1. Kroese, Duncan1, I. Zasada2, N. Grünwald2, J. Weiland2.
Spatial Distribution of Phytophthora rubi and Pratylenchus penetrans in Northwest Red Raspberry Fields
Kroese, Duncan1, I. Zasada2, N. Grünwald2, J. Weiland2.
1Department of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97330
2USDA-ARS Horticultural Crops Research Laboratory, 3420 NW Orchard Ave., Corvallis, OR 97330
INTRODUCTION
RESULTS
The red raspberry industry produces over $63 million annually in the Pacific Northwest, with Washington being the leading producer of processed red raspberries in the country. Phytophthora rubi, an oomycete that causes root rot, and Pratylenchus penetrans, a migratory endoparasitic nematode, are two of the most economically important pathogens in the industry, causing symptoms that can reduce establishment, fruit yield, and quality (Figures 1, 2).
Primary control of these pathogens has been through the use of soil fumigation, fungicides, and nematicides. However, due to health and environmental concerns, these control methods have become increasingly restricted. The objective of this study was to determine the spatial distribution of these pathogens within individual fields. Results from this research may provide useful information to direct and improve management strategies.
Of the 125 total samples collected for one field, 81.6% were positive for the presence of P. rubi DNA and 39.5% were positive for the presence of P. penetrans.
Visual disease ratings of raspberry plants were negatively correlated with relative field elevation (R2 = , p-value = 0.009; Figure 3A).
Phytophthora rubi DNA concentrations showed no correlation with relative field elevation due to large variance (R2 = , p-value = 0.498; Figure 3B).
Pratylenchus penetrans population densities showed weak evidence of being positively correlated with relative field elevation (R2 = , p-value = 0.078; Figure 3C).
No correlation was found between P. rubi , P. penetrans, or visual disease rating with soil texture. This was due to the homogeneity of soil texture within the field (sandy loam).
Figure 1. A, B: Effect of Pratylenchus penetrans on growth of red raspberry. A: Fumigated control plot. B: Non-fumigated plot with P. penetrans present.
Figure 2. Effect of Phytophthora rubi on growth of red raspberry (non-inoculated controls on left).
Figure 4. Interpolated visualizations of the sampled area. A: Relative field elevations. B: Relative P. rubi DNA concentration. C: Relative severity of root rot symptoms. A B C A B N N N
Of the two pathogens, P. rubi has been found to be the more aggressive and devastating. Root rot symptoms caused by P. rubi have been observed to be distributed in clusters within a field. These disease clusters are typically found in lower areas of the field where soils may have higher relative moisture, and where water can more easily gather in standing pools. These conditions are not only favorable for the growth and spread of the pathogen, but can be detrimental to the plant due to unfavorable growth conditions.
This research shows that P. rubi is not only located in areas of high disease, but throughout the field. This may indicate that plants are able to tolerate some amount of pathogen presence, but due to elevated moisture, slightly increased levels of the pathogen, and weakened plants, lower areas of the field are more likely to show symptoms of disease.
A possible explanation for the decrease in P. penetrans population densities with relative elevation may be due to lack of fine root material. Pratylenchus penetrans tend to feed and reside in fine roots. If lower areas are more likely to show symptoms of root rot (one of which is decreased fine root mass), nematodes may be less likely to populate these area.
While more research needs to be preformed, these initial findings provide ideas for potential management actions for these two pathogens, including alteration of a field to create a more uniform topography or measures to increase soil drainage.
CONCLUSIONS
MATERIALS AND METHODS
Four red raspberry ‘Meeker’ (Rubus idaeus L.) fields (4 ha, two each in Oregon and Washington) were sampled. Soil and root samples were collected using two sampling patterns: 1) a uniform grid and 2) a localized concentric, focal sampling scheme. A visual plant disease rating, GPS location, and soil textural data were also obtained for each sampling site. Pratylenchus penetrans was extracted from both soil and roots and the presence of P. rubi was determined from roots with both a greenhouse bioassay and real-time PCR. Phytophthora rubi, P. penetrans, and visual disease rating were further analyzed using linear regression for correlations with site elevation and soil texture. Using GPS data and ArcGIS software, maps were created to visualize the distribution of the pathogens. To date, analyses have only been performed from one field in Washington. Analyses from other field sites are pending.
Figure 3. A: Mean visual disease rating compared to relative field elevation (higher visual disease rating values correspond to more severe symptoms). B: Mean log P. rubi DNA concentrations compared to relative field elevation. C: Mean P. penetrans densities compared to relative field elevation. (Error bars indicate standard error for all figures.) A B
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