Evaluating Soil Fumigation Alternatives in Washington Raspberry Fields

Volume 6 Issue 3

Tom Walters (waltersagresearch@frontier.com), Inga Zasada (inga.zasada@ars.usda.gov), and Lisa Wasko DeVetter (lisa.devetter@wsu.edu).

Trident Ag recently announced that they will only be bed fumigating red raspberry fields with Telone C-35 due to shortages of this fumigant. Given this recent change in fumigation practices, we thought it would be helpful to recirculate reports that summarize research trials comparing broadcast applications of Dominus and Vapam to Telone C-35, as well as bed fumigation trials.

Dominus/Vapam Trial

A trial comparing a conventional Telone C-35 application (shank-injected approximately 16 inches below the soil surface, 35 gallons/Acre) with shallow-applied Vapam (shanks 5 and 10 inches below the soil surface, 75 gallons/Acre) and Dominus (applied same as Vapam, 40 gallons per acre) was established in 2014, and was monitored in 2015 and 2016. Four monitoring sites were established in the Dominus and Vapam plots, and 8 additional monitoring sites were established in the Telone C-35-treated field surrounding the plots. Through 2015, P. penetrans counts were highest in the Telone C-35-fumigated areas, with markedly fewer nematodes in Vapam and Dominus-treated plots (Table 1). In 2016, P. penetrans numbers in the Vapam-treated plot were still less than 20% of those in the C-35 treated and Dominus-treated areas.

In 2015, plant growth (primocanes per hill and height of the longest primocanes in a hill) were better in the Vapam and Dominus-treated plots (Table 2). Differences were less pronounced in 2016; C-35 treated areas had slightly (but not significantly) fewer canes per hill and taller primocanes.

Shallow application of Vapam and Dominus markedly improved nematode control in this trial. When Vapam was used, this effect remained visible two years after fumigation. Most of the nematodes in this field were located in the upper part of the soil profile. As such, results might have been different in a heavier soil where nematodes tend to live deeper in the soil profile.

In this trial, shallow-dwelling nematodes could be found on the winter cover crop planted at fumigation and were a major source of infection in the newly planted raspberry crop.

Cover Crop Trials

Additional trials were established to assess the efficacy of managing P. penetrans on the winter cover crop by either killing the cover crop early (thus starving the nematodes in the roots) or by applying a nematicide to the cover crop. Results were not statistically significant, but the Lannate-treated plots in both trials had numerically lower levels of P. penetrans than untreated plots. Follow-up trials established in 2015 (funded by this project and a WSDA grant) found no effect of insecticide application or time of cover crop kill, but delaying cover crop planting did reduce P. penetrans numbers in the following raspberry crop.
Like the trial above, these trials suggest that shallow-dwelling P. penetrans escape fumigation, find refuge in winter-planted cover crops and readily infect newly planted raspberry. We set up another preplant cover crop trial following fumigation in the fall of 2016. In this trial, cover crops identified as poor P. penetrans hosts (‘Jesup’ fescue, black oats, ‘Wheeler’ rye) are being compared with winter wheat ‘Bobtail’ (provided by CHS) and a fallow check.

A raspberry field that had been left unplanted for a year instead of being immediately replanted was also monitored this past year. The old raspberry crop was removed from this field in 2013. It was planted with a small grain cover crop in the winter of 2013-14, and white mustard (Sinapis alba) was planted, grown and incorporated in 2014. In 2015, we found moderate numbers of P. penetrans on the winter cover crop (250/g root, similar to numbers in fumigated fields), and similarly found moderate P. penetrans numbers on the raspberry plants in September (667/g root). This grower’s strategy of combining a fallow year with white mustard incorporation in place of fumigation appears successful in this moderately infested field.

2015 WSDA Fumigation Trials

A replicated broadcast fumigation trial established in 2015 was evaluated in 2016. This trial included nontarped C-35 (industry standard), tarped C-35, nontarped C-35 with a Vapam cap and shallow-applied Dominus. The tarped C-35 and Vapam cap treatments controlled nematodes better on the preplant cover cover crop than the nontarped C-35 and Dominus treatments did, but no differences were noted on in nematodes on the raspberry plants in June, July or October.

Bed Fumigation Trial

A bed fumigation trial was also established in 2015, with adjoining beds fumigated with C-35, Dominus or Dominus:pic (75% Dominus, 25% chloropicrin). In July 2016, plants in the Dominus:pic-treated beds were slightly (but not significantly) taller than those in C-35 or Dominus-treated beds, and there were slightly more primocanes per hill in the Dominus-treated beds (Figure 1). Root and soil samples for nematode analysis were collected in October 2016. In general, samples contained very few P. penetrans, and there were no significant differences between treatments. The low P. penetrans numbers indicate that these treatments were quite effective in this field. We will continue to follow this trial next year.

Table 1. P. penetrans per g root in cover crop and in spring-planted raspberry following soil fumigation treatments September 2014.

¹average of primocanes per hill, counting 10 hills in each of at least 4 locations
²height of the 3 tallest primocanes in each hill, 10 hills in each of at least 4 locations

Figure 1. A) Raspberry primocane height and B) Primocanes per hill in beds fumigated with Telone C-35, Dominus or Dominus:pic

Table 3. Soil and root P. penetrans in raspberry beds fumigated with Telone C-35, Dominus or Dominus:pic, October 2016.