Abiotic characteristics of soils suppressive to Aphanomyces root
rot
Lars Persson
a,*, S. Olsson
ba
Plant Pathology and Biocontrol Unit, SLU, P.O. Box 7035, S-750 07 Uppsala, Sweden b
Department of Quaternary Geology, TornavaÈgen 13, S-223 63 Lund, Sweden
Abstract
Soils showing suppressiveness to Aphanomyces root rot of pea in bioassays and ®eld experiments were surveyed in an area intensively cultivated with vining pea in southern Sweden. By examining the relationships between disease suppression, soil mineralogy, and selected physicochemical properties of 24 soils with dierent degrees of suppressiveness, the suppressive soils could be divided into two groups, mainly on the basis of their textural characteristics. Both soil groups are developed in unsorted sediments. The ®rst group, S1, consisted of soils with a low content of clay (9±12%), and a high content of sand (56± 73%). The second group, S2, consisted of soils with a clay content of 19±21%, high pH (>6.7), and a high content of calcium (>17 cmol kgÿ1). The ratio of the peak intensity of vermiculite±smectite to the peak intensity of illite±kaolinite in the X-ray diractograms was high in these soils, and an increase in disease suppression was closely related to an increase in this ratio. There were also signi®cant correlations between disease suppression on one hand and content of clay, calcium and pH on the other. The results suggest that soils disease suppressive to Aphanomyces root rot can be found by searching for soils with speci®c abiotic characteristics.72000 Elsevier Science Ltd. All rights reserved.
Keywords: Aphanomyces euteiches; Pea; Disease suppression; Clay mineral; Clay content
1. Introduction
Aphanomyces euteiches Drechs. is a soil-borne pathogen that causes root rot of pea (Pisum sativum L.). There are no eective methods, such as plant re-sistance or fungicides, available for disease control. The only practical method for reducing losses is to avoid ®elds with a high infestation of the pathogen. Recent studies, however, have shown that soils strongly suppressive to the disease occur in southern Sweden. In these soils, disease severity remains low despite the presence of the pathogen and suitable cli-matic conditions for infection. The suppressiveness was measured in a bioassay and was con®rmed in ®eld
ex-periments with pea monocultures on suppressive and conducive soils (Persson et al., 1999). Suppressiveness to Aphanomyces root rot has been studied in only a few earlier investigations (e.g. Oyarzun et al., 1997), but soils suppressive to other diseases caused by, for example, Fusarium spp., Chalara elegans Nag Raj and Kendrick (synonamorph = Thielaviopsis basicola (Berk and Broome) Ferraris), Histoplasma capsulatum Darling, and Pythiumspp., have been found and inves-tigated in several places (Alabouvette et al., 1979; Scher and Baker, 1980; Stotzky and Martin, 1963; Stotzky and Post, 1967; Stutz et al., 1989). In several of these cases, various microorganisms appear to act as antagonists against the pathogen, but in addition, suppressiveness is often related to various physico-chemical properties of the soil (HoÈper and Alabouv-ette, 1996; Stotzky, 1986). Some investigations of disease-suppressive soils have revealed involvement of certain clay minerals in disease suppression (Stotzky
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* Corresponding author. Findus R&D, Box 520, S-267 25 Bjuv, Sweden. Tel.: +46-42-86683; fax: +46-42-81649.
and Martin, 1963; Stotzky and Post, 1967; Stutz et al., 1989). Amendments with clay minerals of the smectite group, such as montmorillonite, have been shown to enhance respiration of bacteria and to increase disease suppression in a conducive soil and also to increase the antagonism towards certain fungi by bacteria (Amir and Alabouvette, 1993; Rosenzweig and Stotzky, 1979; Stotzky 1966a,1966b, 1986; Stotzky and Rem, 1966). Further, soils suppressive to diseases caused by Phytophthora spp. and Pythium spp. often have larger contents of organic matter, and disease suppressiveness to these diseases can be induced by amendment with, for example, composts (Broadbent and Baker, 1974; Hoitink et al., 1996).
The objective of this investigation was to study poss-ible relations between the suppression of Aphanomyces root rot, as evaluated in a bioassay and/or in pea monoculture ®eld experiments, and selected physico-chemical properties of the soils tested.
2. Materials and methods
2.1. Soil sampling and assessment of disease suppression
The disease suppressiveness of soil samples collected from ®elds in the area of vining pea production in
southern Sweden was assessed in a bioassay, as pre-viously described by Persson et al. (1999). Based on preliminary results and observations showing a re-lation between disease suppression and soil type, soil samples were collected from 24 representative ®elds of the most common soil types in the area. Soils sampled within the study area have developed in Weichselian glacial deposits of various genesis (Fig. 1; Table 1). Water-deposited sediments of sand, silt or clay domi-nate on the lowland areas of region 1 (Fig. 1), whereas unsorted glacial sediments (tills) are dominant in the rest of the study area. The lithological character of the latter sediments is strongly controlled by the compo-sition of the bedrock, which is complex with a mosaic of dierent rocks at the surface of the bedrock. Inheri-tance from the local bedrock of Mesozoic, loose clay and sandstones has resulted in mainly sandy or silty, clayey tills (clay content 5±15%) and clay tills (clay content 15±25%) in the north-western part of the study area (3 in Fig. 1). Tills in the eastern part (2 in Fig. 1) normally have lower clay content and high fre-quencies of Palaeozoic shales and acid magmatic rocks. The south-western part (4 in Fig. 1) is domi-nated by calcareous clay tills with a high admixture of Cretaceous and Tertiary chalk and limestones from the local bedrock.
Soil samples were collected in the summer. Fields with a high natural infestation of A. euteiches were avoided if possible, to avoid in¯uence on the results of the bioassay. About 10 subsamples, taken to a depth of 20 cm and mixed to give a general sample, were col-lected from an area of 55 m. Samples were stored in plastic bags at 48C and used in the bioassay within six days. In the assessment of disease suppression, the soils were inoculated with a dry oospore±talcum inocu-lum (800±1000 oospores mlÿ1of soil), giving a disease severity index (DSI) of about 75 in a conducive refer-ence soil, as described by Persson et al. (1999). The inoculated soils were incubated in a growth chamber for seven days, then sown with 10 pea seeds of the cul-tivar ``Tristar'' treated with metalaxyl (Apron 200 LS, 20% a.i.) and watered daily to give optimal conditions for infection (Persson et al., 1999). Six replicates of each soil were used in the bioassay. After four weeks, the roots of the pea plants were examined, and each plant was assigned a DSI as follows: 0 = healthy plant without any symptoms; 5 = discoloration of less than 5 mm on a single root; 10 = discoloration of about 20 mm of the root system; 25 = about 50% of the root system was dark and aected; 50 = the whole root system was dark and aected; 75 = the whole root system, as well as the epicotyl, was dark and aected; and 100 = dead plant (Persson et al., 1997). An average DSI was then calculated for each tested soil.
The ®eld relevance of disease suppressiveness Fig. 1. Map showing the geographic distribution and geologic
classi-®cation of the soils used (clayey tills = clay content, 5±15%; clay tills = clay content, 15±25%).
Disease severity index (DSI) of soils from dierent areas inoculated with oospores ofAphanomyces euteichesin a bioassay and selected physicochemical properties of the soils
Soil Areaa DSI Clay (%) Silt (%) Sand (%) Cb(%) pHc Cad(cmol kgÿ1) Mgd(cmol kgÿ1) Kd(cmol kgÿ1) Groupe
4608 2 5 9 18 73 1.87 6.2 5.5 0.3 0.1 S1
1616 1 49 9 38 47 2.06 6.8 8.5 0.4 0.1
4612 2 21 9 34 56 2.96 8.0 18.0 3.0 0.3 S1
4615 2 29 10 30 58 1.63 6.1 9.0 0.3 0.3 S1
R8 3 76 11 36 45 1.22 6.2 6.0 0.5 0.5
1617 1 18 12 28 59 5.54 6.5 12.5 0.6 0.2 S1
82f 3 66 12 39 48 1.16 6.2 5.5 0.3 0.4
83f 3 77 13 35 44 1.19 6.9 10.0 0.8 0.3
80f 3 58 15 39 44 1.49 6.6 11.0 0.6 0.3
87 3 73 15 34 50 1.28 6.5 8.5 0.5 0.2
81f 3 35 16 40 40 1.68 7.2 14.0 0.5 0.2
85C 3 46 17 44 39 1.68 7.6 13.0 0.8 0.5
L350 4 5 19 28 51 1.93 7.9 36.4 0.9 0.3 S2
85Sf 4 20 19 38 35 1.81 7.7 17.5 0.8 0.2 S2
4517 4 16 20 31 48 1.26 8.0 31.9 1.0 0.4 S2
H511 4 14 21 37 39 1.87 7.9 32.9 0.6 0.4 S2
L553 4 3 21 44 31 1.39 7.5 19.5 0.6 0.3 S2
84f 4 27 21 36 41 1.33 7.3 18.0 0.7 0.3 S2
4529 4 20 21 44 34 2.04 6.8 22.0 1.2 0.4 S2
4534 1 99 35 32 30 1.65 8.1 35.4 3.0 0.7
1521 1 78 35 18 45 1.95 7.4 22.0 1.9 0.4
1615 1 73 40 40 19 2.14 7.0 20.0 2.0 1.0
1613 1 81 41 48 7 2.25 6.7 16.0 1.5 0.5
1611 1 65 41 54 3 2.23 6.0 16.0 1.7 0.8
aSampling area (Fig. 1), 1 = glaci¯uvial deposits, glacial/post-glacial clays; 2 = sandy clayey tills; 3 = clayey tills, clay tills; 4 = clay tills.
b% Organic carbon, dry-weight basis. c
H2O 1.0:2.5, soil:H2O. d
Ammonium lactate extractable. e
Group of suppressive soil, i.e. DSIR30. S1 = sampled in area 1 and 2; S2 = sampled in area 4. f
Pea monoculture ®eld experiments performed.
L.
Persson,
S.
Olsson
/
Soil
Biology
&
Biochemistry
32
(2000)
1141±11
50
measured in the bioassay was assessed in six ®eld ex-periments with pea grown in monoculture for four consecutive years. The experiments were located in ®elds ranging from low to high in soil suppressiveness to pea root rot according to the bioassay, and the results indicated a good relation between the suppres-siveness in the bioassay and in the ®eld (Persson et al., 1999). Each experiment consisted of four plots, each 3 10 m, located beside each other. The infection of A. euteiches in the ®eld experiments was assessed by counting the number of oospores in pea roots (Persson et al., 1999). Each year, at the beginning of the ¯ower-ing stage, 10 plants were collected from a depth of ap-proximately 20 cm at four randomly chosen places in each plot. The roots were cut into pieces and commin-uted in water with a dispersing machine (Polytron, Kinematica AG, Littau, Switzerland), the oospore con-centration in the slurry was counted using a hemacyt-ometer and the number of oospores gÿ1 of root fresh weight was calculated.
2.2. Determination of soil properties
Particle size analysis was done by standard sieving and hydrometer methods (Gandahl, 1952). The con-tents of organic and inorganic carbon were determined by heating dried and homogenized samples from 1008C to 10008C in a Leco furnace (RC 412). The out-put from the CO2-detector of the instrument was
recorded continuously, which allowed the sources of carbon to be dierentiated by the temperature at which they oxidized/volatilized. Soluble potassium, magnesium, and calcium were determined by extrac-tion with acid ammonium lactate (pH 3.75; SIS, 1993). The pH of fresh soil was determined potentiometrically in water (1.0:2.5, soil:water, w/w).
2.3. Determination of clay mineralogy
Analysis of the clay mineralogy was done on 12 of the 24 soil samples, representing the most common soil types of the area. The mineralogical composition of the clay fraction (<2 mm) was determined by X-ray diraction analysis (XRD). The mm fraction less than 2 mm was collected after de¯occulation of the sample in distilled water by ultrasonic treatment and sedimen-tation of the coarse fraction. ``Oriented'' mounts were prepared from the clay suspensions according to the ®lter-membrane peel-o technique (Drever, 1973). The mounts were scanned in a Philips X-ray diractometer (PW 1710) with automatic slits, using CuKa radiation.
Identi®cation of clay minerals was based on the re-sponse to the following pretreatments: (1) satur-ation and drying at room temperature, (2) Mg-saturation and glycerol solvation, (3) heating at 5508C for 1 h, and (4) heating in 1 M HCl for 2 h.
The intensity of the 1.4, 1.0, and 0.7 nm peaks was used for a semi-quantitative evaluation of the relation-ship between smectitic and/or vermiculitic mixed layers, illite, and kaolinite minerals, respectively. Peak intensities were calculated as the product of the maxi-mal peak height and peak width at half maximaxi-mal peak height.
2.4. Estimation of Aphanomyces root rot potentials in ®elds for commercial pea production
Estimations of Aphanomyces root rot potential using greenhouse-grown baiting plants have been per-formed every year since the late ®fties for soils from ®elds in the area to be grown with vining pea (Olofs-son, 1967). Several ®elds have been grown with vining pea on at least ®ve occasions and with approximately a six-year crop rotation. Based on the data of these tests, ®elds could be allocated to dierent categories. Two of these were: (1) ®elds grown with vining pea three times or more, with a six-year crop rotation and with no occurrence of Aphanomyces root rot, and (2) ®elds grown with a six-year crop rotation or longer and with an occurrence of Aphanomyces root rot within three rotations. Crop species used in the ro-tations and the climatic conditions were similar for the ®elds in the two categories. The dominant soil type in every ®eld of the two groups was characterized using geological maps of the Quaternary cover (Adrielsson, et al., 1981; Daniel, 1978; EkstroÈm, 1947, 1953, 1955a, 1955b, 1956, 1960, 1961a, 1961b; EkstroÈm and Moh-reÂn, 1966; MohMoh-reÂn, 1966; Ringberg, 1984).
2.5. Data analysis
The data were analysed using analysis of variance procedures followed by correlation analysis (SAS soft-ware, SAS Institute, Cary, NC).
3. Results
3.1. Selected physicochemical properties of suppressive soils
Correlation analyses performed on disease suppres-siveness and the analyzed factors (Table 1) revealed only weak relations when data from all soil samples were included. However, when the soils were divided into groups, denoted 1±4, depending on their charac-teristics, such as lithological character and texture, sig-ni®cant relations were obtained in correlation analyses performed within these groups.
Particle size analysis showed that the soil samples in the area had a clay content ranging from 9% to 41% (Table 1). The most suppressive soils in the bioassay, L. Persson, S. Olsson / Soil Biology & Biochemistry 32 (2000) 1141±1150
i.e., with a DSI of R30, could be allocated to two groups according to the clay or sand contents, which were the most apparent dierences. The ®rst group S1 consisted of three suppressive soils from area 2 with a clay content of 9±10% and one soil from area 1 with 12% clay. These suppressive soils had a sand content of 56±73% (Table 1). The other distinct group of sup-pressive soils S2 consisted of seven soils, all from area 4, with a clay content of 19±21%, a high pH, and a high content of calcium (Table 1). Correlation analysis performed on the soils from area 3 and 4, which are
similar with respect to lithological character and gen-esis, indicated that increased disease suppression, assessed as DSI in the bioassay, was signi®cantly re-lated to increased contents of clay and calcium and the pH (Fig. 2). There was also a weak but signi®cant re-lation between increased disease suppression and increased organic carbon content R20
:39,
P0:018). For the soil samples from area 3 and 4,
there were also signi®cant interrelations between increased clay content and increases in pH R20
:58,
Fig. 3. Mean numbers of oospores ofAphanomyces euteichesin root tissue after four years of pea monoculture plotted against clay con-tent (A), pH (B), and Ca concon-tent (C) in six ®eld experiments. Bars represent standard error of the mean,n= 4.
P0:015 and the content of calcium R 20
:61,
P0:001).
In the ®eld experiments with pea monocultures (Nos. 80±85), the maximum amounts of oospores gÿ1 of root after pea monoculture were lower on suppres-sive than on conducive soils (Persson et al., 1999). When correlating this number of oospores with con-tents of clay and calcium and the pH, strongly signi®-cant relations were also obtained (Fig. 3).
Soils with clay contents of 35% or more originated from areas with water-deposited, sorted sediments. These samples were conducive to pea root rot in the bioassay and had high contents of organic carbon and soluble nutrients.
3.2. Clay mineralogy and disease suppression
The types of clay minerals were more or less the same in all 12 soils analysed by XRD, but the relative proportion of the main clay minerals varied. Hence, all samples contained kaolinite, illite, and interstrati®ca-tions with vermiculitic layers in addition to non-clay minerals, mainly quartz, plagioclase, and potassium feldspar (Table 2). In some samples, the mixed-layered 1.4-nm phase expanded slightly on glycerol treatment, suggesting that smectite was also present. The occur-rence of clay minerals as well-de®ned vermiculite or as mixed layers of vermiculite and trioctahedral illite± mica in these soils is probably a result mainly of dier-ential weathering. In the subsequent discussion, the denotation of vermiculitic±smectitic mixed layers will be simpli®ed to vermiculite±smectite.
Based on the relative proportion between the major
groups of clay minerals, i.e. clay minerals with a basal spacing of 1.4, 1.0, or 0.7 nm (kaolinite minerals), two end-members could be distinguished: one was domi-nated by vermiculite and the other by kaolinite±illite. The X-ray diractograms of representatives of each of these groups are shown in Fig. 4. The ratio of the intensity of vermiculite±smectite to the sum of the intensities of illite and kaolinite was evaluated semi-quantitatively for each soil (Table 2). The disease sup-pressiveness measured in the bioassay or ®eld exper-iments was signi®cantly related to an increase in this ratio in soils from area 3 and 4 (Fig. 5). The suppres-sive soils, group S2, contained a higher ratio of vermi-culite±smectite to illite±kaolinite than the conducive soils.
3.3. Aphanomyces root rot in commercial ®elds
Results from the last 40 years of yearly estimations of the potential of Aphanomyces root rot in commer-cial ®elds indicated that disease-suppressive soils occur in the area (data not shown). From a large number of ®elds and tests, 53 ®elds were found with no occur-rence of Aphanomyces root rot, despite frequent pea growing, and these ®elds were considered as having a higher degree of suppressiveness. Of these 53 ®elds, 36% had similar lithology and texture compared with the suppressive soils with low clay contents in group S1, as found in the bioassay. Further, 45% had similar lithology and texture compared with the calcareous suppressive soils in group S2. Fields with a low occur-rence of Aphanomyces root rot were found in the same areas as the suppressive soils in groups S1 and
Table 2
Mineralogical composition of the clay fraction of selected soils (relative peak intensitiesaare given in brackets) Soil Minerals
Areab Chlorite Vermiculitic±smectitic mixed layers Smectite Illite Kaolinite Quartz Plagioclase K±feldspar Intensity ratioe
4608 2 Xc(5.8) X (1.3) x (1) X x x 2.5
Calculated as the product of the maximal peak height and peak width at half maximal peak height measured in the X-ray diractograms. b
S2, i.e., areas 2 and 4. The remaining 19% were ®elds with soil types dicult to identify from the maps of the Quaternary cover.
Of ®elds with a history of high infestation of Apha-nomyces root rot, two groups could be distinguished: one group had textural properties similar to the soils from the tills within area 3, which were conducive in the bioassay and in the ®eld experiments; the other group was on glacial/post-glacial heavy clay soils (i.e., area 1).
4. Discussion
Based on their physicochemical properties, soils sup-pressive to Aphanomyces root rot could be grouped into two groups, S1 and S2, with substantial dier-ences in texture and chemical composition. An increase in disease suppression was, for soils from area 3 and 4,
closely related to an increase in the ratio of vermicu-lite±smectite to illite±kaolinite and to increases in clay and calcium content and pH. There were, however, strong interrelations between these factors. The cation-exchange capacity (CEC) and the resulting buering capacity are related to the abundance and nature of the charged constituents, i.e., the clay minerals and or-ganic matter. Calcium is normally the dominant base cation that satis®es the negative charges on clay min-erals and organic matter. The amount and type of clay minerals are controlled not only by the origin of the parent material of the soil, but also by processes (e.g., sorting by size) involved in the genesis of the soil and by pedogenic processes. All soils studied from areas 2± 4 were developed on unsorted Weichselian glacial deposits (tills). The clay tills in the south-western part of the study area (4 in Fig. 1) are derived from parent materials originating in chalk±limestone bedrock mixed with ®ne-grained sediments. These soils are,
thus, highly calcareous when not leached and have high pH values. Also, tills in the north-western part (3 in Fig. 1) include material from limestone bedrock but, in addition, derived much material from the local Mesozoic kaolinitic clays and loose sandstones, result-ing in a lower clay content and lower pH and, gener-ally, in a higher content of kaolin minerals compared with tills in the south-western region (Wiklander and Lotse, 1966). The suppressive soils in group S2, with a high proportion of vermiculite±smectite, were all sampled within the clay till in the south-western region (4 in Fig. 1), whereas the conducive soils with higher proportion of illite and kaolinite were sampled in the north-western region (3 in Fig. 1). Vermiculite and smectite, which in some respects have similar structural characteristics, have a CEC and speci®c surface that are much larger than those for illite and kaolinite. Stotzky and Martin (1963) showed that soils suppres-sive to Fusarium wilt of banana in central America contained smectitic clay minerals, whereas conducive soils did not. Further, the human pathogen,H.
capsu-latum, a soil-borne fungi, was isolated only from soils that did not contain smectite (Stotzky and Post, 1967). In contrast, Stutz et al. (1989) found smectite and illite in a soil conducive to black root rot of tobacco and mostly vermiculite in a suppressive soil. In the growing area of vining pea in southern Sweden, several of the conducive as well as the suppressive soils contained both vermiculite and smectite, but disease suppression was signi®cantly related to the intensity of vermiculite± smectite relative to the intensity of illite±kaolinite. Thus, it appears that a high ratio between clay min-erals with high and with low exchange capacity is an important characteristic for a disease-suppressive soil. The strong relation between disease suppression and abiotic factors does not, however, exclude the possi-bility that other mechanisms are involved. In disease suppression to damping-o of cucumber caused by Pythium splendens H. Braun, a combination of a high calcium content and a high microbial population was suggested to be of importance (Kao and Ko, 1986). Treatment of the suppressive soils in group S2 with gamma-irradiation reduced suppressiveness to Apha-nomyces root rot, which also implies the involvement of a biological mechanism (L. Persson, Unpublished). The results suggest that several complex combinations of abiotic soil factors may be of importance for the disease suppression in soils of group S2, but possibly in combination with groups or communities of micro-organisms.
The S1 group of Aphanomyces-suppressive soils exhibited dierent characteristics as compared with the soils in the S2 group: the sand content was high, the clay content was low and the organic car-bon content was slightly higher than average. As zoospores are the primary infection unit of A. euteiches, the infection of a plant is dependent on a high water content in the soil (Papavizas and Ayers, 1974). Westerlund et al. (1978)) suggested that the low incidence of infection by the zoospore-forming fungus, Olpidium brassicae (Woronin) P.A. Dang, in a sandy loam soil compared with a clay or clay loam soil with a higher incidence of the dis-ease was due to better water drainage and pen-etration in the sandy loam. However, in the case of soils from group S1, elimination of suppressiveness by gamma irradiation also implied a biological mechanism in suppression (L. Persson, Unpub-lished). Soil organic matter is closely related to microorganisms and in¯uences physical and chemical characteristics, such as structure and CEC. Colloids of humus have a very high CEC, similar to or higher than the CEC of vermiculite (Brady, 1984). Soils suppressive to root rot caused by Phytophthora cinnamomi Rands in Australia generally had a high content of organic matter (Broadbent and Baker, 1974), and composts with high microbial activity Fig. 5. Disease suppression to Aphanomyces root rot in soils from
areas 3 and 4 (Fig. 1) plotted against the ratio of the intensity of ver-miculite±smectite to the sum of the intensities of illite and kaolinite. Suppression measured as: (A) disease severity index of soils inocu-lated with oospores in a bioassay; and (B) the mean number of oos-pores in root tissue after the fourth year of pea monoculture in six ®eld experiments. Bars represent standard error of the mean; in A,n = 6; in B,n= 4.)
and microbial biomass suppressed diseases caused by Pythium spp. and Phytophthora spp. (Hoitink et al., 1996). The results suggest that disease suppres-sion in soils of group S1 to Aphanomyces root rot mainly is attributable to a hostile environment for infection by the pathogen but, possibly, also in combination with microorganisms.
The soils with clay contents of 35±40% were sampled in an area with a high incidence of Apha-nomyces root rot, and these soils were conducive in the bioassay, as expected. These soils developed in glacial clays (Daniel, 1978), which were deposited in water and, accordingly, fairly well sorted. Therefore, these soils have dierent physicochemical properties compared with soils on tills, e.g., a higher content of soluble nutrients, such as calcium and potassium, but a lower proportion of vermiculite±smectite. The high clay content and the sorting result in a soil that is compact and has a low permeability, which enhances infection of roots by zoosporic fungi.
Historical data on assessment of infestation of A. euteiches in commercial ®elds con®rmed the strong relations between abiotic characteristics and the dis-ease suppression observed in the bioassay and ®eld experiments. Repeated rotation of pea once in every six years on ®elds with soil characteristics similar to suppressive soils in either group S1 or S2 had not resulted in any infestation of Aphanomyces root rot, and thus, these ®elds had been safe for pea production. In contrast, heavy infestation had devel-oped within three or fewer rotations in ®elds classi-®ed as conducive. The characterisation of the ®elds investigated as group S1 or S2 was facilitated in some areas by use of a speci®c series of Quaternary maps (Serie Ad), where measurements of various properties, such as soil type, pH, loss on ignition, and particle size analyses, had been made on both subsoil and topsoil (EkstroÈm, 1947). The lower inci-dence of Aphanomyces root rot in some of the sup-pressive areas was, to some extent, known by farmers and farm advisers, and they may, thus, be a valuable source of information, as was also noted by Linderman et al. (1983).
Disease suppression to Aphanomyces root rot in southern Sweden was found in two dierent types of soil and was clearly related to abiotic factors of the soil, and these, in turn, were related to the variable ge-ology within the study area. As disease suppression was shown to be related to characteristics, such as clay and sand content, maps of the Quaternary deposits may be a valuable tool for assessing approximate areas of distribution of suppressive soils. In combination with bioassays, they provide possibilities for creating maps of disease-suppressive soils. The results further imply that the abiotic characteristics of the soil and the resulting disease suppressiveness to Aphanomyces
root rot should be taken into account when planning the crop rotation for speci®c ®elds.
Acknowledgements
We thank the farm advisors at Svenska Nestle AB for providing ®eld data, A±K. Arvidsson for technical assistance, Prof. A. Andersson for valuable comments on the manuscript, and Dr. M. WikstroÈm and Prof. B. Gerhardson for critically reading the manuscript. The research was ®nanced by the Swedish Council for For-estry and Agricultural Research, Nestle R&D Center Bjuv AB, and the Foundation for Strategic Environ-mental Research (MISTRA).
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