Further observations on the enhanced degradation of iprodione and vinclozolin in soil

Pesric. Sci. 1987, 21, 219-231

Further Observations on the Enhanced Degradation of Iprodione and Vinclozolin in Soil

Allan Walker

Institute of Horticultural Research, Wellesbourne, Warwick CV35 9EF, UK

(Revised manuscript received 14 April 1987; accepted 21 April 1987)

ABSTRACT

The effects of soi lpH on rates of degradation of iprodione and vinclozolin were measured in a silty clay loam soil. Little degradation of either fungicide occurred a t p H 4.3 or 5.0, and degradation at p H 5.7 was slower than at p H 6.5. I n both of the higher-pH soils, the rate of loss of a second application of either fungicide was faster than that of the first, and a third application degraded even more quickly. In soil with p H 6.5, f o r example, the times f o r SO% degradation of iprodione following the first, second and third applications were about 30, 12 and 4 days, and f o r vinclozolin were 30,22 and 7 days respectively. Iprodione degraded very rapidly in a sandy loam that had been treated three times previously with this fungicide and also degraded rapidly in the same soil pretreated three times with vinclozolin. Vinclozolin degraded rapidly in the vinclozolin pre-treated soil, but its rate of loss in the iprodione pre-treated soil was only slightly faster than in the previously untreated control. Studies of iprodione degradation in 33 soils f r o m commercial fields demonstrated a clear trend towards faster rates of loss in soils with an extensive history of iprodione use. The time f o r 90% loss f rom previously untreated soils varied f r o m 22 to 93 days. I t varied f r o m 16 to 28 days in soils treated once previously and f r o m 5.2 to 23 days in soils treated twice previously. In soils that had received three or more previous doses, the time to 90% degradation varied f rom 3.8 to I5 days.

1 INTRODUCTION

Previous experiments with the dicarboximide fungicides iprodione (3-(3,5-dichlorophenyl)-N-isopropyl-2,4-dioxoimidazolidine-carboxamide) and vinclozolin ((RS)-3-(3,5-dichlorophenyl)-5-methyl-5-vinyl-l,3-oxazolidine-2,4-dione)

219

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220 A. Walker

have shown that their rates of degradation are more rapid in soils treated previously with the particular fungicide than in previously-untreated This phenomenon, which occurs with a number of other soil-applied pesticides, is known as ‘enhanced’ or ‘accelerated’ d e g r a d a t i ~ n , ~ ~ and is thought to be associ- ated with changes in the soil microbial p o p u l a t i ~ n . ~ . ~

There is evidence that degradation of both iprodione and vinclozolin in soil is pH dependent,1s2-6 and some of the earlier results concerning their enhanced degradation were inconclusive because of differences in soil pH between treatments.’ One objective of the present experiments was to examine further the way in which enhanced degradation of the fungicides is influenced by soil pH. Because of the similarity in their chemical structure, it is possible that similar groups of soil micro-organisms may be responsible for degrading both compounds. A further experiment was therefore made to determine whether soils which exhibit enhanced degradation of either compound show cross-enhancement with respect to degradation of the other.

The primary use of iprodione and vinclozolin as soil treatments is for control of white rot disease (Sclerotium cepivorum) of onions. As discussed previously,? although enhanced degradation of the fungicides can be induced by repeated application to soil in the laboratory or the field, current evidence indicates few problems from lack of disease control in practice. In order to determine the extent to which enhanced degradation has been induced in commercial field soils, and hence the potential for a change in biological performance, samples of soil with known histories of iprodione use were collected from 33 different sites and the rate of iprodione degradation in the soils measured under standard laboratory conditions. This experiment was restricted to iprodione because this fungicide has been available commercially for a number of years, whereas vinclozolin was introduced commercially only recently.

2 EXPERIMENTAL METHODS

2.1 Enhanced degradation and soil pH

2. I . 1 Soils and fungicides Four soils were used in these experiments. They were taken from the top 10 cm of plots of a long-term liming experiment started in 1961 in Sawyers field at Rothamsted Experimental Station, Harpenden, Herts. The soils were of the same texture (silty clay loam) with 1.3% organic ~ a r b o n . ~ S o i l pH, measured using a glass electrode in a 1:2.5 suspension of soil in distilled water was 4.3,5.0,5.7 and 6.5 in the different samples. The average moisture content at an applied pressure of 33 kPa (0.33 bar), determined using a pressure membrane apparatus similar to that described by Heining,s was 14.3 (+0.55)%. The fungicides used were commercial wettable powder formulations of iprodione (‘Rovral’, 500 g a.i. kg-’) and vinclozolin (‘Ronilan’, 500 g a.i. kg-’), together with [14C-phenyl]-labelled samples of the two compounds. Specific activities were 20 mCi mmol-1 (iprodione) and 10.8 mCi mmol-1 (vinclozolin).

Enhanced degradation of iprodione and vinclozolin in soil 22 1

2.1.2 Incubation procedure Three subsamples (500 g) of each soil were air-dried overnight while further subsamples (25 g) were dried at 110°C to determine soil moisture content. [14C]iprodione dissolved in dichloromethane ( 5 ml containing 1 pCi) was added to one of the 500-g amounts of each soil and the solvent allowed to evaporate. A suspension of the commercial formulation of iprodione in water was then incor- porated into all twelve soil samples to give an initial concentration of 4.0 mg kg-I and soil moisture of 14.3%. The samples containing [14C]iprodione were divided into two equal amounts (approximately 250 g) and incubated in 500-ml capacity polypropylene containers at 20°C. The remaining two samples of each soil were incubated in 1-litre containers at the same temperature. Soil moisture contents were maintained by periodic additions of water as necessary. The whole procedure was repeated with a further three subsamples of each soil and vinclozolin. Immediately after sample preparation, and at intervals during the subsequent SO days, samples of soil (30 g) were removed from the treatments containing labelled iprodione and vinclozolin and stored at - 15°C until analysis.

One hundred days after the incubation began, all of the fungicide-treated soils that had not been sampled received a second dose of the appropriate fungicide. They were removed from their containers and spread separately in trays on the laboratory bench. They were weighed periodically and when 15 g water had evaporated, the mixing procedure using labelled and unlabelled fungicides as outlined above was repeated. This provided two replicate samples of each soil (250 g) containing [14C]-labelled plus unlabelled iprodione, two replicate samples of each soil containing labelled plus unlabelled vinclozolin, and one sample of each soil (500 g) for each fungicide containing unlabelled chemical only. The samples were incubated at 20°C as before, and the soils containing labelled fungicide were subsampled at intervals during the next 85 days. After a further 100 days (200 days from the start of the experiment), the remaining eight treated soils that had not been sampled received a third dose of the appropriate fungicide. They were removed from their containers, dried until 15 g water had evaporated, and prepared for re-incubation with the appropriate labelled plus unlabelled fungicide as before. The soils were incubated at 20°C and samples were once more taken at intervals during an SO-day incubation period.

Throughout the above experimental procedure, in order to prevent cross contamination between samples, all of the equipment used in preparation of the soil/fungicide mixtures, and in their subsequent subsampling, was either dispos- able and used once only, or steam-sterilised before and after use.

2.1.3 Extraction and determination of fungicide residues The subsamples of soil (30 g) which had been removed from each treatment were extracted with acetone+water (50 ml, 9+1 by volume) by shaking for 1 h on a wrist-action shaker. They were allowed to stand until the soil had settled when clear supernatant (25 ml) was removed. This was evaporated to near dryness on a vacuum rotary evaporator and transferred to a separating funnel with sodium sulphate solution (25 ml, 50 g litre-' in water) and dichloromethane (25 ml). The

222 A. Walker

samples were shaken and the dichloromethane layer removed. The extraction was repeated with further dichloromethane (25 ml) and the combined dichloromethane extracts were evaporated to dryness and the residue redissolved in ethyl acetate (10 ml). Duplicate subsamples (2 ml) of the ethyl acetate solutions were transferred to counting vials and toluene-based scintillation fluid added (10 ml, Scintran-T, BDH Chemicals Ltd, Poole, Dorset, UK). The samples were counted using a Rackbeta liquid scintillation counter and quench corrections were made by external standard calibration. The ethyl acetate extracts which remained following preparation of samples for counting were evaporated to dryness, redissolved in dichloromethane (0.1 ml) and spotted on to silica gel FZs4 precoated thin-layer plates. The plates were run for 10 cm above the baseline in a dichloromethane+acetone (99+ 1 by volume) solvent system, dried, and examined using a Birchover Instruments radiochromatogram spark chamber to locate the zones of radioactivity. These were scraped from the plates, dispersed in ethyl acetate (2 ml) in scintillation vials, scintillation fluid added, and the samples counted as before.

2.2 ‘Cross-enhancement’ of degradation

The above procedure of sequential incubations of soil with iprodione and vinclozolin was repeated with duplicate subsamples (500 g) of a sandy loam for each fungicide. The soil was taken from the surface 10 cm of Wharf Ground field at the Institute of Horticultural Research, Wellesbourne. It had an organic matter content of 2.5% (loss on ignition), pH 6.2 and 33 kPa soil water content of 12.8%. After three successive incubations with the commercial formulations of the fungicides at an initial concentration of 4.0 mg a.i. kg-’, each lasting 80 days, the soils were reincubated with labelled and unlabelled fungicides as before. There were four incubation samples (250 g) involving iprodione-pretreated soil, two containing labelled plus unlabelled iprodione, and two containing labelled plus unlabelled vinclozolin. Similarly, there were four incubation samples involving vinclozolin-pretreated soil, two containing vinclozolin and two containing iprodione. A further four samples (250 g) of previously untreated soil were prepared for incubation in the same way, two containing iprodione and two containing vinclozolin. All twelve samples were incubated at 20°C. Immediately after sample preparation and at intervals during the subsequent 60 days, subsamples (30 g) were removed from each treatment and the fungicide residues extracted and determined using the radioassay procedures described before (Section 2.1.3).

2.3 Iprodione degradation in soil samples from commercial fields

2.3.1 Incubation procedure Soil samples were collected from 33 fields with known histories of iprodione use. Twelve soils had not been treated previously with the fungicide, five had been treated once previously, four twice previously, three three times previously, and nine had received multiple previous applications in recent years. The location of the sites and the properties of the soils are given in Table 1. The soils within each pre-treatment category are listed in order of increasing organic matter content, Organic matter, pH, and soil water content at an applied pressure of 33 kPa were

Enhanced degradation of iprodione and vinclozolin in soil 223

TABLE 1 Soil Properties and Histories of Iprodione Use

Soil Site Soil texture Organic pH Water Previous iprodione use number location matter content

1 2 3 4 5 6 7 8 9

10 11 12

13 14 15 16 17 18 19 20 21 22 23 24

25 26 27 28 29 30 31 32 33

Oxfordshire Warwickshire Hampshire Warwickshire N. Yorkshire Lincolnshire Lincolnshire Lincolnshire Lincolnshire Lincolnshire Kent Hampshire

Lincolnshire Lincolnshire Lincolnshire Lincolnshire Lincolnshire Lincolnshire Lincolnshire Lincolnshire Lincolnshire Warwickshire Lincolnshire Lancashire

Kent Essex Kent Lincolnshire Kent Kent Lancashire Lancashire Lancashire

Sandy loam Sandy clay loam Silt loam Sandy loam Clay loam Silty loam Silty loam Silty loam Silty loam Silty loam Sandy clay loam Silt loam

Silty loam Silty loam Silty loam Silty loam Silty loam Silty loam Silty loam Silty loam Silty loam Sandy loam Silty loam Clay loam

Sandy clay loam Sandy loam Sandy clay loam Silty loam Sandy clay loam Sandy clay loam Sandy loam Sandy loam Sandy clay loam

(%)

1.49 2.12 2.43 2.49 2.86 3.39 3.41 3.50 3.66 3.82 3.94 6.66

3.33 3,45 3.46 3.49 3.61 3.28 3.41 3.43 3.50 2.49 3.69 7.47

2.55 3.13 3.50 3.67 3.69 3.69 3.74 3.84

10.09

.-

6.4 9.9 6.9 12.2 6.2 24.7 6.2 12.8 7.3 23.8 7.5 19.2 7.8 18.5 7.8 19.5 7.6 19.8 7.6 20.0 5.7 21.9 7.2 32.8

7.4 20.0 7.2 19.6 7.3 20.2 7.2 21.8 7.3 20.7 7.5 20.0 7.2 19.6 7.1 19.7 7.4 19.8 6.2 12.8 6.9 19.5 6-6 19.8

7.6 20.7 7,2 13.6 7-3 22.7 7.3 19.5 6.9 20.7 7.6 18.9 6.0 10.6 7.0 11.4 6.6 24.3

-__ _ _ None None None None None None None None None None None None

Once in: 1985 1983 1984 1982 1981

Twice in: 1985 1980, 1985 1983, 1984 1984. 1985

Three times in 1986 1981, 1984, 1985 Three times in 1986

More than three in past 5 years More than three in past 5 years More than three in past 5 years More than three in past 5 years More than three in past 5 years More than three in past 5 years More than three in past 5 years More than three in past 5 years More than three in past 5 years

determined as before. Duplicate subsamples (400 g) of each soil were prepared for incubation as described previously (Section 2.1.2). The fungicide used was the commercial wettable powder formulation of iprodione, the initial concentration was 8.0 mg a.i. kg-', and soil moisture was adjusted to the appropriate 33 kPa value (Table 1). All of the samples were incubated at 20°C and subsamples (30 g) were removed immediately after preparation and at intervals during the subsequent 95 days.

2.3.2 Extraction and determination of fungicide residues Residues of iprodione were measured by reversed-phase high performance liquid chromatography (h.p.1.c.) using a modified form of the method described by Cabras et aL9 The soil samples were extracted by shaking with acetone+water (50 ml, 9+ 1 by volume) as before and, following evaporation of clear supernatant

224 A . Walker

(20 ml) to near dryness and addition of sodium sulphate solution (50 ml, 50 g litre-' in water), the iprodione was partitioned to ethyl acetate (20 ml). A portion of the ethyl acetate solution (10 ml) was evaporated to dryness, and the residue redissolved in the mobile phase (10 ml, acetonitrile+water, 60+40 by volume). The concentrations of iprodione in the acetonitrile/water solutions were measured by h.p.1.c. using a Merck Hibar Lichrosorb RP18 ( 2 5 0 ~ 4 . 0 i.d.) column, 1.0 ml min-' flow rate, and an Altex-Hitachi variable U.V. detector at 210 nm. Retention time was approximately 6 min. Recovery of iprodione from soil samples fortified in the range 0.2-2.0 mg kg-' was 94( +5.5)% and the limit of detection was approximately 0.10 mg kg-I.

3 RESULTS AND DISCUSSION

3.1 Enhanced degradation and soil pH

The R , value for iprodione on the t.1.c. plates in the solvent system used was 0.45 and for vinclozolin was 0.90. These were the main zones of radioactivity associ- ated with the zero-time soil extracts. Other zones of radioactivity were located on the chromatograms prepared from the soil extracts at the other sampling times although their relative importance varied considerably between treatments. With iprodione, these additional zones were located at R , values of 0.20 and 0.75. The radioactivity at R, 0.20 was the principal degradation product during the early stages of degradation, and that at R, 0.75 was the main component when extensive degradation had occurred. With vinclozolin, the additional radioactive zones were located at R, values of 0.35 and 0.75, with the radioactivity at R , 0.35 predominant in the earlier samples, and that at R, 0.75 predominant at later sampling times in those treatments where degradation was extensive. The inter- mediate degradation products (those at R, 0.20 for iprodione and 0.35 for

100 2 E - 80 m e E

$ 60

r 6 40 E f $ 20

I=

L

u

01

u 01

0 2 0 4 0 60 80 I 0 0

(b)

20 40 60 80 100 Incubation time (days)

Fig. 1. Degradation of (a) iprodione and (b) vinclozolin in soil with pH (W) 4.3. (A) 5.0, ( A ) 5.7, and (0) 6.5.


vinclozolin) were not identified but the main degradation product which was common to both fungicides (R , 0.75), chromatographed in an identical manner to pure, unlabelled 3,s-dichloroaniline. It has been suggested previously that 3,5- dichloroaniline may be an important degradation product of several dicarboximide fungicide^,^^'" and the present results agree with this suggestion.

The effects of soil pH on iprodione and vinclozolin degradation are illustrated in Figs l a and l b respectively. The data show the amounts of extractable radioactivity that could be characterised as parent fungicide following thin layer chromatography (t.l.c.), and are expressed as percentages of the amounts recovered at time 0. The mean initial recovery of iprodione was 93*1(*2.33)% and of vinclozolin was 87.4( k0-91)%. The data indicate negligible degradation of iprodione at pH 4.3 or 5.0 during the 80 day incubation period, and only limited degradation of vinclozolin in these acid soils. At pH 5.7, both fungicides degraded more rapidly and the times for SO% loss of iprodione and vinclozolin were about 60 and 75 days respectively. Degradation was more rapid still at pH 6.5 with times for 50% loss of 30-35 days for both compounds. There is evidence that iprodione is prone to hydrolysis with increasing soil pH. For example, Cayley and Hide6 could not make realistic adsorption measurements of the fungicide by soils with pH greater than 7.9 because the chemical was degraded during the equilibration period. They also demonstrated that iprodione was degraded completely within 14 h at pH 8.7. It is unlikely, however, that increased rates of hydrolysis with increasing soil pH can explain the present results with iprodione since studies in aqueous solution with pH in the range 3-8 have indicated that significant hydroly- tic degradation occurs only when the pH is greater than 7 (May and Baker Ltd, personal communication). It is probable, therefore, that the present results can be explained by differences in microbial activity in the different soils. The implica- tion is that soil bacteria may be particularly important since their activity is generally reduced when soil pH falls below 6 whereas the activities of soil fungi are often less affected by low soil pH." The results from the study of degradation following repeated application to the different soils also indicate a significant microbial involvement. When the soils with pH 4.3 and 5.0 were treated with the appropriate fungicide for a second time after 100 days and for a third time after 200 days, there was no change in degradation rate from that observed following the initial fungicide application, and little degradation of either fungicide occurred. However, in the other two soils, there was a progressive increase in degradation rate with successive doses of either iprodione (Figs 2a and 3a) or vinclozolin (Figs 2b and 3b). In the soil with pH 5.7, the times for 50% degradation of iprodione following the first, second and third applications were approximately 60,25 and 5 days respectively (Fig. 2a), and for vinclozolin they were 70, 30 and 6 days respectively (Fig. 2b). In the soil with pH 6-5, similar times for 50% degradation of iprodione were 30, 12 and 4 days for the first, second and third doses respectively (Fig. 3a), and for vinclozolin, they were approximately 30,22 and 7 days (Fig. 3b). These results are in general agreement with those reported previously1.2 and demonstrate significant enhancement of degradation rates by repeated application of the fungicides to soil. They further illustrate the problem of limited degradation in soils of low pH, and provide additional evidence that in

226 A. Walker

$ 80-

I 1 I I l 2b 40 ’ 60 80 100 0 20 40 60 80 100 Incubation time (days).

60

OI - , LL s o

Fig. 2. Degradation of sequential applications of (a) iprodione and (b) vinclozolin in soil with pH 5.7. (O), first treatment: (H), second treatment; (A) , third treatment.

Incubation time (days)

Fig. 3. Degradation of sequential applications of (a) iprodione and (b) vinclozolin in soil with pH 6.5. (O), first treatment; (B), second treatment; (A). third treatment.

soils with pH above 5 - 5 4 , enhanced degradation is a common occurrence, a suggestion discussed again in Section 3.3.

3.2 ‘Cross-enhancement’ of degradation

The results from the experiments in which iprodione degradation was examined in vinclozolin pre-treated soil and vinclozolin degradation was examined in iprodione pre-treated soil are shown in Fig. 4. In previously-untreated soil, the time to 50% degradation of both fungicides was between 25 and 30 days. When iprodione was incubated in the iprodione pre-treated soil, its rate of loss was rapid, with 40% degradation in 1 day and over 95% degradation in 4 days. In a similar way, vinclozolin degraded very rapidly in the vinclozolin pre-treated soil with almost 80% degradation in 1 day and 90% degradation in 4 days. In both of these treatments, virtually all of the extractable radioactivity in the samples taken at 4 days and after, was located on the t.1.c. plates at the R , value of 3 3 -


Incubation time (days)

Fig. 4. Degradation of (a) iprodione and (b) vinclozolin in a sandy loam soil. (A) , previously untreated soil; (m). soil treated three times previously with iprodione; (O) , soil treated three times previously

with vinclozolin.

dichloroaniline (0.75), further confirming that this is an important degradation product of the fungicides in soil. When iprodione was incubated in the vinclozolin pre-treated soil (Fig. 4a), its rate of degradation was somewhat slower than in the iprodione pre-treated soil, but was still considerably faster than in the previously untreated control. The time for 50% loss was about 4 days and the time for 90% loss about 15 days. Vinclozolin degradation in the iprodione pre-treated soil (Fig. 4b), however, was comparatively slow and the times to 50% and 90% loss were about 18 and 38 days respectively. The change in degradation rate from that in previously untreated soil was relatively small. In a recent review of the enhanced degradation of herbicides in soil, Roethl* concluded that such differences in degradation rate in studies of cross-enhancement are not unusual. He commented that when compounds within the same family are tested, degradation is often faster in a soil previously exposed to any member of the family than in previously- untreated soil, but that the degradation rate is usually slower for the ‘cross- challenge’ compound than for the ‘history’ compound. The extent of the differ- ence depends upon the specific compound being tested. Results similar to those in Fig. 4 were obtained previously for the related carbamate herbicides EPTC and butylate.” EPTC degradation was fully enhanced by prior treatment of soil with EPTC or butylate, whereas butylate degradation was enhanced by prior treatment with butylate but only slightly affected by prior treatment with EPTC. There are, therefore, no simple relationships where cross-enhancement is concerned, and it seems probable that the only way to understand such phenomena fully will be to isolate the specific organisms and enzyme systems responsible for degradation.

3.3 Iprodione degradation in soils from commercial fields

There was considerable variation in the rate of iprodione degradation among the 33 soils examined. In some of them, no parent compound was extracted after incubation for 7 to 11 days, whereas, in others, significant amounts were still

228 A. Walker

present after 95 days. In order to make comparisons between degradation rates in the different soils, the results were evaluated by curve fitting procedures. The data from 26 of the 33 soils gave good fits to the exponential decay equation:

C, = C,, e-kr (1) where C,is the residual concentration at time t, C, is the initial concentration and k the rate constant. Appropriate data were fitted to this equation by linear regres- sion analysis of the logarithms of the residual concentrations against time of incubation. The correlation coefficient ( r ) was always greater than 0.97 and statistically significant at P~0.001. The degradation data which did not fit eqn (1) (those for soils 1, 2, 3, 4, 6, 9 and 11 in Table 1) showed an initial period of relatively slow degradation, similar to the lag-phase described previously for other compounds,12.14 followed by more rapid rates of loss. The results were best described by an equation of the form:

C ,=a -b ekr (2) where a and b are constants. Appropriate data were fitted to eqn (2) using a maximum likelihood computer program. Examples of data which fitted eqns (1) and (2) are shown in Figs 5a and 5b respectively. The residual concentrations, on a logarithmic scale, are plotted against time of incubation. The data in Fig. 5b for soil 11 illustrate an apparent increase in the relative rate of degradation with increasing incubation time. This pattern of loss has been explained previously by the need for soil micro-organisms to adapt to the pesticide as substrate before rapid degradation can proceed.12.*4 In the present experiments, all of the soils for which this type of degradation curve was observed. had no history of prodione use. This indicates that in these soils, the activity of the degrading micro-organisms was low initially, but increased with time of incubation. Degradation in all of the previously-treated soils was best described by the simple exponential rate equation (eqn 1) suggesting no change in degradation rate with time, and hence indicating that degrading organisms were already active in these soils. This

.z 2 0

E 10 - 1 L

T 7 - 7

0 10 20 30 40 0 20 40 60 80 100 Incubation time (days)

Fig. 5. Degradation of iprodione in (a) soil 14, Table 1 and (b) soil 11. Table 1 illustrating fit to (a) equation 1 , and (b) equation 2.


Fig. 6. Times for 90% degradation of iprodione (DTw, days) in 33 soils plotted on a logarithmic scale in ascending order. Pretreatment histories (Table 1) annotated as 0, 1 .2 ,3 and M to indicate zero, one,

two, three and multiple pretreatments respectively.

explanation is clearly an over-simplification since there were five soils with no known history of iprodione use (Nos 5, 6, 7 , 8 and 12 in Table 1) in which no noticeable lag-phase was observed. As suggested previously (Section 3.2), detailed study of the degrading micro-organisms might indicate the reasons for these apparently anomalous results.

In order to compare the rates of loss in the different soils, the appropriate equation of best fit to the degradation data for each soil was used to estimate the time to 90% loss (DT,) of parent fungicide. The results are shown in Fig. 6 in which the times for 90% degradation in the 33 soils are plotted in ascending order. The appropriate soil number (Table 1) and frequency of prior iprodione use is also indicated in the Figure. Although there were some marked variations in pH between the different soils, these had little apparent effect on the rates of iprodione degradation in this experiment. The pH of the soils showing the slowest and the most rapid rates of loss were similar (6.4 and 6.2 respectively). There was a clear trend, however, towards faster rates of degradation in the soils with an extensive pre-treatment history and much slower rates of degradation in the previously untreated soils (Fig. 6). This is illustrated further in Table 2 where the maximum, minimum and mean DT, is presented for each group of similar

TABLE 2 Summary of Iprodione Degradation Data for 33 Soils

Previous Number Time .for 90% loss (days) ~. treatments of soils Minimum Maximum Mean fSD

None 12 22 93 50 23.7 One 5 16 28 20 4.9 Two 4 5.2 23 17 8.2 Three 3 3.8 15 7.8 5.83 More than three 9 4.8 13 8.1 3.21

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pretreatment history soils. The mean time to 90% disappearance varied from 50 days in the soils which had not been treated previously with iprodione in the field to about 8 days in soils which had been treated previously on three or more occasions.

These results therefore demonstrate that enhanced degradation of iprodione in field soils following repeated use is a common phenomenon. The very fast rates of loss recorded in some of the soils which had received regular applications of the fungicide in recent years indicate a clear potential for loss of biological activity against the target organism. The information received with two of the soil samples suggests that this lack of control is beginning to occur in practice. With one of them (soil 26, Table 1 ; DT,,, 4.8 days) the comment was made that ‘the grower is having great difficulty controlling onion white rot’; with the second (soil 24, Table 1; DT,,, 5.1 days) the comment made was ‘the soil sample was taken from the vicinity of onion plants treated with the fungicide but showing symptoms of the disease’. Although the use of dicarboximide fungicides for the control of soil- borne diseases is very small compared with their use for control of foliar diseases, the potential loss of disease control is important for the individual growers concerned. The present results indicate that in the absence of effective methods to counteract enhanced degradation, it is essential to avoid repeated use of either iprodione or vinclozolin at the same site in successive seasons if the problem is not to become more widespread.

ACKNOWLEDGEMENTS

Thanks are expressed to Mr P. H. Nicholls, Rothamsted Experimental Station and to staff of the Agricultural Development and Advisory Service for providing most of the soil samples used in this work. Thanks are also given to May & Baker Ltd and BASF United Kingdom Ltd for providing samples of labelled and unlabelled fungicides. The technical assistance of Miss Pauline Brown and Miss Sarah Welch is gratefully acknowledged, as is the assistance of Mrs Kathleen Phelps with mathematical analysis of the data.

REFERENCES

1. Walker, A.; Entwistle, A. R.; Dearnaley, N. J. British Crop Protection Council Monograph No. 27, Soils and Crop Protection Chemicals, 1984, 117-123.

2. Walker, A.; Brown, P. A.; Entwistle, A. R. Pestic. Sci. 1986, 17, 183-193. 3. Obrigawitch, T.; Martin, A. R.; Roeth, F. W. Weed Sci. 1983,31, 187-192. 4. Kaufman, D. D.; Edwards, D. F. Proc. 5th lnt. Congr. Pestic. Chem. 1983,4,117-123. 5. Kaufman, D. D. ; Katan, Y .; Edwards, D. F.; Jordan, E. G. Beltsville Agric. Res. Cent.

Agricultiiral Chemicals of the Future. Symp. No. 8, (Hilton, J. L . , Ed.), Rowman and Allanheld, Totowa, New Jersey, 1984, pp. 437451.

6. Cayley. G. R.; Hide, G . A. Pestic. Sci. 1980, 11, 15-19. 7. Nicholls, P. H . ; Evans, A. A. Proc. 1985 Br. Crop Prot. Conf.-Weeds, 1,333-339. 8. Heining, B. J . Agric. Eng. Res. 1963, 8, 48-49.

Enhanced degradation of iprodione and vinclozolin in soil 23 1

9. Cabras, P.; Diana, P.; Meloni, M.; Pirisi, F. M.; Pirisi, R. J . Chromatog. 1983, 256,

10. Pirisi, F. M. ; Meloni, M. ; Cabras, P.; Bionducci, M. R. ; Serra, A. Pestic. Sci. 1986,17,

11. Stevenson, I. L. In: Chemistry ofthe Soil, (Bear, F. E., Ed.), Reinhold, New York,

12. Roeth, F. W. Reviews of Weed Science, 1986, 2 , 4 5 4 5 . 13. Wilson, R. G . Weed Sci. 1984, 32, 264-268. 14. Audus, L. J. PI. Soil 1951,3, 170-192.

176-181.

109-118.

1964, pp. 242-291.

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Further observations on the enhanced degradation of iprodione and vinclozolin in soil