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RaspberryBy, Lisa DeVetter (Washington State University), Tom Walters (Walters Ag Research), and Inga Zasada (USDA-ARS)

 

Washington State leads national production of processed red raspberry with a single county (Whatcom) responsible for 97 percent of in-state production. Growers in Whatcom County produced just over 68 million pounds of fruit in 2017 with raspberry production contributing to the vibrancy of the rural economy in northwestern Washington. Despite the scale and economic impact of the raspberry industry, a microscopic nematode poses a significant threat to growers.  This nematode is Pratylenchus penetrans, more commonly referred to as Root Lesion Nematode (RLN).

Root lesion nematode is a migratory endoparasitic nematode that spends its life in both the soil and plant roots. It is inside those fine roots where it feeds and causes damage leading to reductions in plant productivity, and in extreme cases, plant death.

Raspberry growers have struggled to successfully manage RLN since the phase out of methyl bromide. Fumigant alternatives to methyl bromide have been inconsistent and sometimes ineffective at controlling RLN. Furthermore, post-plant management options are limited and have not been shown to be very effective across a wide range of situations. Viable solutions for RLN management were needed by the raspberry industry to protect their high-value crop from this destructive nematode.

In response to growers’ needs, researchers teamed up across institutions with the goal of generating information and data-driven management practices that would allow growers to successfully manage RLN. This research started approximately eight years ago and the information generated now allows growers to manage RLN based on knowledge of RLN biology, at-planting population densities, soil type, and fumigant chemistry and application methods.

 

Determine Population Densities

Growers can collect soil or root samples to get an estimate of RLN population densities in their field. This can be helpful in determining if densities are low, medium, or high, which will inform future management practices. Although soil samples are typically collected by growers, they can often underestimate population densities. Root samples are more reflective of population densities and they should be collected before the planting is removed for renovation to ensure the roots are alive and contain nematodes. Regardless of sample type, samples should be collected near the crown of the plant and to a depth of 12 to 18 inches. 

Action thresholds for plant-parasitic nematodes in perennial fruit crops are challenging to generate because perennial crops like raspberry can compensate for nematode parasitism depending on their internal physiological state and/or environmental conditions. However, McElroy (1992) generated guidelines for soil samples. These thresholds suggest management should ensue if pre-plant soil populations are >250 RLN/250 cm3 or post-plant populations are 500 to >2,000 RLN/250 cm3.

Importance of Soil Type

Raspberry is commonly grown on sandy loam to loamy sand soils. Soil type and proportion of sand has been shown to influence RLN distribution in the soil profile. Therefore, it is important to understand where nematodes reside in soil and how fumigants will behave in different soil types.

Research has shown RLN resides predominately in the top 18 inches in high sand content soils (70 percent sand), whereas they reside deeper in the soil profile with decreasing sand content (e.g. 50 percent and below) (Kroese et al., 2016). Furthermore, soil type influences fumigant efficacy. High sand content soils where RLN resides at more shallow depths respond better to shallow applications of fumigants. In this situation, shallow applications of Vapam (metam sodium) would be a good approach. For heavier soils where RLN resides deeper in the soil profile, deep-shank applications of 1,3-dichloropropnene (1,3-D) containing products are appropriate.

Compound Selection

There are only a few fumigant compounds that are registered and effective against RLN. One of the most widely used compounds is 1,3-D, which is a nematicide and is often mixed with chloropicrin that has fungicidal activity. A common 1,3-D containing fumigant that raspberry growers use is Telone® C-35 (65 percent 1,3-D:35 percent chloropicrin). 

Increasingly growers are using metam sodium (e.g. Vapam), which liberates methyl isothiocyanate when exposed to water. Methyl isothiocyanate is an organosulfur compound that has nematicidal activity. Metam sodium is less expensive and has smaller buffer zones than fumigant products that contain chloropicrin. It also has a broader spectrum of activity. Metam sodium can be injected at shallow depths, which can be a benefit or downfall of the application method depending on soil type and where RLN resides in soil.

A few other fumigant products are available to growers, including Basamid (another methyl isothiocyanate liberator) and Dominus (allyl isothiocyanate). These products have smaller buffer zones, but efficacy has been variable and likely attributed to limited soil mobility. 

Broadcast or Bed Fumigation?

Bed Fumigation 2 Credit DeVetter

 

Raspberry growers traditionally broadcast fumigated their fields. Now, however, an increasing number of acres are bed fumigated. Bed fumigation provides many benefits over broadcast fumigation and can improve RLN management. Less fumigant is applied on a per area basis because only raised beds are fumigated, which in turn reduces fumigant buffers. There is currently a global shortage of Telone, therefore, bed fumigation will allow for more acreage to be fumigated with Telone relative to broadcast fumigation.

Because alleyways are untreated with bed fumigation, growers were concerned that RLN may move from alleyways to their raised beds and subsequently infect newly planted raspberry plants. However, results from field trials have shown few RLN reside in alleyways and re-colonization of roots in raised beds is negligible (Walters et al., 2017). Furthermore, bed fumigation has outperformed broadcast fumigation in terms of RLN suppression in many trials.

One caveat with bed fumigation is that it may not be effective for soilborne diseases that are able to travel through water, such as Phytophthora root rot. This could be particularly problematic in fields that experience flooding. This potential scenario underscores that knowledge of the field site and its history can also be powerful in informing management decisions.

Tarps

Tarps are agricultural films that are totally impermeable or virtually impermeable (TIF and VIF, respectively). Tarps are strongly recommended for fields with high RLN population densities or a history of other soilborne diseases, as they enhance fumigant efficacy by retaining a fumigant in the soil longer. Tarps also qualify for buffer zone reduction credits and listed by active ingredient(s) (see https://www.epa.gov/soil-fumigants/tarps for more information). Research to date has underscored the value that tarps can have in reducing RLN population densities compared to untarped fumigation. Although tarps add cost to the expense of fumigation, the additional expenditure can be worthwhile.

Table or Registered Fumigants

Alternatives to Fumigants

Alternatives to chemical soil fumigants have been an active area of research for many cropping systems. In northwestern red raspberry, we have studied application of brassica seed meals, solarization, and different cover crop management practices. While some of these alternatives have worked in fields with low- to medium- RLN infestations, they do not have the reliability of soil fumigation. Additional research efforts are needed to find cost effective management strategies for RLN, which would benefit growers that would prefer not to fumigate their fields.

pc 4 head Credit Zasada

Pratylenchus Stylet 2 Credit ZasadaPlant Resistance

Our research has demonstrated that all of the raspberry cultivars commonly planted in northwestern Washington are susceptible to RLN (Zasada et al., 2015). This leaves growers with no options to use plant resistance to manage RLN.

Post-plant Management

Even though pre-plant is the most important time to implement RLN management practices, sometimes pre-plant management efforts are unsuccessful, and growers need additional tools to try to keep RLN in check. Unfortunately, it appears that many new nematicides are not effective against RLN in raspberry. Trials with Nimitz, Salibro, and Velum Prime applied to newly planted raspberry and in the year following planting demonstrated the inability of these compounds to reduce populations of RLN. While speculative, either RLN is protected from these contact nematicides by the root or these compounds are not toxic to RLN. However, Vydate (oxamyl) can be applied to non-bearing raspberry and has shown efficacy (Walters and Zasada, 2017). Efficacy is likely maximized in fields with low RLN densities, so fields with higher densities will need to consider other or combined RLN management approaches.

Research Goes on…

While collaborating scientists have made progress in better understanding RLN in the red raspberry productions system and generated data-driven recommendations, the research continues. The fumigant landscape continues to evolve and growers increasingly need more information on how to optimize management of this significant plant parasitic nematode. Collaborations between researchers and raspberry growers will continue to be imperative and allow problems to be addressed, helping to ensure the continued vibrancy of this important agricultural industry. 

A decision-aid tool has been generated that synthesizes information about nematode and soilborne disease management for raspberry. It may be accessed at:  http://www.nwberryfoundation.org/SFU/2017/September%20Special%20Edition%20SFU.pdf

Disclaimer statement 

For any of the pesticides mentioned in this article, be sure to read the label for specific rates and application recommendations.

References:

Kroese, D.R., J.E. Weiland, and I.A. Zasada. 2016. Distribution and longevity of Pratylenchus penetrans in the red raspberry production system. Journal of Nematology 48(4):241-24.

McElroy, F.D. 1992. A plant health care program for brambles in the Pacific Northwest. Journal of Nematology 24(3):457-462.

Walters, T.W., M. Bolda, and I.A. Zasada. 2017. Alternatives to current fumigation practices in western states raspberry. Plant Health Progress 18:104-111.

Zasada, I.A. and T.W. Walters. 2016. Effect of application timing of Oxamyl in nonbearing raspberry for Pratylenchus penetrans management. Journal of Nematology 48:177-182.

Zasada, I.A., J.E. Weiland, Z. Han, Z., T.W. Waters, and P. Moore. 2015. Impact of Pratylenchus penetrans on establishment of red raspberry. Plant Disease 99:939-946.