Abstract
The majority of winegrape (Vitis vinifera) vineyards in Washington are planted with own-rooted grapevines, as opposed to grapevines grafted onto rootstock varieties. The plant-parasitic nematode Meloidogyne hapla (common name: northern root-knot nematode) is commonly found in Washington winegrape vineyards, and own-rooted grapevines are susceptible to this nematode. Before rootstocks are used to manage M. hapla or other horticultural characteristics in Washington, their host status for M. hapla should be defined. In greenhouse experiments, 10 commercially available rootstock varieties were evaluated for their M. hapla host status. Additionally, the reproductive potential of different M. hapla populations collected from Oregon and Washington, and of another root-knot nematode, M. chitwoodi, on rootstock varieties and own-rooted V. vinifera Chardonnay was evaluated. The rootstocks Salt Creek, Freedom, Harmony, St. George, Riparia Gloire, 101-14 Mgt, 3309C, 110R, 420A, and Matador were poor hosts for M. hapla. Populations of M. hapla varied in reproductive potential and virulence on own-rooted Chardonnay. An M. hapla population collected from a V. vinifera vineyard in Paterson, WA had 33 to 78% greater reproduction than the other M. hapla populations. An M. hapla population collected from a V. vinifera vineyard in Alderdale, WA was consistently more virulent than the other M. hapla populations. Own-rooted Chardonnay and the rootstock Matador were poor hosts for M. chitwoodi. This is the first report of the host status of several grapevine rootstocks for M. hapla.
Over 30 different winegrape varieties are cultivated on ~21,043 ha of vineyards in Washington (NASS 2017). Most of these vineyards are planted with own-rooted varieties of Vitis vinifera, as opposed to grapevines grafted onto rootstock varieties. The periodic occurrence of sub-zero cold winter temperatures, particularly rapid drops in temperature during vine cold hardiness acclimation and deacclimation, can result in cold injury to vines (Ferguson et al. 2014). Recent examples of such weather events occurred during the “Halloween Freeze” (31 Oct) of 2002 and the “Thanksgiving Freeze” (24 Nov) of 2010, when temperatures dropped to −11.5 and −17.3°C, respectively (AgWeatherNet; weather.wsu.edu). When vines are own-rooted, vineyards can be retrained during the season immediately following cold damage, resulting in only a one-year loss in crop (Moyer et al. 2011). However, when cold damage occurs to vines that are grafted onto a rootstock variety, the growing season immediately following a cold event is either spent field-grafting a scion onto the rootstock variety or removing the remaining rootstocks entirely and replanting. This process can result in a crop loss for up to two to three years following a damaging cold event.
The modern Washington winegrape industry underwent its first rapid vineyard expansion in the 1980s, followed by an additional period of rapid growth from 1993 to 1999 (NASS 2017). Thus, many vineyards are either past or approaching the end of their productive lifespans and are scheduled for replanting within the next several years. Plant-parasitic nematodes are commonly found in Washington vineyards and could be a concern for replanting. Surveys conducted in eastern Washington found Meloidogyne hapla, the northern root-knot nematode, to be the most abundant nematode present, found in 60% of the surveyed vineyards (Zasada et al. 2012). The proposed threshold is 100 M. hapla/250 g soil (Santo, unpublished data, 2000), a density exceeded in 26% of surveyed winegrape vineyards in Washington. While M. hapla is the predominant species found in the region, M. chitwoodi, another other common Pacific Northwest Meloidogyne species, is also widespread in other crop production systems (Zasada et al. in press). Own-rooted V. vinifera varieties have been shown to be good hosts for M. hapla (Howland et al. 2015). Unfortunately, given the preference for own-rooted vines in Washington, replant situations where susceptible vines are placed into sites with high nematode pressure is a concern for vineyard establishment and productive lifespan.
Meloidogyne spp., or root-knot nematodes, are a significant production and economic constraint to grapevines worldwide (Jenser et al. 1991, Arredondo 1992, Nicol et al. 1999). As sedentary endoparasites, these nematodes remain stationary inside the roots of a host plant for the majority of their lifespan. Adult females lay their eggs outside the roots in a gelatinous matrix and a single egg mass can contain up to 400 to 500 eggs. The infective stage is the second-stage juvenile, which hatches from eggs and migrates through the soil in search of a root tip to penetrate. Once within the root tip, the juvenile migrates up the root, where it ultimately establishes a feeding site and completes its lifecycle. In the United States, Meloidogyne spp. have been reported to reduce grapevine yields by up to 20% (Anwar and McKenry 2000). Seven species of Meloidogyne are found on grapevines, but only four species, M. incognita, M. hapla, M. javanica, and M. arenaria, are considered damaging (Esnard and Zuckerman 1998, Esmenjaud and Bouquet 2009).
Most winegrape-producing regions use rootstocks to manage plant-parasitic nematodes when they are identified as a production constraint. Breeding for resistance to Meloidogyne spp. has been the primary goal of some rootstock programs over the years. The cultivars Harmony and Freedom were the first Meloidogyne-resistant rootstocks to come from a breeding program (Weinberger and Harmon 1966). 101-14 Mgt and Ramsey (= Salt Creek) are also considered resistant to Meloidogyne spp. (Nicol et al. 1999, Ferris et al. 2012). Other rootstocks more recently developed with resistance to Meloidogyne spp. include UCD GRN1, 2, 3, 4, and 5 (Ferris et al. 2012), USDA 10-17A, USDA-23B, USDA 6-19B, RS-3, and RS-9 (Anwar et al. 2002, Gu and Ramming 2005a, 2005b), and Matador, Minotaur, and Kingfisher (Cousins 2011). In a summary of the literature on nematode-resistant rootstocks, M. hapla was not included (Ferris et al. 2012). Very little is known about the response and host status of rootstocks to M. hapla, and no breeding programs focus on developing rootstocks with resistance to M. hapla.
The host status of rootstocks for the industry-prevalent M. hapla must be known for the Washington wine industry to deploy rootstocks effectively for management of nematodes and other desired horticultural characteristics. The research presented here is a first step in this direction. The objectives were: 1) to determine the host status of Vitis rootstocks for M. hapla, 2) to determine whether M. hapla populations from Washington and Oregon differ in virulence on Vitis rootstocks and own-rooted V. vinifera Chardonnay, and 3) to compare the ability of M. hapla to parasitize own-rooted Chardonnay and the rootstock Matador with that of M. chitwoodi.
Materials and Methods
Experiment 1: Determining host status of rootstocks for M. hapla
Nine rootstocks, including Salt Creek, Freedom, Harmony, St. George, Riparia Gloire, 101-14 Mgt, 3309C, 110R, and 420A (Sunridge Nurseries, Inc., Bakersfield, CA) (Table 1), were evaluated for host status to a single population of M. hapla. Own-rooted V. vinifera Riesling was included as a susceptible control (Howland et al. 2015). In March 2014, dormant, non-rooted cuttings of each rootstock and the own-rooted Riesling were grouped relative to stem diameter to ensure vine uniformity. Using pruning shears, vines were cut into three node segments, with the basal internode cut diagonally. The basal internode was dipped in rooting hormone (1% indole-3-butyric acid, 0.5% 1-napthalaneacetic acid; Dip’N Grow) to stimulate root growth. Cuttings were inserted in a perlite and vermiculite mixture (Santo and Hackney 1980) and placed on a bench with a heating pad for two months, where they were misted with water every 30 min.
Parentage of Vitis rootstocks evaluated against Meloidogyne hapla and M. chitwoodi.
In April 2014, the grape cuttings were removed from the mist bench and placed in a greenhouse under a shade cloth to be hardened-off. A week later, established grape cuttings of each rootstock or own-rooted Riesling with uniform root systems were transplanted into 3.7 L pots containing a steam-pasteurized 1:1 sand:Willamette loam soil. Buds were removed until only a single bud/shoot remained, and any developing inflorescences were removed to promote root growth. The grapevines were fertilized initially with a 9-45-15 NPK starter fertilizer (Jack’s Professional) at a rate of 4 g/L, delivering 336 mg/L N. Four weeks later, the grapevines were fertilized with a 20-20-20 NPK fertilizer (Jack’s Professional) at a rate of 16 g/L, delivering 150 mg/L N; vines were fertigated biweekly thoughout the experiment. The grapevines were grown in a greenhouse under a 16 hr photoperiod for the duration of the experiment; temperatures were set to 25°C during the day and 20°C at night.
In late May 2014, vines were inoculated with nematodes. The M. hapla population was originally collected from a V. vinifera vineyard in Veneta, OR (designated Veneta). To establish the population in culture, soil was collected from the vineyard, placed in a 2 L clay pot, and a 3- to 4-wk-old tomato (Solanum lycopersicon Mill. Rutgers) was planted in each pot. After approximately four to five months, plants were removed from the pots, roots were washed free of adhering soil, and single egg masses were picked and placed on new tomato plants. These plants were maintained for an additional three to four months with these single-female lines used as inoculum. Inoculum was obtained by destructively harvesting tomato plants and collecting eggs from washed roots by agitating the root system in a 0.05% NaOCl solution for 3 min (Hussey and Barker 1973). The egg suspension was then poured over nested 250- and 25-µm-sieves, with eggs being retained on the 25-µm-sieve. A 1 mL subsample of the egg suspension was placed on a counting slide to determine the total inoculum concentration. The suspension was then diluted until the concentration equaled 9000 eggs/3.7 L pot, or a density of three Meloidogyne eggs/g of soil. The inoculum was applied to each grapevine by pipetting 5 mL suspension into four holes, 6 cm deep around the base of the vine. The holes were covered and plants were watered regularly starting the next day. The rootstocks and own-rooted Riesling were arranged in a randomized block design on a greenhouse bench with treatments replicated six times; the experiment was conducted twice with trials separated in time (inoculation was offset by a week) and space (trials were conducted in different greenhouses).
Plants were destructively harvested in October 2014. For each vine, the shoot was removed, placed in a paper bag, dried at 70°C for five days, and weighed. Roots were shaken free of soil and a 50 g subsample of soil from each pot was collected to extract second-stage juveniles (J2) using the Baermann funnel method (Ingham 1994). Roots were then gently rinsed free of soil. M. hapla eggs were extracted from the entire root system as described above. The number of eggs in 1 mL of the 50 mL egg suspension was determined using an inverted microscope. The remaining roots were oven-dried like the shoots and weighed.
Experiment 2: Determining M. hapla population virulence differences
Four rootstocks, Harmony, St. George, 3309C, and Riparia Gloire (Sunridge Nurseries, Inc.), were evaluated for host status to four populations of M. hapla. Own-rooted V. vinifera Chardonnay was included as a susceptible control (Howland et al. 2015). The Veneta population was used, as well as three other M. hapla populations: two collected from V. vinifera vineyards in Paterson, WA and Alderdale, WA, respectively (designated Paterson and Alderdale), and the third collected from a Vitis labruscana Concord vineyard in Prosser, WA (designated Prosser). The establishment of nematode cultures was as described in Experiment 1. In March 2015, dormant, unrooted cuttings of each rootstock and own-rooted Chardonnay were grouped relative to stem diameter to ensure vine uniformity, and rooted as described above. The same experimental methods described in Experiment 1 were used to root, establish, and maintain vines in pots, and for nematode inoculation of vines. The genotype and M. hapla population treatment combinations were arranged in a randomized block design on a greenhouse bench with treatments replicated five times; the experiment was conducted twice, and trials were separated in time (inoculation was offset by a week) and space (different greenhouse benches). Plants were destructively harvested in October 2015 as described above.
Experiment 3: Comparing host status of M. hapla versus M. chitwoodi
The rootstock Matador (Inland Desert Nursery, Benton City, WA) was evaluated for host status for a single population each of M. hapla and M. chitwoodi. Own-rooted V. vinifera Chardonnay was included as a susceptible control. The M. hapla Paterson population was used, as well as an M. chitwoodi Race 1 population originally collected from a potato field in Prosser, WA. The establishment of nematode cultures was as described in Experiment 1. In March 2017, dormant, unrooted cuttings of each rootstock and own-rooted Chardonnay were grouped relative to stem diameter to ensure vine uniformity and rooted as described above. The same experimental methods as in Experiment 1 were used to establish and maintain vines in pots and for nematode inoculation of vines. The genotype and M. hapla/M. chitwoodi treatment combinations were arranged in a randomized block design on a greenhouse bench with treatments replicated six times. The experiment was conducted twice, and trials were separated in time (inoculation was offset by a week) and space (different greenhouse benches). Plants were destructively harvested in October 2017 as described above.
Data analysis
Meloidogyne data are presented as eggs/g root. In addition, reproduction factor values, where RF = final nematode population (eggs + J2)/initial nematode population (9000 eggs/pot), were calculated. An RF value > 1 indicates that the plant is a good host, while an RF value < 1 indicates a poor host (Oostenbrink 1966). Data were analyzed using a mixed linear model analysis of variance (ANOVA) in JMP (SAS Institute Inc.). In all analysis, trial was considered a random factor while all other treatments were fixed factors. When the trial × treatment interaction was significant (p < 0.001), the trials were analyzed separately. To meet ANOVA assumptions, nematode data were log10 (x+1)-transformed prior to analysis. Statistically significant differences among treatments were computed by Tukey’s honest significant difference test with significance at p < 0.05.
Results
Experiment 1: Determining host status of rootstocks for M. hapla
Differences were observed among the rootstocks in above- and below-ground biomass (Table 2). Shoot weight of Freedom was significantly smaller than that of Salt Creek, 420A, and own-rooted Riesling, which did not differ from each other. Riparia Gloire had the largest root system, which was similar to that of Freedom, 101-14 Mgt, and 420A. 3309C had the smallest root system, which was similar in size to that of Salt Creek, Harmony, St. George, and 110R. The susceptible control, own-rooted Riesling, had a significantly greater density of M. hapla eggs/g of root and RF value than the rootstocks (Table 2). Among the rootstocks, there were no differences in the measured M. hapla parameters: all rootstocks were poor hosts (RF < 1; less-then-replacement reproductive rate) for M. hapla.
Reproduction of Meloidogyne hapla on Vitis rootstocks and on own-rooted Vitis vinifera Riesling in Experiment 1.
Experiment 2: Determining M. hapla population virulence differences
In both trials, the rootstocks evaluated against the four M. hapla populations, 3309C, Riparia Gloire, St. George, and Harmony, were all poor hosts for the populations, with RF values ranging from 0 to 0.38 and M. hapla eggs/g root ranging from 0 to 565. To determine if the M. hapla populations varied in virulence on a susceptible host, the data from the own-rooted Chardonnay was analyzed independently of the other rootstock varieties. The results from the trial repetitions were significantly different (p = 0.001), therefore, they were analyzed separately (Figure 1); however, similar trends were observed. In the first trial of the experiment, root parasitism by M. hapla Alderdale re sulted in a significantly smaller root system at the end of the experiment than the other M. hapla populations (Figure 1A). In this trial, the M. hapla Paterson population had a greater final population density on own-rooted Chardonnay than the other populations, with 41% more eggs/g root recovered then the next highest population density in M. hapla Alderdale. The RF value of M. hapla Paterson was at least two times greater than that of the other M. hapla populations (Figure 1B). While M. hapla Alderdale produced more eggs/g root than M. hapla Prosser and Veneta, the RF values were similar.
Reproduction of Meloidogyne hapla populations collected from Washington (Alderdale, Paterson, and Prosser) and Oregon (Veneta) on own-rooted Vitis vinifera Chardonnay in Experiment 2. Reproduction factor (eggs on roots + second-stage juveniles in soil)/initial nematode population density (9000 eggs) values are shown at the top of graphs C and D. Values presented numerically and as columns are the mean + standard error of five observations. Nematode data was log10 (x + 1) transformed prior to analysis; nontransformed means are presented. Mean or columns within a graph panel followed by the same letter are not significantly different according to Tukey’s honest significant difference test with significance at p < 0.05.
In the second trial, similar to the first trial, the root system of the own-rooted Chardonnay was the smallest under M. hapla Alderdale parasitism; however, this was only significantly different from the largest root system parasitized by the M. hapla Prosser population (Figure 1C). While the highest density of eggs/g root and RF value were again observed in the M. hapla Paterson population in the second trial, this density and value were not significantly different from the next highest density or two next highest RF values, respectively (Figure 1D). Again, in the second trial, M. hapla Veneta had the numerically lowest eggs/g root and lowest RF value.
Experiment 3: Comparing host status of M. hapla versus M. chitwoodi
Growth of the rootstock Matador differed from that of own-rooted Chardonnay (p < 0.001; Table 3); Matador had ~52% more shoot biomass. The opposite was observed for root biomass. The root system of Matador was 64% smaller than own-rooted Chardonnay. Neither Meloidogyne species impacted shoot or root biomass of Matador or own-rooted Chardonnay (p > 0.05). Matador was not a good host for either M. chitwoodi or M. hapla Alderdale with RF values < 0.03 (Table 3). On own-rooted Chardonnay, the final population density was 6000 times greater than the final population density of M. chitwoodi (p < 0.001; Table 3).
Reproduction of Meloidogyne hapla and M. chitwoodi on own-rooted Vitis vinifera Chardonnay and the Vitis rootstock Matador in Experiment 3.
Discussion
Our data provides additional information on the relative susceptibility of commercially available rootstocks to plant-parasitic nematodes (Ferris et al. 2012); specifically, those that are present in the Pacific Northwest. Few studies have evaluated the host status of Vitis rootstocks to M. hapla or M. chitwoodi (Lider 1954, Stirling and Cirami 1984, Ramsdell et al. 1996). Therefore, these data are very important for broadening knowledge of the host status of rootstocks for this nematode. Our results indicate that all the rootstocks considered, Riparia Gloire, 101-14 Mgt, Salt Creek, Freedom, Harmony, St. George, 3309C, 110R, 420A, and Matador, are poor hosts for M. hapla. Salt Creek was reported to be resistant to M. hapla (Lider 1954) and another report found both Salt Creek and Freedom to be resistant to M. hapla (Stirling and Cirami 1984). Contradictory to our findings, two studies found Riparia Gloire (Dalmasso and Cuani 1976) and 3309C (Ramsdell et al. 1996) to be susceptible to M. hapla.
Most of these rootstocks have been evaluated for host status to other Meloidogyne spp., including M. incognita, M. javanica, and M. arenaria. Widespread use of Harmony and Freedom rootstocks has resulted in aggressive pathotypes of Meloidogyne spp. that are capable of feeding on N-allele grapevine rootstocks (Cousins 2011); many rootstocks resistant to other populations of Meloidogyne are susceptible to these pathotypes, designated as M. arenaria Harmony A and M. incognita Harmony C (Cain et al. 1984, Anwar et al. 1999). The rootstocks 3309C and St. George are considered susceptible to M. incognita Race 3, M. javanica, M. arenaria, M. arenaria Harmony A, and M. incognita Harmony C (Nicol et al. 1999, Cousins and Walker 2002, McKenry and Anwar 2006, Ferris et al. 2012). Freedom and Harmony are resistant to most populations of M. incognita, M. javanica, and M. arenaria, except for the ones stated previously (Chitambar and Raski 1984, McKenry et al. 2001). Salt Creek (also known as Ramsey) was found to be a non-host to a mixed population of M. incognita, M. arenaria, and M. javanica, but is a host to M. arenaria Harmony (McKenry et al. 2001). The Matador rootstock was developed to be resistant to an M. arenaria Harmony A, but there is little other information on host status for other nematodes for this rootstock (Cousins 2011). Riparia Gloire is considered resistant to M. arenaria Harmony A and M. incognita Harmony C, but is susceptible to M. incognita Race 3, as is St. George (Cousins and Walker 2002, Ferris et al. 2012). 101-14 Mgt is resistant to M. arenaria Harmony A, M. incognita Harmony C, M. incognita, M. arenaria, and M. javanica (Sauer 1967, Nicol et al. 1999, Ferris et al. 2012). Both 110R and 420A are resistant to M. arenaria Harmony A and M. incognita Harmony C (Ferris et al. 2012), but 420A is susceptible to M. javanica, and 110R is reported susceptible to field populations of M. incognita, M. javanica, and M. arenaria in Spain (Sauer 1967, Téliz et al. 2007).
While the majority of the Vitis rootstocks evaluated in this trial are poor hosts for M. hapla, the mechanism of resistance may differ among rootstocks. Resistance mechanisms in grapevines may occur at nematode penetration, feeding, development, or reproduction (Ferris et al. 1982, Anwar and McKenry 2000, McKenry and Anwar 2006, Ferris et al. 2012). In Harmony, a hypersensitive response in the grape to Meloidogyne spp. prevents development (Ferris et al. 2012). Due to Salt Creek’s widespread root system, there is reduced penetration and success of Meloidogyne spp (McKenry and Anwar 2006).
The major grapegrowing region of Washington State, east of the Cascade Mountains, is marked by hot, dry summers and cold winters. A major concern with rootstocks for this region is tolerance to cold, both directly for the rootstock and indirectly on the scion. High-vigor rootstocks may delay cold-acclimation of the scion and result in vines that are more susceptible to fall cold events (Cousins 2005). In one of the few rootstock evaluations in Washington State, the rootstock 99R (V. berlandieri × V. rupestris) performed poorly over winter, which was attributed to its long growing period and late cold acclimation (Keller et al. 2012). This may indicate that 110R (V. berlandieri × V. rupestris) or 420A (V. berlandieri × V. riparia) may also fare poorly under Washington conditions. Rootstocks with V. berlandieri heritage, which is native to the southern United States, may be less cold tolerant and have delayed maturity. Very high-vigor rootstocks, such as St. George, Salt Creek, and Freedom, may also have delayed cold-acclimation in fall and may be less cold-hardy as a result. Generally, rootstocks with V. riparia heritage are more likely to be cold-tolerant, but are less drought-tolerant (Pongrácz 1983). Rootstocks with V. champinii heritage, which is from central Texas, like Freedom, Harmony, and Salt Creek, may not be particularly cold-hardy. Rootstocks with V. rupestris heritage, including St. George (V. rupestris) and 110R (V. berlandieri × V. rupestris), have high drought tolerance (Carbonneau 1985, Serra et al. 2014). Riparia Gloire and 101-14 Mgt are considered to have low drought tolerance, 3309C and 420A have low to medium drought tolerance, and Salt Creek (= Ramsey) has medium to high drought tolerance (Carbonneau 1985, Serra et al. 2014). Matador, a cross of 101-14 Mgt and V. mustangensis and V. rupestris parents, has not been evaluated for cold hardiness or drought tolerance.
To further explore the poor host status of the rootstocks for M. hapla observed in Experiment 1, we challenged a subset of the rootstocks to three additional populations of M. hapla collected from Washington and Oregon. There is evidence in the literature that races or pathotypes of M. hapla are present in Washington (Ogbuji and Jensen 1972, 1974, Santo and Hackney 1980). Nematode species can be differentiated into pathotypes and races on the basis of host range, pathogenicity or virulence, mode of reproduction, or genetic differences. Two proposed races of M. hapla were differentiated by chromosome number: Race A, which reproduces by facultative meiotic parthenogenesis, and Race B, which is pentaploid parthenogenetic (Triantaphyllou 1966). In the Pacific Northwest, five pathotypes of M. hapla were identified based upon their varying ability to reproduce on a range of hosts (Ogbuji and Jensen 1972). In Concord grape (V. labruscana), the presence of M. hapla pathotypes was considered after the observation that an M. hapla population collected from alfalfa (Medicago sativa) was a poor host on Concord grape, contrary to field observations where M. hapla was associated with vines exhibiting poor growth (Santo and Hackney 1980). To determine if M. hapla populations vary in virulence and reproduction on Concord grape, three populations of M. hapla, all identified as Race A based upon chromosome number, were collected from alfalfa, currant (Ribes sp.), and Concord grape in Washington (Santo and Hackney 1980). When inoculated onto Concord grape, the M. hapla populations varied in reproduction rate, with higher final population densities of the currant and Concord grape M. hapla populations than of the alfalfa M. hapla population (Santo and Hackney 1980). Additionally, the M. hapla population from Concord grape reduced root biomass compared to that observed for the alfalfa and grape M. hapla populations.
Our study again demonstrates that the reproductive potential of M. hapla populations varies. The M. hapla Paterson population consistently had the numerically greatest reproduction (eggs/g root) of the M. hapla populations evaluated. In one trial, the final RF value for this population was more than two times higher than the other M. hapla populations. However, it is important to note that all of the M. hapla populations would be considered successful parasites on V. vinifera, with >13-fold increase in population densities over a six-month period. We also observed consistent trends in root biomass outcomes with the different M. hapla populations. The M. hapla Alderdale population is potentially more virulent on V. vinifera than other M. hapla populations. This demonstrates that there is reproductive and virulence diversity among M. hapla populations in Washington as previously observed (Santo and Hackney 1980), and may explain why other researchers reported resistance/susceptibility results for Vitis rootstocks that were contrary to our findings (Dalmasso and Cuani 1976, Ramsdell et al. 1996).
Due to the potential for expanding winegrape vineyards to fields once cropped with agronomic hosts of M. chitwoodi like potato, small grains, or corn, an understanding of the ability of M. chitwoodi to parasitize V. vinifera and Vitis rootstocks is required to guide vine selection. M. hapla and M. chitwoodi are commonly found in mixed populations in the Pacific Northwest. Across the region, M. chitwoodi was more commonly detected in diagnostic samples from 2012 to 2016, with 60% occurrence compared to 25% for M. hapla when present (Zasada et al. in press). When root and soil samples from potato were analyzed (Nyczepir et al. 1982), the dominant species in the region was M. chitwoodi (56 to 93% incidence), with M. hapla present at an incidence of 0 to 39%. The greater incidence of M. chitwoodi was attributed to a cool growing season and increased acreage of small grain rotation crops, which are better hosts for M. chitwoodi than M. hapla. Plants in the Vitaceae are moderate to poor hosts for M. chitwoodi (EPPO 1991). M. chitwoodi did not produce high densities of eggs/g root on own-rooted V. vinifera Cabernet Sauvignon compared to the densities observed for M. arenaria Harmony A and M. incognita on the same host (Anwar et al. 2002); however, abundant M. chitwoodi second-stage juveniles were found in soil surrounding roots of own-rooted Cabernet Sauvignon. In this same study, the host status of nine rootstocks for M. chitwoodi was considered. Some of these rootstocks were poor hosts for M. chitwoodi (USDA 6-19B, 10-23B, and 10-17A, and RS-2, RS-3, and Harmony) and some were moderate hosts (Ramsey, Teleki 5C, Freedom, and Harmony). From a Washington viticulture perspective, it appears that own-rooted Chardonnay is a poor host for M. chitwoodi, indicating that there should be minimal risk to planting new V. vinifera own-rooted vineyards into areas where M. chitwoodi is present. However, if rootstocks are deployed, M. chitwoodi may be able to increase in population density, depending upon rootstock selection. The impact of M. chitwoodi on vine productivity is unknown.
Conclusions
This is the first comprehensive greenhouse evaluation of the host status of many commercially available Vitis root-stocks for M. hapla. Our results indicate that many rootstocks are poor hosts for M. hapla. These results were confirmed when Vitis rootstocks were challenged with four different populations of M. hapla collected from vineyards in Oregon and Washington. It was also found that M. hapla populations varied in reproductive potential and virulence on V. vinifera, and that own-rooted Chardonnay and the rootstock Matador were not hosts for M. chitwoodi. While rootstocks resistant to Meloidogyne spp. in greenhouse experiments also showed resistance in the field (Stirling and Cirami 1984), the next step in this research is to establish field evaluations of Vitis rootstocks in Washington to determine if similar results are obtained to those reported here.
Acknowledgments
The authors thank Duncan Kroese and Mariella Ballato for technical assistance. This research was funded, in part, by the Washington Grape & Wine Research Program and USDA-ARS CRIS project #2072-12220-004-00D. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.
- Received March 2018.
- Revision received July 2018.
- Accepted July 2018.
- Published online January 2019
This is an open access article distributed under the CC BY 4.0 license .
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