ABSTRACT

A three-year field study was conducted to evaluate the resistance of mustard and oilseed rape to feeding damage from crucifer flea beetles, Phyllotreta cruciferae. Tests included 40 lines of Sinapis alba, Brassica carinata, B. juncea, B. rapa and B. napus. Entries were first grown in isolation tests in the field to standardize seed quality. Seedlots were harvested and sieved to obtain uniform-sized seed which was standardized in each species. Sized seeds were planted without chemical protectants in six-row plots at 200 seeds per 6.0m row. Flea beetle damage to the cotyledons, averaged over three test years, ranged from 20% to 70% depending on the entry. Damage was lowest in lines of S. alba and B. carinata and highest in lines of B. juncea and B. rapa. Tests identified lines in each species with superior antixenotic resistance to flea beetles. Seedling counts 21 days after planting (DAP) ranged from 30 to 120 seedlings per row. Counts were highest in lines of S. alba and lowest in lines of B. rapa and B. juncea. Lines in each species with the highest seedling establishment were deemed most tolerant to flea beetle damage. Rates of cotyledon/leaf growth 14-21 DAP and dry matter content 28 DAP also varied among entries. Growth rates were 5-15 times higher in lines of S. alba than lines of B. napus and B. rapa. Tolerant lines with the highest growth rates usually had the highest seed yield.

INTRODUCTION

The crucifer flea beetle, Phyllotreta cruciferae (Goeze), is a serious pest of mustard and oilseed rape in western Canada (Lamb 1989). After overwintering in non-crop areas, adults migrate into commercial fields and feed on the cotyledons, leaves and stems of young seedlings. Feeding damage results in seedling mortality, reduced seedling growth, delayed crop development, uneven maturity and lower seed yield (Putnam 1977; Lamb 1984). Crop rotation and biological agents provide limited control of flea beetles so producers are dependant on several methods of chemical control including seed treatments, granular insecticides and field sprays (Lamb 1984, 1989).

Host plant resistance has been proposed as an alternative to chemical control of flea beetles (Lamb 1989). Studies have shown that accessions of Sinapis alba, Brassica carinata, B. juncea, B. rapa and B. napus differ in their attractiveness or palatability to flea beetles, tolerance to feeding damage and growth rates (Putnam 1977; Lamb 1984, 1988; Bodnaryk and Lamb 1991; Palaniswamy et al. 1992, 1997; Brandt and Lamb 1994). The objectives of this study were to evaluate the resistance and tolerance of mustards and oilseed rapes to flea beetles and identify lines with superior agronomic attributes including seedling establishment, growth rate and seed yield.

MATERIALS AND METHODS

Field tests were conducted at Saskatoon in 1995, 1996 and 1997. Tests included nine lines of S. alba and B. carinata (test 1), 15 lines of B. juncea and B. rapa (test 2) and 17 lines of B. napus (test 3). Entries were grown in isolation tents in the field in 1994 to standardize seed quality. Seed was harvested and sieved through graded screens of 0.2mm increments to identify the most common seed-size fraction in each species (Table 1). Seeds from this fraction were planted without chemical protectants in six-row plots at 200 seeds per 6.0m row and 0.3m row spacing. Tests were replicated four times and employed a randomized complete block design (test 3) or randomized split-plot design (tests 1 and 2) with species as main plots and entries as subplots. Seeds were planted at 1.5-2.5cm depths with a four cone, double-disc drill during the second or third week of May.

The glucosinolate content of the seed was determined by gas chromatography using the modified procedure of Raney and Rakow (1995). Allyl isothiocyanate and benzyl glucosinolate were used as standards for Sinapis and Brassica species, respectively. Flea beetle damage to the cotyledons (n=20 seedlings/plot) was evaluated 21 days after planting (DAP) using a 10-point visual rating system (after Palaniswamy et al. 1992). Ratings corresponded to the percentage of cotyledon surface eaten by flea beetles. Seedlings along the centre rows of each plot were counted 21 DAP. To estimate rates of cotyledon-leaf growth, 10 plants were collected from each plot 14 and 21 DAP. Samples were transported on crushed ice to the laboratory, cleaned and dissected. Surface areas of the cotyledons and leaves were measured with a computer-based imaging system (Decagon Devices Inc.). Ten plants were collected from each plot 28 DAP to evaluate compensatory growth. After the roots were removed, samples were dried at 60EC for 5-7 days and weighed. The four centre rows of each plot were harvested at maturity with a small-plot combine to determine seed yield.

Table 1. Flea beetle damage (+SE) and agronomic attributes of lines of Sinapis alba and four Brassica species grown from sized seeds in 1995-1997.

RESULTS AND DISCUSSION

Antixenosis or non-preference to feeding damage has been shown to be an important factor in the resistance of Brassica and Sinapis species to flea beetles. In feeding bioassays conducted at the cotyledon stage, lines of S. alba and B. carinata had moderate to high antixenosis whereas lines of B. juncea, B. rapa and B. napus had little or no antixenosis (Bodnaryk and Lamb 1991: Palaniswamy et al. 1992, 1997). In the present study, flea beetle damage to the cotyledons of S. alba and four Brassica species ranged from 28% to 69% (Table 1). Damage was lowest in lines of S. alba and B. carinata, moderate in lines of B. napus and highest in lines of B. juncea and B. rapa. Results indicated that S. alba and B. carinata had the highest antixenosis whereas B. juncea and B. rapa had the lowest antixenosis. Seedling survival, which has been used to evaluate the susceptibility of seedlings to flea beetle damage (Lamb 1988), varied from species to species. Seedling counts 21 DAP ranged from 42 seedlings per row in B. rapa to 100 seedlings per row in S. alba. Counts indicated that B. rapa was the most susceptible to damage whereas S. alba was the least susceptible to damage.

Tolerance to feeding damage has also been shown to be an important factor in resistance of Brassica and Sinapis species to flea beetles. Evaluation of growth rates and dry matter content in damaged seedlings indicated that lines of S. alba and B. juncea were more tolerant to feeding damage than lines of B. rapa and B. napus (Bodnaryk and Lamb 1991; Brandt and Lamb 1994). In the present study, rates of cotyledon-leaf growth 14-21 DAP and dry matter content 28 DAP varied among species (Table 1). Growth rates and dry matter were 3.4 to 9.4 times higher in S. alba than Brassica species. Results indicated that lines of S. alba were the most tolerant to flea beetle damage. Among Brassica species, B. juncea was the most tolerant to damage whereas B. napus was the least tolerant to damage. However, seed yields were similar in B. napus, B. juncea and S. alba indicating that B. napus was able to compensate for damage later in development.

Flea beetle damage and agronomic attributes differed among lines of S. alba (Table 2). Flea beetle damage was nearly two times higher in an experimental low glucosinolate (GS) line than AC Pennant and Ochre. Differences in antixenosis among lines may relate to glucosinolates in the seed. Hydroxybenzyl glucosinolate or sinalbin has been shown to inhibit feeding in flea beetles at concentrations found in the cotyledons of newly emerged seedlings of Ochre (Bodnaryk 1991). Concentrations of hydroxybenzyl glucosinolate were substantially lower in seeds of the low GS line (5 Fmoles/g) than seeds of AC Pennant (147Fmoles/g) and Ochre (156 Fmoles/g). Therefore, lower antixenosis in the low GS line may be partly due to reduced concentrations of hydroxybenzyl glucosinolate in the seed and young cotyledons. Low and high glucosinolate lines also differed in their agronomic attributes and tolerance to flea beetles. Seedling counts, growth rates, dry matter content and seed yield were 1.3 to 2.8 times higher in Ochre and AC Pennant than the low GS line. If these attributes can be incorporated into low GS lines then development of S. alba as a canola mustard would improve agronomic performance and resistance to flea beetles.

Lines of B. carinata had similar damage (38+2%) and moderate antixenotic resistance to flea beetles. Concentration of sinigrin, the major glucosinolate in the seed, was marginally lower in Dodolla (73 Fmoles/g) than S67 (90 Fmoles/g) and C90-1088 (90Fmoles/g). Dodolla and C90-1088 had the highest seedling counts and were less susceptible to flea beetle damage than S67. Dodolla has the highest growth rate, dry matter content, seed yield and tolerance to flea beetle damage.

Flea beetle damage varied among lines of B. juncea ranging from 54% in AC Cutlass to 71% in J90-4253/1, an experimental line of canola mustard. Concentrations of sinigrin, the major glucosinolate in the seed, were higher in AC Vulcan (116 Fmoles/g) and Cutlass (107 Fmoles/g) than J90-4253/1 (1.0Fmoles/g). High glucosinolate lines were agronomically superior to the low glucosinolate line. Seedling counts, growth rate, dry matter content and seed yield were 1.5 to 2.4 times higher in Cutlass and AC Vulcan than J90-4253/1. Growth rates and dry matter also indicated that Cutlass and AC Vulcan were more tolerant to flea beetle damage than J90-4253/1.

Flea beetle damage was similar in lines of B. rapa and not correlated with glucosinolate concentrations in the seed. However, seedling counts ranged from 62 seedlings per row in Echo to 32 seedlings per row in IMP 8618, a double low alkenyl line. Growth rates, dry matter content and seed yield and tolerance to flea beetles were higher in Echo and Tobin than IMP 8618.

Flea beetle damage in lines of B. napus ranged from 47% in Topas to 55% in Bronowski. Damage was not correlated with glucosinolate concentrations in the seed. Seedling counts differed among lines ranging from 50 seedlings per row in Bronowski to 89 seedlings per row in Argentine. Counts indicated that Argentine and Topas were less suceptible to flea beetle damage than Westar and Bronowski. Bronowski had the lowest growth rate, dry matter content, seed yield and tolerance to flea beetle damage. Argentine had the highest growth rate, dry matter content and tolerance to flea beetle damage. Topas and Argentine had the highest seed yield.

Table 2. Glucosinolate content of seed, flea beetle damage and agronomic attributes of selected lines of Sinapis alba and four Brassica species in 1995-1997 ^a.

^a means within species followed by the same letter are not significantly different (LSD, p=0.05)

ACKNOWLEDGEMENTS

We thank Larry Mann and Mike Robinson for their assistance in the field tests and Phil Raney for conducting the glucosinolate analyses. The research was supported by a grant from the Canola Agronomic Research Program and was funded by the Saskatchewan Canola Development Commission and Alberta Canola Producers Association.

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