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Keywords:

  • Daucus carota var. sativa;
  • fungicide;
  • persistence in soil;
  • Pythium sulcatum

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Metalaxyl was used to control Pythium diseases of carrots in experiments on farms with a history of cavity spot. The first experiment compared the method of application (sprayed, banded or broadcast) and rate (0, 1.5, 3 or 6 kg a.i. ha−1) one week after sowing. Three additional experiments compared the rate (0, 0.75, 1.5 or 3 kg a.i. ha−1) and time (sowing, 1- to 2- or 4- to 5-true-leaf stage) of application. In expt 1, the application of metalaxyl, but not the method by which it was applied, increased yield by 20% and significantly reduced the incidence of cavity spot, forking and misshapen carrots. In expts 2, 3 and 4, neither the rate nor time of application affected yield or reduced the incidence of Pythium diseases. Comparison of the sites showed that they differed in past metalaxyl usage. Metalaxyl had not been used on the site of expt 1, but had been used previously at sites 2, 3 and 4. Laboratory experiments were conducted to determine whether these differences in efficacy resulted from reduced sensitivity of Pythium isolates to metalaxyl, or reduced persistence of metalaxyl in soil. ED50 values showed that there was no reduction in metalaxyl sensitivity. The half-life of metalaxyl was 82 days in soil from expt 1, but was 10 days or fewer in soils from expts 2, 3 and 4. Thus the failure of metalaxyl to control Pythium diseases was associated with reduced persistence in soil, not reduced sensitivity of the target fungi.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Carrots are the most important horticultural export from Western Australia (WA), with a value of $32 million in 1997/98 ( Anon, 1998). They are grown with irrigation on coarse, sandy soils in coastal areas in the south-west of WA. Carrots are grown all the year round, maturing in 4–6 months depending on the time of year. Marketable yields average 50 tonnes ha−1. Their value, the high level of mechanization, and lack of suitable, alternate crops mean that carrots are often grown without rotation. Export markets are expanding, and this increasing demand can be met by increasing the area under production and increasing marketable yield.

One of the most important constraints on carrot production on existing sites is the Pythium disease, cavity spot. A survey of over 200 carrot crops carried out in 1990/91 showed that cavity spot reduced marketable yield by more than 10% in 16% of the crops ( Galati & McKay, 1996).

Cavity spots are sunken, brown, circular to elliptical lesions 1–10 mm wide, sometimes surrounded by a pale halo. The slow-growing species P. violae and P. sulcatum are the most important causal organisms in Europe, Japan and North America ( Nagai et al., 1986 ; White, 1986; Montfort & Rouxel, 1988; Vivoda et al., 1991 ; Breton & Rouxel, 1993; Benard & Punja, 1995). In WA, P. sulcatum is the most important species ( Davison & McKay, 1998), although P. coloratum has also been implicated ( El-Tarabily et al., 1996 ).

Other Pythium diseases of carrots include damping off, which results in lower root numbers at harvest, and root dieback, resulting in forked and misshapen carrots ( White, 1986; Liddell et al., 1989 ).

The systemic fungicide metalaxyl is effective at controlling both root dieback and cavity spot when applied shortly after sowing ( Lyshol et al., 1984 ; Wheatley et al., 1984 ) or during the life of the crop ( Walker, 1988; Walker, 1991). In WA, however, it has not consistently controlled these diseases ( Galati & McKay, 1996). In six field experiments, metalaxyl was applied in different combinations of rate, time after sowing and time of year, but the incidence of cavity spot was reduced in only one experiment.

In this paper, field experiments on farms where metalaxyl was applied to control cavity spot at different rates, by different methods and at different times after sowing are reported. As the results from these experiments were also inconsistent, laboratory experiments that sought to determine whether these inconsistencies resulted from decreased sensitivity ( Federation of British Plant Pathologists, 1973) of local isolates of P. sulcatum to metalaxyl, or to reduced persistence of metalaxyl in soil, are reported.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Field experiments

All experiments were carried out on farms with a history of cavity spot, 100 km north of Perth. Site characteristics are shown in Table 1. In all experiments, seeds of carrot cv. Ivor (Clause Seeds, France) were sown in four double rows per raised bed to give a target density of 60 plants m−2. The experiments were managed as part of a commercial crop with daily irrigation when evaporation exceeded rainfall. For each crop, total applied nitrogen ranged from 215 to 320 kg ha−1, total phosphorus ranged from 80 to 120 kg ha−1, and total potassium ranged from 240 to 460 kg ha−1. Sowing dates are shown in Table 1. Experimental plots were 4 m long.

Table 1.  Site characteristics and crop parameters of the four experiments Thumbnail image of

Experiment 1 was a factorial combination of three methods of application − (i) sprayed as Apron 350SD (Novartis Crop Protection Australasia Limited) (350 g a.i. metalaxyl kg−1); (ii) broadcast as Ridomil 50G (Novartis Crop Protection Australasia Limited) (50 g a.i. metalaxyl kg−1); (iii) banded along rows as Ridomil 50G − by four rates: 0, 1.5, 3.0 or 6.0 kg a.i. metalaxyl ha−1. Apron 350SD was applied by boom spray with fan nozzles (Hardi 4110–12) applying 250 L water ha−1 at 1 bar; matching controls were sprayed with water only. Ridomil 50G was applied mixed with dry yellow sand; matching controls were treated with sand only. Fungicide treatments were applied 1 week after sowing and each treatment was replicated four times.

Experiments 2, 3 and 4 were factorial combinations of three times of application − (i) at sowing; (ii) at the 1- to 2-true-leaf stage; (iii) at the 4- to 5-true-leaf stage when the roots were starting to swell − and four rates: 0, 0.75, 1.5 or 3.0 kg ha−1. Application times were 5 and 10 weeks after sowing in expt 2, 4 and 8 weeks after sowing in expt 3, and 3 and 6 weeks after sowing in expt 4. Metalaxyl, as Apron 350SD, was applied by boom spray with fan nozzles applying 243 L ha−1 at 1 bar; controls were sprayed with water only and each treatment replicated four times.

At harvest, the carrots from two 1-m lengths of row (representing areas of 0.91–0.96 m2) from the middle rows were hand-harvested, machine-washed, and then stored at 1°C for up to 7 days before assessment. The mean number of carrots sampled per plot was 52 in expt 1, 64 in expt 2, 60 in expt 3, and 66 in expt 4.

The harvested carrots were separated into those with and those without cavity spot. Each group was assessed by weight into the following categories: export marketable (> 150 mm long, 25- to 50-mm crown diameter), short marketable (120–150 mm long, 25- to 50-mm crown diameter), undersized (< 120 mm long, or < 25-mm crown diameter), oversize (> 50-mm crown diameter), forked, misshapen, or split.

Fungal isolations were made at the final harvest from all cavity spots from a subsample of 20 carrots from each plot in the 0 and 3.0 kg ha−1 treatments in each experiment. Isolations were made onto Pythium-selective agar ( Davison & McKay, 1998). The number of spots sampled ranged from 67 in expt 3, to 303 in expt 4.

Sensitivity of P. sulcatum to metalaxyl

A preliminary experiment using metalaxyl at 0, 5, 50 and 100 μg mL−1 in corn meal agar (CMA) showed that the ED50 was less than 5 μg mL−1. A dilution series of 0, 2.5, 5, 10 and 20 μg mL−1 metalaxyl in CMA was used for the main experiment ( White et al., 1988 ). A Perimatic GP pump plate pourer (Jencons [Scientific] Ltd) was used to pour 9-cm plates of uniform thickness.

Hyphal tip cultures of P. sulcatum from carrots from five carrot farms in WA and from field expts 1, 2, 3 and 4 were used. The number of isolates is shown in Table 4. A 5-mm disc cut from the margin of a 2-week-old culture of the test fungus was placed at one side of each plate, and the plates incubated at 20°C. There were 5 replicate plates for each metalaxyl concentration. The colony radius was measured after 3, 6, 9 and 13 days. The daily growth rate for the 6- to 9-day period, when radial growth was fastest, was used for calculating ED50 values.

Table 4.  Comparison of ED50 values for metalaxyl for Pythium sulcatum isolates from different locations. Thumbnail image of
  • a

    aED50 values with the same letter do not differ significantly (P > 0.05).

  • Persistence of metalaxyl in soil

    Soil was collected from the exact location of the four experimental sites in March 1998. A bulked sample of about 5 kg of soil was collected from the top 15 cm of the profile, using a 2-cm internal diameter soil corer. Samples were stored for 2 weeks at 4°C until required.

    The moisture content of all soils was adjusted to 8%, approximately field capacity. The equivalent of 1 kg oven dry weight of each soil was dispensed into a 2-L wide-necked, screw-topped glass jar. Metalaxyl solution (1 mL, 0.857% Apron 350SD) was added to each jar to give a final concentration of 3 mg metalaxyl per kg of soil. There were three replicate jars for each soil. The jars were sealed with a screw-top lid, and the contents mixed overnight on a tumble mixer.

    The following day, the lids were replaced with new lids which had a 3-cm hole sealed with glass-fibre paper to allow gas exchange. A 100-g subsample was removed from each jar, frozen and stored at −20°C. The jars were incubated at 20°C in the light. Additional subsamples of soil were removed 8, 15, 22 and 37 days after the addition of metalaxyl, frozen and stored at −20°C.

    The metalaxyl content of the soil samples was determined by acetone extraction followed by dual capillary column gas chromatography with thermionic detection ( Caverly & Unwin, 1981).

    Statistical analysis

    The results from the field experiments were analysed by analysis of variance ( anova) with GENSTAT 5 (1993), using appropriate transformations if the data was heteroscedastic.

    ED50 values for P. sulcatum isolates were calculated in the following manner. An exponential curve for mean daily growth rate against metalaxyl concentration was fitted for each isolate using GENSTAT 5 (1993), and the parameters A, B and R were used in the following calculation:

    inline image

    where C is − ln R. This curve reaches a maximum of A + B at a concentration of zero. ED50 was defined as the concentration at which growth is reduced by half to (A + B) / 2, i.e.

    inline image

    or

    inline image

    General anova was used to compare ED50 values from the different experiments.

    The proportion of metalaxyl remaining in the soils was calculated for each harvest time. An exponential curve was fitted to these values using GENSTAT 5 (1993). The half-life of metalaxyl in the different soils was calculated in a similar way to the calculation of ED50 values, using the formula:

    inline image

    where A, B and R are values from the fitted curve, and C is −ln R.

    Results

    1. Top of page
    2. Abstract
    3. Introduction
    4. Materials and methods
    5. Results
    6. Discussion
    7. Acknowledgements
    8. References

    Field experiments

    In expt 1, the application of metalaxyl, but not its method of application, significantly increased the total weight (per 2 1-m lengths of row) and the proportion of exportable yield ( Table 2). The estimated total weight increased by 20%, from 53.8 to 64.8 tonnes ha−1, and exportable yield (irrespective of cavity spot) increased from 28.7 to 56.0 tonnes ha−1. Metalaxyl also decreased the proportion of forked, misshapen and cavity spot-affected carrots ( Table 2). There was no difference (P > 0.05) between the 1.5 and 6.0 kg a.i. ha−1 treatments ( Table 2).

    Table 2.  Experiment 1: comparison of the method and rate of application of metalaxyl on the weight and grade of carrots, irrespective of cavity spot, and proportion of carrots with cavity spot irrespective of grade. Back-transformed values are presented Thumbnail image of
  • a

    ***P < 0.001; **P < 0.01; *P < 0.05; NS: P > 0.05.aComparison of weights was made on log transformed data.bComparisons of proportions were made on angular transformed data.cValues within each column with the same letters do not differ significantly (P > 0.05).

  • In expts 2, 3 and 4 there was no effect of either the rate or time of metalaxyl application on the weight, grade or incidence of cavity spot ( Table 3).

    Table 3.  Experiments 2, 3 and 4: comparison of the rate and time of application of metalaxyl on total weight and incidence of cavity spot. Back transformed values are presented Thumbnail image of
  • a

    aComparisons of weights were made on log-transformed data.bComparisons of proportions were made on angular-transformed data.

  • Pythium isolations were attempted from cavity spots from a subsample of the harvested carrots in the 0 and 3 kg ha−1 treatments. Pythium spp. were isolated from between 60 and 81% of a total of 789 spots. All of the isolates in expts 2, 3 and 4 and all but one of the isolates from expt 1 were P. sulcatum.

    Sensitivity of P. sulcatum to metalaxyl

    A comparison of mean ED50 values shows that the highest value was from the site of expt 1, where metalaxyl had not been used previously, while the lowest value was from site 2, where metalaxyl had been used before ( Table 4). The upper ED50 values do not show any isolates with low sensitivity to metalaxyl ( Table 4).

    Persistence of metalaxyl in soil

    The half-life of metalaxyl varied among sites. It was 82 days in soil from the site of expt 1, and 10 days or fewer in soil from the sites of expts 2, 3 and 4 ( Table 5).

    Table 5.  Estimated half-life of metalaxyl in soil from the experimental sites Thumbnail image of

    Discussion

    1. Top of page
    2. Abstract
    3. Introduction
    4. Materials and methods
    5. Results
    6. Discussion
    7. Acknowledgements
    8. References

    Lyshol et al. (1984 ) and Sweet et al. (1989 ) showed that metalaxyl at rates between 0.2 and 1.2 kg a.i. ha−1 are effective in controlling cavity spot when applied at or shortly after sowing. It is likely that the target organism in their experiments was P. violae, the commonest cause of cavity spot in Europe ( White, 1988). In WA, cavity spot is caused by P. sulcatum ( Davison & McKay, 1998), a species that is less sensitive to metalaxyl than P. violae. White et al. (1988 ) and Breton & Rouxel (1993) give ED50 values of 2.8–9.0 μg mL−1 for P. sulcatum and 0.07–0.25 μg mL−1 for P. violae. WA isolates of P. sulcatum have an ED50 of between 1.8 and 4.6 μg mL−1 ( Table 4). These values indicate that a higher rate of metalaxyl might be required to control cavity spot in WA than in Europe.

    Field expt 1 showed that cavity spot and other Pythium diseases could be controlled with metalaxyl at the lowest rate, 1.5 kg a.i. ha−1 ( Table 2). There was no indication of increasing efficacy with increasing rate of fungicide applied. In the other experiments, metalaxyl was ineffective ( Table 3), but the reason for this appears to be associated with its poor persistence in these soils ( Table 5). Kookana et al. (1995 ) examined the persistence and mobility of pesticides in Karrakatta sand, the soil type at sites 1, 2 and 3. They showed that the half-life of metalaxyl was 10 weeks, similar to the calculated half-life in soil from the site of expt 1, but much longer than its half-life in soils from the sites of the other experiments ( Table 5).

    When fungicides are applied to soil, they can be lost from the surface horizons by leaching, chemical transformation or microbial decomposition. Enhanced biodegradation of a fungicide may occur if it is used repeatedly ( Walker, 1993), and Bailey & Coffey (1985) and Droby & Coffey (1991) showed that metalaxyl is subject to such a process. The history of metalaxyl usage is the main difference between the sites used for the field experiments ( Table 1); between 1989 and 1995, metalaxyl was used 18 times on site 2, while it was used six times between 1990 and 1994 on site 3. Details of its usage on site 4 are not available. As metalaxyl has been used frequently in the past, enhanced biodegradation is the most likely cause of its poor persistence on sites 2 and 3.

    These experiments show that although metalaxyl can be a very effective way of reducing cavity spot and other Pythium diseases of carrots in WA, it is most likely to be effective on sites where it has either not been used previously or has been used infrequently. It may not offer a long-term solution for growers.

    Acknowledgements

    1. Top of page
    2. Abstract
    3. Introduction
    4. Materials and methods
    5. Results
    6. Discussion
    7. Acknowledgements
    8. References

    We thank WA carrot growers for allowing us to conduct experiments on their properties and collect soil samples, and the Chemistry Centre (WA) for mixing the soil and carrying out the metalaxyl analyses. We thank J. Speijers for statistical advice, D. R. Mueller, P. Murphy and R. A. Deyl for technical assistance and J. G. White for helpful discussions throughout this work. Funding from the Horticultural Research and Development Corporation is gratefully acknowledged.

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    1. Top of page
    2. Abstract
    3. Introduction
    4. Materials and methods
    5. Results
    6. Discussion
    7. Acknowledgements
    8. References
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