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The root-lesion nematode Pratylenchus penetrans (Cobb) Filipjev & Schuurmans-Stekhoven, an endoparasite, is highly damaging to numerous plants other than turfgrasses. In Canada, surveys of plant-parasitic nematodes have shown that root-lesion nematodes are commonly associated with turfgrass on golf courses (Fushtey and McElroy 1977; Simard et al. in press; Yu et al. 1998;). In Ontario, Yu et al. (1998) found four species of root-lesion nematodes, including P. crenatus Loof, P. fallax Seinhorst, P. neglectus (Rensch) Filipjev & Schuurmans-Stekhoven, and P. thornei Sher & Allen. A recent survey on 38 golf courses in Quebec and Ontario revealed the presence of P. crenatus, P. penetrans and P. thornei on greens, fairways and roughs (Simard et al. in press). Annual bluegrass (Poa annua L.) is a major turfgrass species on golf courses in Canada and the United States (Beard 2002; Royal Canadian Golf Association 2003). Currently, the pathogenicity of the root-lesion nematode to annual bluegrass is not well documented in the literature.

The root-lesion nematode is able to reproduce on various crops, including perennial ryegrass (Lolium perenne L.) (Bélair et al. 2002). Troll and Rohde (1966) reported the capability of this nematode to infect turfgrass species such as annual ryegrass (Lolium multiflorum Lam.), creeping red fescue (Festuca rubra L.) and Kentucky bluegrass (Poa pratensis L.). However, Townshend et al. (1973) showed that several cultivars of Kentucky bluegrass are not favourable to the reproduction of the root-lesion nematode. Consequently, Townshend et al. (1984) proposed that Kentucky bluegrass could be used as a cover crop to control root-lesion nematode populations in orchards. Yu et al. (1998) suggested that bluegrass turf species probably repress the development of lesion nematodes and can eventually eliminate them on golf courses.

This study was undertaken to assess the effect of various population densities of the root-lesion nematode on the growth of annual bluegrass under controlled conditions. In 2004, two growth chamber trials were carried out in the research facilities of Agriculture and Agri-Food Canada, Horticulture Research and Development Centre, in St-Jean-sur-Richelieu, Quebec. Both experiments were conducted with annual bluegrass collected on a golf course located in the Montreal area (45°23’N; 73°21’W) using a golf course hole cutter (10.8 cm diam, 7.7 cm deep). Annual bluegrass cores were cut to standardize core depth to around 2.5 cm in order to remove most of the golf course soil and preserve the integrity of the root system. Turfgrass cores were subsequently placed into pots (11 cm diam, 9.5 cm deep) filled with a 4:1 (v:v) mix of pasteurized sand and peat. A second experiment was performed using growth tubes (Tube RLC-7, 3.8 cm diam, 14 cm deep, Stuewe and Sons Inc., Corvallis, OR) containing the same sand:peat mix. Annual bluegrass tillers were then separated from a single core and transplanted individually in the tubes. In both experiments, annual bluegrass was grown for 1 wk prior to nematode inoculation. Annual bluegrass was evenly watered as needed and fertilized once per wk, with (20-20-20) (N-P2O5-K2O) + micronutrients, at N rate of 300 µg L‑1. The plants were maintained in a growth chamber at 20 ± 2°C and a 12-h photoperiod during the study.

The experimental design was a randomized complete block design with six treatments in both experiments, and each treatment was replicated 6 or 20 times for pots and tubes, respectively. Two treatments were included as controls: water only (spray volume of 75 mL per pot and 15 mL per tube), and oxamyl, a systemic nematicide formulated as Vydate® L, at 0.2 g a.i. m‑2 per application (spray volume of 5 mL per pot and 1.6 mL per tube). For the nematicide treatment, one application was made concurrently with the other treatments, and one more application was made 3 wk after nematode inoculation. The nematodes used in both trials were obtained from a pure culture of P. penetrans reared on tobacco cv. Delgold at 25°C in the greenhouse. The nematodes were inoculated at concentrations of 100, 500, 1000 or 5000 nematodes 100 cm‑3 soil using 75 mL per pot and 15 mL per tube water suspension. After 9 wk, the number of nematodes in the roots was recorded. The entire root system from each container was washed under running tap water, weighed, and placed in a misting chamber for a 2-wk extraction period at 22°C (Seinhorst 1950). Nematodes were counted using a stereomicroscope and expressed as numbers per pot or tube. Mowed clipping dry weight (Pot: wk 1 to 9; Tube: wk 5, 8 and 9) and visual assessment of turfgrass quality (Pot: wk 1 to 9; Tube: wk 2 to 9) were also evaluated weekly. Visual quality, including ratings for colour, density and uniformity of turfgrass in tubes or pots, was evaluated using a scale of 1 to 9, where 1 = dead turfgrass, 9 = best visual quality, and 6 = lowest acceptable quality rating. Analyses of variance followed by a protected LSD (P = 0.05) were performed to compare treatments for nematode counts, mowed clipping dry weight, and visual rating of quality (SAS Institute Inc. 1999). Results of both experiments showed that annual bluegrass is a host conducive to the development of the root-lesion nematode (Table 1). Nine wk post-inoculation, root P. penetrans populations had increased linearly with initial nematode concentrations in both experiments (Pot: F = 4.40, P < 0.01; Tube: F = 27.82, P < 0.0001) (Table 1). In the presence of 1000 and 5000 nematodes 100 cm‑3 soil, the quality of annual bluegrass was slightly reduced in tubes, and a significant reduction in top growth was recorded 5 wk after plant establishment in tubes (F = 2.53, P < 0.05). In pots, a similar effect from the highest nematode concentration was observed on the quality of annual bluegrass, with a significant reduction in top growth during the first (F = 2.96, P < 0.05) and second (F = 2.77, P < 0.05) wk after inoculation. However, 9 wk after transplanting, growth and quality of turfgrass were uniform for all treatments, with no significant difference from the controls. The similarity between the nematicide and water control treatments indicates that nearly zero nematodes were introduced on the planting material and that the observed population build-up in the inoculated treatments is due to nematode inoculation only.

Table 1

Effect of inoculation rate (per 100 cm3 soil) on Pratylenchus penetrans populations and on dry weight of clippings and visual ratings of Poa annua in two experiments

Treatment

Clipping weighta,b

(g)

P. penetrans/

pot or tubea,c

Visual ratinga,d

Colour

Density

Uniformity

Experiment 1: Pot

Water

0.97 (0.27)

< 1 be

8.5 (0.3)

8.7 (0.3)

8.6 (0.4)

Nematicide

1.04 (0.28)

0 b

8.5 (0.4)

8.7 (0.3)

8.5 (0.4)

100 p. penetrans

1.03 (0.26)

9 (4) b

8.6 (0.3)

8.7 (0.2)

8.7 (0.2)

500 p. penetrans

0.96 (0.27)

102 (26) b

8.5 (0.3)

8.7 (0.3)

8.6 (0.3)

1000 p. penetrans

0.96 (0.25)

385 (66) b

8.5 (0.3)

8.6 (0.3)

8.0 (0.4)

5000 p. penetrans

1.00 (0.28)

1426 (661) a

8.5 (0.3)

8.5 (0.2)

8.0 (0.3)

Experiment 2: Tube

Water

0.32 (0.12)

< 1 d

8.5 (0.2)

7.8 (0.4)

NRe

Nematicide

0.34 (0.12)

0 d

8.4 (0.2)

7.7 (0.3)

NR

100 p. penetrans

0.32 (0.13)

125 (60) cd

8.2 (0.2)

7.4 (0.4)

NR

500 p. penetrans

0.32 (0.13)

569 (60) c

8.3 (0.2)

7.5 (0.5)

NR

1000 p. penetrans

0.31 (0.12)

1310 (221) b

8.1 (0.2)

7.3 (0.4)

NR

5000 p. penetrans

0.28 (0.12)

2602 (427) a

7.8 (0.2)

7.0 (0.4)

NR
a

Data are means (± SE). Within a column, means followed by the same letter are not significantly different as determined by LSD test at P = 0.05. The absence of letters in a column indicates that there were no significant differences.

b

Means of dry clipping collected after each turfgrass mowing (pot = 9 times; tube = 3 times).

c

Means of 6 replicates (pot) and 20 replicates (tube) after 9 wk.

d

Means of 9 visual ratings (including colour, density and uniformity) for pots, and 8 (colour) and 4 (density) visual ratings for tubes.

e

No uniformity rating for tubes.

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Damage thresholds are extremely complex and difficult to establish because they vary with the type of soil, the season, the growth stage and vigour of the plant, and cultural practices. Moreover, nematodes nearly always live in communities made up of more than one species. Surveys performed on golf courses in Quebec and Ontario revealed the exact same situation in turfgrass (Simard et al. in press; Yu et al. 1998), so that one or several species present may be contributing to the damage.

Typically, nematodes are associated with a disease complex in which significant turfgrass injury is the result of a combination of stresses such as diseases, environmental stress, and/or parasitic nematodes (Beard 2002). Nematode lesions can be enlarged by fungi and bacteria that invade and multiply in the wounds (Powell 1971; Taheri et al. 1994). In Canada, wheat root rot was significantly more severe when P. neglectus occurred together with the fungus Rhizoctonia solani kühn (Benedict and Mountain 1956). Physiological changes occurring in plants infected with nematodes can enhance their susceptibility to attacks by fungi, whether pathogenic or non- pathogenic (Powell 1971). In Germany, Kemper (1966) observed severe root-lesion nematode damage on wheat in a sandy soil exhibiting a low pH. Usually, thresholds are lower in sandy soils than in heavy- textured soils that are more fertile and moisture-retentive. As with other plants, turfgrasses are more tolerant of a given nematode population level during favourable growth periods in spring and fall than during summer when grass is exposed to numerous physical stresses (Smiley et al. 2005). Shallow-rooted grass plants tend to be less tolerant than deeper- rooted plants. Damage threshold levels are considerably higher on grass that is well fertilized and watered, such as in the current study, than on grass that is under nutrient and/or moisture stress. Close mowing practices place severe physiological stress on turfgrass, thus substantially lowering threshold levels (Smiley et al. 2005). Further investigation is needed to establish the damage thresholds of P. penetrans on turfgrass species under different soil and environmental conditions, and also to determine the effect of P. penetrans on disease development under various turfgrass management programs in Canada.