Research Update: Root Inoculum Removal as a Tool to Improve Soilborne Disease Management in Red Raspberry

L.W. DeVetter1, I. Zasada2, J. Weiland2, T. Walters3, R. Rudolph4, and S. Watkinson5

1Assistant Professor, Small Fruit Horticulture, Washington State University Northwest Research Extension Center (WSU-NWREC) in Mt. Vernon, WA

2Research Plant Pathologists, United States Department of Agriculture Horticultural Crops Research Unit in Corvallis, OR

3Agricultural Researcher, Walters Ag Research, Anacortes, WA

4Graduate Student and 5Scientific Technician, WSU-NWREC

One of the primary goals of the WSU Small Fruit Horticulture program and collaborating scientists is to develop improved tools for soilborne disease management for red raspberry systems in the Pacific Northwest (PNW).  To achieve this, we have begun investigating a wide variety of techniques that could have application in this endeavor.  The potential tools we have begun investigating are diverse and include alternative fumigants and methods of fumigant application, targeted selection of cover crop species that could have disease suppressive properties, methods of cover crop management, biofumigation, and root inoculum removal.  The focus of this newsletter article is to provide an update on one of these projects initiated in August of 2014 focusing on root removal as a pre-plant method to improve management of soilborne diseases.


The concept for this project emerged when it was observed that large volumes of raspberry root and crown material remain in fields being replanted to raspberry (Fig. 1).  Although these fields are typically fumigated, many fumigants are not labeled to penetrate tissue larger than 5 millimeters in diameter.  Much of the plant material remaining in raspberry fields exceeds this size and could serve as a source of inoculum in replanted fields, thereby promoting disease.  The primary objective of this project is to demonstrate and evaluate the efficacy of raspberry root inoculum removal as a pre-plant management technique for reducing soilborne pathogen and pest populations.  Long term, this project will contribute to increasing our knowledge of disease-causing organisms and how to effectively manage them. 


This project involved two experiments.  In Experiment 1, we compared three root removal devices for speed and efficacy of root removal in commercial raspberry fields in Whatcom County, WA.  Devices tested includes: Lundby plant lifter, beach cleaner, and potato harvester (Fig. 2).


Experiment 2 focused more closely on the effects of root removal on soilborne diseases and plant development.  To evaluate this, we established a split-split plot experiment in a commercial ‘Chemainus’ field being re-planted to ‘Meeker’ raspberry in Whatcom County, WA.  The main plot factor was fumigation (with or without, using Telone ® C-35) and the split plot factor was root removal (with or without removal using a Lundby plant lifter), replicated six times.  Main plots were 100 x 30 ft and the splitFigure 1. Residual root and crown material in a field to be replanted with raspberry prior to soil-incorporation and fall fumigation. plots 50 x 30 ft in size.  Data collected includes: changes in soilborne disease [Fusarium and Pythium, proxies for Phytophthora rubi (causal agent of Phytophthora root rot)] and root lesion nematode (Pratylenchus penetrans) populations, plant growth, and yield. This experiment will continue until 2018.  An additional component of this project is to look at the economic impact of root removal with Ms. Suzette Galinato of the WSU School of Economic Sciences.  Work on this aspect of the project is ongoing.






Preliminary Results and Discussion

Experiment 1.

The percent of plant material removed was 91%, 96%, and 98% for the beach cleaner, potato harvester, and plant lifter, respectively.  This was estimated by excavating 4 ft3 of soil after one pass by each device and weighing the remaining plant material in the soil.  The average speed of the plant lifter, beach cleaner, and potato harvester were 0.25 mph, 0.37 mph, and 1.0 mph, respectively.  Thus, the plant lifter removed the most plant material, but was the slowest among all devices evaluated.


Experiment 2.

Monthly samples of raspberry roots were collected and used to monitor changes in populations of root lesion nematodes across all treatments starting in March of 2015 (Fig. 3).  Root lesion nematode populations were negligible in March and April.  Populations began to increase in May, with the combined fumigation plus root removal treatment (Fum – Roots) having the lowest population across all treatments in May and June.  The fumigation with roots (Fum + Roots) had the lowest populations in July.  Data suggests that the combined effects of fumigation and root removal is reducing populations of root lesion nematodes, particularly in May and June.






Soil populations of Fusarium and Pythium were monitored immediately following treatment implementation, six weeks after fumigation, and in Feb. of 2015.  Populations of Fusarium and Pythium were reduced in both root removal treatments (with and without fumigation) six weeks after fumigation, but were similar across all treatments by Feb. 2015 (data not presented).  Although not statistically significant, the combined treatment of fumigation plus root removal had the lowest number of these pathogens.  Colonization by Fusarium and Pythium in large root material >5 millimeters was reduced by fumigation, but were not different by Feb. 2015 (data not presented).  These data demonstrate that even in a fumigated field setting, large plant material remaining in replanted fields serves as a source of inoculum for these pathogens.  This suggests plant material remaining in replanted fields may also be a source of inoculum for another major disease-causing organism of raspberry, P. rubi, which causes Phytophthora root rot.


Raspberry root material that was removed from the field site of Experiment 2 was composted at Smit’s Compost, a local composting facility in Lynden, WA.  Screening of the composted material for Fusarium and Pythium revealed composting was effective at reducing these organisms to undetectable levels.   A greenhouse bioassay utilizing this composted material is currently underway and results on how composting effects root lesion nematodes in greenhouse-grown raspberry (tissue-cultured ‘Tulameen’) are pending.


Cane number and heights were measured in the replanted field in early July of 2015.  No statistical differences in cane number nor heights have currently been observed (data not presented).  However, treatment differences may become clearer over time as the treatments and plants become established.


Conclusions to Date

This project is in its early stages.  However, some of the preliminary results indicate root material remaining in replanted fields serves as a source of disease inoculum.  Moreover, preliminary data suggests that the combined effects of fumigation and root removal are reducing populations of disease-causing organisms.  A better understanding of the effects of root removal on disease suppression potential and plant performance will be determined as the plants become established and additional results emerge as this study continues.










Portions of this project was funded by the Northwest Center for Small Fruits Research.  Special thanks to Jon Maberry of Maberry Packing, LLC, fellow grower cooperators, Smit’s Compost, and Tim Purcell of Trident Agricultural Products for project assistance.  Acknowledgements also go to Rachel Weber for assistance with field work.

Figure 2. Three root removal devices tested in 2014, including: A) potato harvester; B) Lundby plant lifter; and C) beach cleaner. Both the potato harvester and plant lifter were locally available, whereas the beach cleaner was purchased by a local grower.

Figure 3. Changes in root lesion nematode populations over time, 2015.  Axes symbols: Pp = P. penetrans; Fum + Roots = fumigation with roots; Fum – Roots = Fumigation with root removal; Nonfum + Roots = No fumigation with roots; Nonfum – Roots = No fumigation without roots.  Figure courtesy of I. Zasada.