Hop cones from the hop plant (
Although several bacterial, fungal, and viral diseases have been documented on hop (Mahaffee et al., 2009), there are relatively few reports on the plant-parasitic nematode species associated with this crop. The hop cyst nematode (
As a result of the high demand for locally produced hop from the micro-brewing industry, hop plants were recently introduced into Florida as an alternative crop. In early 2016, the University of Florida planted hop rhizomes at the Gulf Coast Research and Education Center in Hillsborough County, Florida to evaluate the potential of this crop. By late 2016, some of the hop plants were showing stunted growth and yellow leaves. Uprooted plants exhibited severe root galling, and females extracted from galls, as well as J2s extracted from soil, were subsequently identified as
In Florida, root-knot nematodes (
The studies were conducted at the Gulf Coast Research and Education Center research farm in Wimauma, Florida, United States. Soil at the site is an Arredondo fine sand (95% sand, 3% silt, 2% clay). The field had been fallow for over 10 years under natural grass cover prior to conducting the following trials.
In 2016, a new hop yard was established at the research farm to evaluate plant-parasitic nematode population development on 10 hop cultivars from two planting material sources. Planting rows were 82-m long and were spaced 4.88-m apart, with a 0.91-m wide strip of black geotextile mulch covering the center of each row. Hills were spaced 0.91-m apart within the planting row. A 6.1-m tall trellis wire was installed above each row. Plants were irrigated with two drip tapes (30-cm emitter spacing), each located 0.46-m feet away from the row center along both sides of the planting row. Plants were fertilized with all-purpose fertilizer (20:20:20) at a rate of 149 kg N/ha/season. Ground cover between rows consisted of perennial ryegrass (
The experimental design was a randomized complete block with three blocks and 12 cultivar treatments. In total, 36 plots were overlaid on to three planting rows within the hop yard. Plots consisted of four hills (3.66-m long) and were planted at a density of two rhizomes per hill. A 0.91-m buffer zone was located between plots to minimize edge effects. On April 12, 2016, triplicate plots were planted with rhizomes from each of the following cultivars: (i) ‘CTZ’, (ii) ‘Cascade’ (source: Oregon), (iii) ‘Cascade’ (source: Washington), (iv) ‘Centennial’ (source: Oregon), (v) ‘Centennial’ (source: Washington), (vi) ‘Chinook’, (vii) ‘Fuggle’ (source: Oregon), (viii) ‘Magnum’ (source: Washington), (ix) ‘Nugget’ (source: Washington), (x) ‘Sorachi Ace’ (source: Washington), (xi) ‘Triple Perle’ (source: Washington), or (xii) ‘Willamette’ (source: Washington).
Plants were harvested in August 2016 and December 2016 by cutting the bines by hand at ground level. During the growing season, a number of the cultivars displayed symptoms of a viral disease, which was subsequently confirmed as
In December 2016, plants were uprooted from the soil after harvest using a shovel and assessed for root galling. Entire root systems were assessed for galling and given a root gall index rating on a scale from 0 to 10 (Bridge and Page, 1980), with 0 indicating no visible galling and 10 indicating 100% galled. The abundance of plant-parasitic nematodes in soil was also determined for each plot. Eight soil cores (20 cm in length, 2.5 cm in diameter) were obtained from each plot directly from the previous rooting zone (two soil cores per hill). Soil samples were placed into plastic bags and were stored at 4°C for a maximum of 14 days prior to subsequent processing. Nematodes were extracted from a 200-mL subsample of soil from each plot using the Baermann pan technique (Forge and Kimpinski, 2007), with a two-day incubation period. After collecting the nematodes over a 25-µm sieve, the nematodes were transferred in water into plastic scintillation vials and store at 4°C for a maximum of 14 days prior counting on an inverted compound microscope.
In 2017, a new hop planting was established in the previously renovated hop yard. On August 15, 2017, metam potassium (K-Pam® HL; AMVAC Chemical Corporation, Los Angeles, CA) was drip applied to the previous planting rows at a rate of 584 L/ha. The experimental design was a randomized complete block with three blocks and 12 cultivar treatments overlaid on the previous plots from the first field experiment in 2016. Each hill within a plot was planted with two transplants. On September 13, 2017, triplicate plots were planted with tissue-cultured transplants from each of the following cultivars: (i) ‘Cascade’ (source: Washington), (ii) ‘Cascade’ (source: Florida), (iii) ‘Cashmere’, (iv) ‘Centennial’ (source: Washington), (v) ‘Centennial’ (source: Florida), (vi) ‘Chinook’ (source: Washington), (vii) ‘Chinook’ (source: Florida), (viii) ‘Comet’ (source: Washington), (ix) ‘Magnum’ (source: Washington), (x) ‘Nugget’ (source: Washington), (xi) ‘Triple Perle’ (source: Washington), or (xii) ‘Zeus’ (source: Washington).
In February 2018, artificial lights (Philips Flowering Lamp 2.0; Philips, Amsterdam, the Netherlands) were installed every 2.74 m along the trellis wire to extend the daylight length by 5 hr from March to May and from September to November. Plants were harvested by hand in December 2017, July 2018, December 2018, and July 2019 by cutting the bines at ground level.
The abundance of plant-parasitic nematodes in soil was determined on four sampling dates (December 2017, July 2018, December 2018, and July 2019). Soil was sampled and nematodes were quantified as described previously.
In 2017, an additional new hop planting was established on a single planting row adjacent to the previous hop yard from 2016. On August 15, 2017, metam potassium was drip applied to the planting row, as described above. Planting rows were covered with a 0.91-m wide strip of metallic plastic mulch. The experimental design was a randomized complete block with four blocks and five cultivars overlaid on the previous plots from the first field experiment in 2016. Each hill within a plot was planted with two transplants. On September 15, 2017, plots were planted with tissue-cultured hop transplants of each of the following cultivars sourced from Washington: (i) ‘Canadian Red Vine’, (ii) ‘Galena’, (iii) ‘Mt. Rainier’, (iv) ‘Tahoma’, or (v) ‘Willamette’.
Artificial lighting was installed every 2.74 m along the trellis wire to extend the daylight length during March to May and September to November. Plants were harvested by hand in December 2017, July 2018, December 2018, and July 2019 by cutting the bines at ground level.
The abundance of plant-parasitic nematodes in soil was determined on three sampling dates (December 2017, July 2018, December 2018, and July 2019). Six soil cores were obtained from each plot directly from the previous planting holes (two cores per hill). Soil samples were processed, and nematodes were quantified as described previously.
In 2017, a greenhouse experiment was performed to evaluate the susceptibility of 14 hop cultivars to
At harvest, plants were carefully uprooted from the soil and assessed for root galling, as described above. The abundance of eggs on each root system was determined by extracting eggs from entire root systems using the sodium hypochlorite technique, as described above. Eggs were collected over a 25-µm sieve, transferred in water into plastic scintillation vials, and store at 4°C for a maximum of 14 days prior counting on an inverted compound microscope.
In July 2019, dagger (
Nematode data from the field and greenhouse experiments were subjected to a one-way ANOVA in SAS Studio (SAS University Edition; version 3.3; SAS Institute Inc., Cary, NC, USA) using the PROC GLM procedure. Differences among treatment means were examined using Tukey’s HSD test (
Plant-parasitic nematode species recovered from the root zone of hop plants grown in a Florida field soil during the three-year trial period included
Molecular identification of plant-parasitic nematode species in the rooting zone of hop cultivars in a Florida field soil.
Common name | GenBank accession | Closest accession | Percent identity (%) | Species |
---|---|---|---|---|
|
||||
Dagger | MN922336.1 | KX931065.1 | 97.5 |
|
Lesion | MN922338.1 | MG745329.1 | 98.8 |
|
Ring | MN922337.1 | FN433872.1 | 98.1 |
|
Spiral | MN922339.1 | AB602601.1 | 99.1 |
|
Stubby-Root | MN922340.1 | MG938546.1 | 100 |
|
|
||||
Lesion | MN911166.1 | KY677821.1 | 98.8 |
|
Spiral | MN911167.1 | MK796435.1 | 98.8 |
|
Stubby-Root | MN911168.1 | KJ934126.1 | 100 |
|
Notes: aAmplified with D2A/D3B primer set. bAmplified with 1813/2646 primer set.
Soil population densities of
Effect of hop cultivar and planting material source on soil populations of plant-parasitic nematodes and root galling in December 2016 in Hop Yard Planting No. 1.
Nematodes per 200 mL of soil (No.) | |||
---|---|---|---|
Cultivar (Source) |
|
|
Gall rating (0-10) |
‘CTZ’ | 46 | 19 | 1.44 c |
‘Cascade’ (OR) | 277 | 42 | 3.92 abc |
‘Cascade’ (WA) | 102 | 26 | 2.53 bc |
‘Centennial’ (OR) | 336 | 23 | 5.94 ab |
‘Centennial’ (WA) | 389 | 27 | 4.42 abc |
‘Chinook’ | 526 | 21 | 6.67 a |
‘Fuggle’ | 127 | 11 | 5.83 ab |
‘Magnum’ | 31 | 69 | 0.72 c |
‘Nugget’ | 56 | 29 | 1.75 c |
‘Sorachi Ace’ | 75 | 19 | 3.92 abc |
‘Triple Perle’ | 20 | 7 | 1.17 c |
‘Willamette’ | 175 | 31 | 3.08 abc |
|
0.651 | 0.904 | <0.001 |
Root galling varied considerably among the hop cultivars in Hop Yard Planting No. 1. The most severe root galling was observed on ‘Chinook’, ‘Centennial’ (OR), ‘Fuggle’, and ‘Centennial’ (WA). The lowest amount of root galling was observed on ‘Magnum’, ‘Triple Perle’, ‘CTZ’, and ‘Nugget’.
In December 2017,
Effect of hop cultivar on soil populations of
|
||||
---|---|---|---|---|
Cultivar (Source) | December 2017 | July 2018 | December 2018 | July 2019 |
‘Cascade’ (WA) | 16 | 68 ab | 208 | 20 |
‘Cascade’ (FL) | 48 | 3 b | 155 | 29 |
‘Cashmere’ | 2 | 22 ab | 89 | 34 |
‘Centennial’ (WA) | 36 | 106 a | 320 | 132 |
‘Centennial’ (FL) | 48 | 69 ab | 372 | 37 |
‘Chinook’ (WA) | 1 | 17 ab | 99 | 9 |
‘Chinook’ (FL) | 67 | 0 b | 37 | 5 |
‘Comet’ | 27 | 13 b | 278 | 69 |
‘Magnum’ | 1 | 6 b | 234 | 21 |
‘Nugget’ | 5 | 10 b | 85 | 1 |
‘Triple Perle’ | 1 | 1 b | 23 | 23 |
‘Zeus’ | 0 | 46 ab | 33 | 26 |
P-value | 0.225 | 0.004 | 0.922 | 0.689 |
Soil population densities of
Effect of hop cultivar on soil populations of
|
||||
---|---|---|---|---|
Cultivar (Source) | December 2017 | July 2018 | December 2018 | July 2019 |
‘Cascade’ (WA) | 0 | 4 | 23 | 13 |
‘Cascade’ (FL) | 85 | 40 | 24 | 3 |
‘Cashmere’ | 0 | 13 | 25 | 5 |
‘Centennial’ (WA) | 0 | 3 | 6 | 10 |
‘Centennial’ (FL) | 40 | 22 | 21 | 5 |
‘Chinook’ (WA) | 0 | 33 | 23 | 13 |
‘Chinook’ (FL) | 33 | 7 | 17 | 4 |
‘Comet’ | 34 | 8 | 14 | 7 |
‘Magnum’ | 2 | 17 | 98 | 8 |
‘Nugget’ | 0 | 15 | 19 | 3 |
‘Triple Perle’ | 1 | 21 | 25 | 7 |
‘Zeus’ | 1 | 71 | 87 | 18 |
|
0.142 | 0.085 | 0.557 | 0.643 |
Soil population densities of
Effect of hop cultivar on soil populations of
|
||||
---|---|---|---|---|
Cultivar | December 2017 | July 2018 | December 2018 | July 2019 |
‘Canadian Red Vine’ | 88 | 128 | 486 | 19 |
‘Galena’ | 8 | 146 | 93 | 9 |
‘Mt. Rainier’ | 31 | 153 | 522 | 5 |
‘Tahoma’ | 80 | 427 | 339 | 18 |
‘Willamette’ | 12 | 190 | 275 | 10 |
|
0.274 | 0.337 | 0.309 | 0.677 |
Effect of hop cultivar on soil populations of
|
||||
---|---|---|---|---|
Cultivar | December 2017 | July 2018 | December 2018 | July 2019 |
‘Canadian Red Vine’ | 1 | 10 | 14 | 8 |
‘Galena’ | 11 | 17 | 60 | 18 |
‘Mt. Rainier’ | 3 | 18 | 63 | 20 |
‘Tahoma’ | 2 | 4 | 20 | 7 |
‘Willamette’ | 8 | 5 | 28 | 3 |
|
0.559 | 0.165 | 0.305 | 0.217 |
Root galling differed significantly among the hop cultivars evaluated in the greenhouse experiment (Table 7). The most severe root galling was observed on ‘Comet’, ‘Cashmere’, ‘Canadian Red Vine’, and ‘Chinook’. The lowest amount of root galling was observed on ‘Galena’, ‘Willamette’, and ‘Cascade’. The number of
Effect of hop cultivar on root galling and the abundance of
Cultivar | Gall rating (0-10) |
|
---|---|---|
‘Canadian Red Vine’ | 5.33 abc | 6,267 c |
‘Cascade’ | 1.83 def | 17,253 abc |
‘Cashmere’ | 5.50 ab | 29,960 ab |
‘Centennial’ | 3.17 b-e | 23,107 abc |
‘Chinook’ | 5.00 abc | 16,493 abc |
‘Comet’ | 6.67 a | 38,653 a |
‘Galena’ | 0.50 f | 4,707 c |
‘Magnum’ | 3.33 bcd | 2,453 c |
‘Mt. Rainier’ | 3.33 b-e | 10,893 bc |
‘Nugget’ | 3.00 cde | 7,307 bc |
‘Tahoma’ | 3.33 b-e | 8,880 bc |
‘Triple Perle’ | 3.17 b-e | 12,707 bc |
‘Willamette’ | 1.67 def | 2,949 c |
‘Zeus’ | 4.67 abc | 13,360 bc |
|
<0.001 | <0.001 |
A diverse range of plant-parasitic nematodes inhabit Florida soils (Porazinska et al., 1999), likely as a result of favorable biotic and abiotic conditions provided by the sandy soils and warm sub-tropical climate characteristic of this growing region. As a result, plant-parasitic nematodes present a significant obstacle to most crops planted in Florida soils. In this study, plant-parasitic nematode species recovered from the root zone of hop planted in a Florida field soil included
The abundance of particular plant-parasitic nematode species in the root zone differed considerably among the different hop cultivars planted. Root-knot nematodes are widespread in Florida (Garcia and Rich, 1985), and likely pose one of the most significant obstacles to successful production of hop in the growing region. All hop cultivars that were evaluated were hosts to
No consistent differences in lesion nematode (
Plant-parasitic nematodes present an obstacle to any crop grown in Florida and finding suitable hop cultivars that can thrive in the sub-tropical climate as well as tolerate or resist nematode feeding will be vital to the establishment of a sustainable hop industry in the region. In a recently conducted field trial in Central Florida, the cultivars ‘Columbus’ and ‘Chinook’ showed the greatest bine lengths and yield, followed by ‘Neo1’ and ‘Amalia’ (Pearson et al., 2016). In addition to finding vigorous growing hop cultivars, developing effective pest management strategies will also be key to sustaining the hop industry. As a herbaceous perennial crop, opportunities for soilborne pest management are limited on hop. In this study, pre-plant fumigation only provided temporary nematode relief at the time of planting and did not prevent subsequent nematode population development later on in the growing season, as has been observed in other studies on perennial crops (Watson et al., 2017). Nematicides may be able to provide effective management options for use by growers (e.g. fluopyram), alongside other management tools, such as the development of tolerant/resistant hop cultivars through hop breeding programs and adopting management practices that promote soil health and disease suppressive soils.
Overall, our study provides the first report of the plant-parasitic nematode species recovered from soil in the rooting zone of hop in Florida and provides a basis for selecting hop cultivars that restrict plant-parasitic nematode population development in Florida field soils.