors between fields. This research shows that P. penetrans are not only surviving fumigation, but also provides potential answers to how and where they are surviving. The research also demonstrates that P. rubi is typically distributed throughout a field and its presence may be dependent on certain factors within a field, but no universal factors were related to P. rubi concentrations between fields. With this information, the knowledge base of the distribution dynamics of these two important red raspberry soilborne pathogens has been increased. Along with future research, this information has the potential to help growers target management practices and more effectively control these pathogens.
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