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In Quebec, the root lesion nematode, Pratylenchus penetrans (Cobb) Filipjev and Schuurmans-Stekoven, is a common and dominant species in small fruit crops, including strawberry (Fragaria spp.) (Bélair and Khanizadeh 1994). The pathogenicity of P. penetrans on strawberry plants has been demonstrated under both controlled and field conditions (Morgan 1964; Townshend 1963; Townshend et al. 1966). The root lesion nematode has been shown to increase the symptoms and damage caused by Verticillium wilt (Verticillium dahliae Kleb.) (McKinley and Talboys 1979). This is even more drastic on cv. Jewell (F. ananassa Duch.), which is highly susceptible to Verticillium wilt (Maas et al. 1989). The withdrawal of numerous soil fumigants from the market, includ-ing dichloropropene, and the raising environmental awareness of growers generated the need for alternative management strategies for P. penetrans and Verticillium wilt in strawberry production. The purpose of this study was to monitor the impact of crop rotation on the management of root lesion density and its incidence on Verticillium wilt in a strawberry plantation.

The field experiment was conducted at the MP Vaillancourt Farm on Île d’Orléans, Quebec. The soil was sandy loam with a coarse texture (44% sand, 33% silt, 23% clay). In 2005, the following crop treatments were performed: corn (Zea spp.), canola (Brassica napus L.) cv. Hyola followed by white mustard (Sinapsis alba L.) cv. Caliente, oats (Avena spp.) (positive control), and forage pearl millet (Pennisetum glaucum L.) hyb. CFPM 101. On May 15, 2005 oats, canola and white mustard were seeded at a rate of 100, 6, and 15 kg ha-1, respectively. Forage millet was sown on June 15, 2005 at a rate of 10 kg ha-1. Each plot was 5.2 m wide and 20 m long. The experimental design was a complete randomized block with six replicates. The cruciferous plots were plowed up as follows: canola (late flowering stage, early silica formation) at the end of July and white mustard during the first week of October. In spring 2006, all plots were rototilled and 1.5-m plant beds were made in the middle of each plot. Strawberry plants cv. Jewel were transplanted in double rows (30-cm rows, 40 cm between the rows) in early May 2006. A 1.5-m band of oats between the blocks and plots was planted and mowed as necessary between the plots to ensure vegetation cover on the ground. Total mineral fertilization was carried out according to the reference grids of the different crops in the Fertilization Reference Guide (CRAAQ 2003). Weeds were mechanically and manually controlled. Soil samples for nematode counts were taken on June 15 and September 15, 2006. Verticillium wilt symptoms were assessed on August 28, 2006 using the following index: Verticillium wilt (1 = dead plant, 5 = normal). Canopy/strawberry growth was monitored by count-ing the total number of crowns and daughter plants per 3-m row on October 11, 2006.

On June 15, 2006, nematode populations (P. penetrans) were significantly higher in plots grown in corn or oats the previous year (ANOVA: p = 0.05) (Table 1). The lowest nematode density was found in previous forage pearl millet and cruciferous crops. Nematode density recorded for both sampling dates generally was below 500 P. penetrans kg-1 soil, which is the known pest threshold for strawberry (Tremblay and Baker 1990). Nematodes had an impact on Verticillium disease symptom expression and plant stunting. Green manure of cruciferous plants and forage pearl millet reduced the incidence of Verticillium wilt and increased the growth of strawberry plants (Table 1). In October, the number of crowns and daughter plants was significantly higher following forage pearl millet or cruciferous than corn. The highest wilt symptoms and the lowest strawberry growth were observed in previous corn plots, which also harboured the highest spring populations of P. penetrans. These results support a positive interaction between P. penetrans and V. dahliae, and more importantly on susceptible cultivars such as Jewell (McKinley and Talboys 1979). It also supports the idea that the man- agement of P. penetrans density will have an impact on Verticillium wilt symptoms and damage. Sakuma et al. (2011) concluded that B. juncea L. as green manure could decrease damage caused by the root lesion nematode to subsequent crops although it had no effect on population, thus suggesting it could be used to improve soil conditions rather than be used as a control method. In two subsequent field trials in the same strawberry production area, the same Brassica green manure treatments failed to reduce P. penetrans density and had no significant impact on Verticillium wilt symptoms (G. Bélair, unpublished data). Because Brassica crops are also very good hosts for P. penetrans, this nematode multiplies on their root system before the plants are chopped and plowed under (Bélair et al. 2012). In Quebec, this procedure is performed from mid- to late July for the first crop, and from mid- to late October for the second crop. Further field trials are needed to confirm the efficacy of Brassica green manure for controlling Verticillium wilt and for improving growth in strawberry plants. Previously, forage pearl millet cv. CFPM 101 was shown to decrease P. penetrans density and increase potato yield in Quebec (Bélair et al. 2005). Thus, the results from the current trial would support the cropping of forage pearl millet as an alternative management strategy to control P. penetrans in the strawberry production system. Also, it is suggested to avoid corn as a rotation crop in strawberry fields with a history of Verticillium wilt problems. With the highest count of P. penetrans at strawberry planting, this crop generated the highest wilt symptoms and the lowest strawberry growth, even lower than oats, our standard positive host.

Table 1

Effect of cropping on P. penetrans density, Verticillium wilt symptoms and canopy growth of strawberry cv. Jewel in 2006

Effect of cropping on P. penetrans density, Verticillium wilt symptoms and canopy growth of strawberry cv. Jewel in 2006

* Values within the same column followed by the same letter are not significantly different (ANOVA: p ≤ 0.05; Tukey’s test: a ) Wilt = Verticillium wilt index (1 = dead plant, 5 = normal); b ) Canopy = total number of daughter plants and crowns per 3-m row); ns: values not significantly different (ANOVA: p > 0.05).

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