Plant and Soil 171: 235-244, 1995. © 1995 KluwerAcademicPublishers. Printedin theNetherlands.

Oat residue and soil compaction influences on common root rot (Aphanomyes euteiches) of peas in a fine-textured soil

V. A. Fritz 1, R.R. Allmaras 2, E L. Pf lege r 3 and D. W. Davis 4 1Southern Experiment Station, University of Minnesota, 35838 120th St., Waseca, MN 56093, USA, 2Agricultural Research Service, U.S. Department of Agriculture, St. Paul MN 55108, USA, 3Department of Plant Pathology, University of Minnesota and 4Department of Horticultural Science, University of Minnesota, St. Paul, MN 55108, USA

Received 28 April 1994. Accepted in revised form 14 September 1994

Key words: bulk density, cultural control, green pea yield, root disease rating, soil hydraulic properties, traffic patterns, vine growth

Abstract

Management of common root rot (Aphanomyces euteiches Drechs.) in peas (Pisum sativum L.) is sought primarily by host crop avoidance for several years. Soil compaction is known to aggravate A. euteiches disease in peas but effects on infection and subsequent symptom development are not sufficiently known to assist in cultural control. Several isolated observations have noted that oat crop residues may suppress A. euteiches infection and disease in pea roots. The individual and combined influence (a factorial combination of two factors each at two levels) of a prior oat crop and soil compaction were studied for their effects on common root rot severity in processing peas grown in an A. euteiches disease nursery on a fine-textured soil in the northern Corn Belt of the USA. A previous crop of summer oats relative to prior-year peas significantly suppressed common root rot and increased pea fresh vine weight 210% at peak bloom stage. Both fresh vine weight and green pea yield were reduced as much as 63% by soil compaction and increased as much as 48% by a prior oat crop. Greater soil bulk density at the 10 to 25-cm depth identified wheel traffic compaction patterns in each year. A 10-fold reduction of saturated hydraulic conductivity in the 10 to 25-cm compacted zone and high soil-water potentials within the upper 60 cm both confirmed an impaired water drainage, especially during infiltration events. These observations support the use of a previous full season or summer oat crop jointly with chisel plowing, plus the prevention of excessive traffic during secondary tillage and planting, to reduce common root rot in a field infested with A. euteiches. Shallow incorporation of oat shoot and root residue by chiseling could be a crucial component of the cultural control of the disease.

Introduction

Common root rot in peas (Pisum sativum L.), caused by Aphanomyces euteiches Drechs., has been recognized as a serious soilborne disease since 1925 (Papavizas and Ayers, 1974). The disease is widespread not only in fine-textured/poorly drained soils in subhumid to humid climates (Papavizas and Ayers, 1974), but has also been observed in irrigated, coarse-textured soils (Kraft et al., 1990; Pfender and Hagedorn, 1983). Man- agement of common root rot is sought primarily by avoidance of the host crop for six years or longer, yet crop loss remains unpredictable. Soil compaction

aggravates common root rot and often results in greater disease severity (Burke et al., 1969a, 1970; Tu and Findlay, 1986; Vigier et al., 1983). Preliminary work suggested that reduced soil compaction and a prior oats crop are two cultural practices that could help to sus- tain pea production on infested soils (Papavizas and Ayers, 1974; Tu and Findlay, 1986). A. euteiches also can be one component in a complex of fungi which individually can cause pea root disease and which are aggravated by soil compaction. Thus compaction also contributes to chick pea (Cicer arietinum L.) and pea root diseases caused by Fusarium solani (Mart.) Sacc. f. sp. pisi (ER. Jones) W.C. Snyder and H. N. Hanson

236

(Bhatti and Kraft, 1992; Burke et al., 1969a, 1970; Kraft and Allmaras, 1985). Furthermore, root disease caused by Pythium ultimum Trow is aggravated by compaction because the pathogen is often associated in a root-disease complex with either A. euteiches (Tu, 1987) or E solani f. sp. pisi (Kraft and Allmaras, 1985; Tu, 1987).

Effects of soil compaction on root disease devel- opment depend on the soil ecology imposed and the requirements of the pathogen. Soil compaction may be produced within a tilled layer or at the base of a tilled layer and usually has a spatial pattern that is related to traffic/tillage patterns (Allmaras et al., 1988a, b). Spa- tial patterns of compaction can be characterized by bulk density or penetrometer measurements, but associated hydrothermal, aeration, and mechanical resistance in the root zone are the factors that impact infection and disease development (Allmaras et al., 1988a). A. eute- iches and F. solani f. sp. pisi inoculum predominance in the plow layer of pea fields in Wisconsin (Burke et al., 1969a, b, 1970) was related to the extent of pea rooting, as well as to tillage mixing of soil and organ- ic debris above a plow pan. Burke et al. (1972a, b) demonstrated a similar influence of moldboard-plow and disk pans on the rooting of field beans (Phaseolus vulgaris L.) and on the incidence of Fusarium solani (Mart.) Sacc. f. sp. phaseoli (E R. Jones) W. C. Snyder and H.N. Hanson above zones of bulk density suffi- cient to impede rooting. Inoculum of A. euteiches and F. solani f. sp. pisi occasionally has been found below the Ap layer (Burke et al., 1970; Kraft and Allmaras, 1985; Kraft et al., 1990).

Burke et al. (1969a, b) showed that infection of A. euteiches on pea roots was not influenced by temper- ature but was hastened by brief soil saturation. Brief localized saturation is likely to occur in compacted soil located in plow pans or even in traffic lanes in the Ap layer (Allmaras et al., 1988b). However, the expression of symptoms in peas is influenced by tem- perature. Soil moisture conditions for P ultimum and A. euteiches infection are not greatly different, but P. ultimum symptoms usually occur at lower tempera- tures (Alconero and Hagedorn, 1967) than those of A. euteiches. Damping-off of sugar beet (Beta vulgaris L.) seedlings shows a similar time development pattern for the disease complex of A. cochlioides Drechs. and Pythium ultimum (Payne and Asher, 1989). Burke et al. (1969) demonstrated a rapid spread of A. euteiches infection from pea roots that had grown from non- infested into infested soil. They contrasted this rapid spread ofA. euteiches with a much slower spread of E

solani f. sp. pisi infection along the root from zones of infested to noninfested soil.

Oat (Avena sativa L.) residues have reduced com- mon root rot severity and increased seed yield of peas in the greenhouse (Davey and Papavizas, 1961), and in the field (Tu and Findlay, 1986). A saponin found in oat roots and their extracts may lyse zoospores of Aphanomyces spp and P ultimum oospores (Deacon and Mitchell, 1985).

In the quest for effective cultural management of A. euteiches, we field tested the individual and combined influence of a prior oat crop, and an induced soil com- paction (produced during secondary tillage and plant- ing operations) on a fine-textured soil in a long-term pea-disease nursery in the northern Corn Belt of the USA. The test of oats as a cultural control measure was suggested by observations in the nursery as early as 1986 (D.W. Davis and F.L. Pfleger), that this crop could be used to reduce A. euteiches severity at a level suitable for evaluating germplasm susceptibility.

Methods and materials

The field experiment was conducted in an approximate area of 50 × 10 m (Fig. 1) within a long-established, heavily infested nursery for evaluating pea response to common root rot caused by A. euteiches. The exper- imental site, a moderately to poorly drained Web- ster clay loam (fine loamy, mixed, mesic Typic Hap- laquoll), has about 2% slope. It is located at the Uni- versity of Minnesota, Southern Experiment Station, Waseca. The Ap layer consists of 348, 326, and 326 g kg-x respectively of sand, silt, and clay; the organic matter content is 47.4 g kg- i. The respective compo- sition of the adjacent subsoil is 320, 342,338, and 33.0 kg- 1. The site had been fall moldboard plowed annu- ally for more than 10 yr, including the fall of 1986.

The sequence of treatments and measurements was initiated at pea (vat. Bolero) planting (12 May) and oat (vat. Steele) planting (1 May) in 1987 and ter- minated at pea harvest in 1989 (Table 1). One half of the experimental area was selected randomly to be planted to oats in 1987 and the other one-half to peas. This treatment assignment began the crop histo-

2 of the pea vines were ry to be tested in 1988. About deposited back onto respective plots after pea harvest in July 1987; oats were harvested on 4 August and straw removed within several days. (Current tillage practices for pea production in the Midwest are almost

N

I

21:

21 m

Fig. 1.

Qoppl r~ Sequence

1987 1988 1989 Plot Test Host Test Host

clc n/n NO,' Crop 2 Crop CrOp ~ Crop

I oats peas oats peas

• . . 1 . . . . . 2 . . 2 O O t S p e g s n o n e p e o s

3 oats peas none peas

clc rVn ~ IX:Its p e a s oo ts p e o s

5 peas peas oots peas 3 4

6 peas peas none peas

7 peas peas none peas

8 peas peas oats peas

i C/C: compacted rn 1988 and 1989 before secondafy~llage and planting

n/n c/c n/n: not coml:x3cted

primary tillage in fall of 1987 and 1988 - 5 6 c~sel plow

2 flJl seosc~ c lop

sul-nn"~r crop rVn c/c

7 8

I+- 2.5 m--N

Sequence of treatments and tests for period from planting 1987 to harvest 1989.

237

Table 1. Pea growth and disease development (1989) as influenced by soil compaction and prior crop treatments

Date

Main-effect response Stage of Compaction vs Oats vs

growth no compaction peas in 1988 a

1 June 2-3 node 10% hr b

7 June 3-5 node 20% hr with some diseased/dead plants

15 June 6-7 node 30% hr with more

diseased/dead plants

29 June Bloom 50% hr with diseased/dead plants

No response No hiC; necrosis of lower leaves after peas

25% hi and no disease symptoms after

oats; increased necrosis of lower leaves after pea

More wilting, yellowing, necrotic leaf margins after peas than oats

a Peas with fallow after harvest in 1988 vs peas followed by a summer crop of oats in 1988 (see Fig. 1). b hr = height reduction due to compaction. c hi = height increase due to prior oats crop.

so le ly compr i sed o f p lant ing into a p rev ious ly chisel

p l o w e d field hav ing min ima l crop residue as f rom soy-

bean; two or three secondary t i l lages are used with a

disk or spr ing tooth cul t iva tor pr ior to plant ing.) The

who le exper imenta l area was chisel p lowed in autumn.

In spr ing o f 1988 two levels o f compac t ion (c/c and

n/n) were r andomly ass igned wi th in each of the 1987

crop histories. The compac ted plots were each uni-

formly packed us ing a med ium-s i zed tractor (5000-kg

axle load, 200 kPa tire pressure), and the who le area

t i l led with a spr ing- tooth cult ivator. Peas were planted

across the entire site on 6 M a y in 25 -cm rows us ing a

drill wi th a double-disk opener to ach ieve about 123

238

plants m -2. Seed was treated with Captan I Postemerge weed control consisted of two applications of a mix of bentazon (Basagran 1 at 0.8 kg a.i. ha - l ) and sethoxy- dim (Poast 1 at 1.1 kg a.i. ha- l ) . Irrigations of 2.5 cm each were applied over a 4 hr period on 24 May and 1, 6, 18 June. Peas were harvested on 5 July 1988.

After pea harvest, two treatments were randomly assigned within each of the rectangular areas (i.e., pre- vious test crop of oats in 1987 and compaction in 1988 in upper left rectangle in Fig. 1). These were bare fal- low and summer oats (var. Steele) planted 29 August and supplementally irrigated when necessary to sus- tain growth. At killing frost on 5 October, the oats were about 50 cm tall and had not reached anthesis. The whole experimental area was chisel plowed on 17 October 1988.

In spring of 1989, wheel compaction was imposed on the same plots and in the same manner as 1988; the whole experimental site was then tilled as in 1988 and planted to (Captanl-treated) peas on 16 May. Weeds were controlled with a pre-emerge application of propachlor (Ramrod 1 at 4.5 kg a.i. ha -1) and prom- etryn (Caparol 1 at 1.1 kg a.i. ha- l ) . Irrigations of 2.5 cm each were applied on 8 and 19 June, and 5 and 11 July. Peas were harvested on 11 July 1989.

In 1988 field notes were recorded on pea plant growth and disease symptoms. Plant heights and dis- ease symptoms were made on 10 randomly selected plants within each of the four treatments at various stages of growth. Soil related measurements were also made. Tensiometers were installed on 16 May and read three mornings per week until 10 July, using the proce- dure of Marthaler et al. (1983). There were three ten- siometers at each of two depths (25 and 40 cm) within each of the compacted and noncompacted treatments (Fig. 1). The bulk density profile (2-cm increments to 50-cm depth) was measured in the compacted and noncompacted treatments during May and June, using a system of composited cores each 18 mm in diam- eter (Allmaras et al., 1988c). Precipitation and mean air temperature were recorded from a central weather station located less than 1 km from the nursery.

More detailed plant data were obtained in 1989 by repeated observation of symptoms and growth of ran- domly selected plants. Disease symptoms and growth

l Mention of a trademark or proprietary product does not consti- tute a guarantee or warranty of the product by USDA/University of Minnesota and does not imply its approval to the exclusion of other products that may also be suitable. This article reports the results of research only.

of peas were observed over the period from two nodes to peak bloom. Root disease severity data was obtained from three samples of 10 randomly dug plants within each of the eight plots (Fig. 1) when the plants were at peak bloom on 29 June. Roots were washed and rated visually to assess the severity of common root rot using a scale of 1 to 5. The five-point scale was based on an estimate of the percent roots infected (1 = 0-10%, 2 = 11-25%, 3 = 26-50%, 4 = 51-75%, 5 = 76-100% ). Fresh vine weight and yield of green peas were obtained from the same plants sampled on 29 June for determining root disease severity. Varia- tion among the three samples within each of the eight plots in Figure 1 was used to compute a standard error to compare the four treatments.

Fresh vine weight and yield of green peas were obtained on 11 July from a harvest area of 3 m 2 selected randomly within each of the eight plots (Fig. 1). Vine height/length was measured on three sets of five plants, and the total number of plants was observed in each of three separate row lengths within each plot (Fig. 1) outside the harvested area. An error term for comparing the four treatments was generated from eight sets of yield versus vine length/height.

Soil bulk density was measured on 16 June using the same procedure as in 1988. Triplicate cores (5 cm diam. and 5 cm long) for saturated hydraulic conductivity(Ksat were taken on 10 August at 10, 25, and 40 cm at each of two sites within the compacted and noncompacted treatments. The four sites (Fig. 1) were at the south end of plots 3 and 4 and north end of plots 5 and 6; all sites were within a 12-m 2 area. Ksat was measured in these cores using a falling head technique (Klute and Dirksen, 1986). In 1989 soil- water-potential sensors (Watermark 1) were installed at 10, 25, 40, and 60 cm at each of two locations with- in the compacted and noncompacted treatments. Their signal was automatically recorded hourly from 1 June until pea harvest. These sensors become erratic near saturation and have a + 10% accuracy in the range of -2 to -150 kPa water potential (McCann et al., 1992; Spaans and Baker, 1992). Their accuracy is improved when rewetted to saturation after a soil drying excur- sion.

Results

The period 1 April to 15 July 1988 had mean daily air temperatures 3.8 °C above and a precipitation sum 168 mm below the long-term normals of 13.3 °C and

239

Table 2. Root disease index and fresh vine weight of peas at peak bloom (29 June 1989) as related to treatments of soil compaction and prior crop of oats

Previous Soil Root disease Fresh vine

crop ~' compaction index b weight (g/10 plants)

Oats 2.0 b c 280 a

Oats + 2.0 b 210 b

Peas 2.7 a 92 c

Peas + 2.7 a 66 c

Std. error 0.08 17.9

a Peas with fallow after harvest in 1988 vs peas followed by a summer crop of oats in 1988 (see Fig. 1). b Disease index (pea roots infected): 1 (0 - 10%), 2(11 - 25%), 3(26 -50%), 4(51 - 75%), and 5(76 - 100%). c Duncans multiple range using p = 0.01.

April May June July

G

E

g

<

/~J[ II IVl~y June JUly

E

5_

g

Fig. 2. Precipitation and average daily air temperature during the 1 April to 15 July 7 period of 1988 and 1989 at the Southern Minnesota Agric. Expt. Station.

322 mm (Fig. 2). In 1989, the mean daily air tem- perature was 15.0 °C and precipitation sum was 215 rnm. The four irrigations of 2.5 cm each restored the precipitation deficit in 1989 but not in 1988.

Plant growth and disease

At the 5-node stage on 30 May 1988 no disease symp- toms were visible. At the 12-node stage on 20 June most pea plants in plots 6 and 8 (Fig. 1, peas 1987 and compacted in spring 1988) were dead; the remaining plants were severely diseased showing classical symp- toms of A. euteiches infection (Pfender,1984). Plants in plots 1, 2, 3, 4, 5, and 7 (Fig. 1) as a group had nearly the same growth and disease symptoms, i. e. necrosis and yellowing of the lower 4 to 6 leaves; these plots had either not been compacted, or had a 1987 oat histo- ry with/without compaction. Soil samples at this time showed pea rooting activity to 50 cm in plots 5 and 7, whereas rooting in plots 6 and 8 was much more reduced (data not shown).

Plant growth and disease responses to compaction and prior oat cropping were observed in more detail in 1989, after the preliminary field observations and extreme drought of the 1988 season (Fig. 2). Field observations of plant height and symptoms following compaction (Table 1) showed an increase in root rot symptoms (specific to A. euteiches) (Pfender, 1984) and a decrease of symptoms following a prior oat crop. There was, however, some interaction observed as ear- ly as the 3 to 5-node stage, in which the prior oat crop suppressed disease symptoms better in the absence of compaction. Soil samples taken on 7 June showed a much wetter soil in the 30 to 50-cm layer of the com- pacted treatments.

At anthesis the prior oat crop effect was evident as a significant (p --- 0.01) reduction in root disease index (Table 2); however compaction had no effect on

240

the index. A response of root disease index to com- paction may have been observed had the sample been taken earlier, because more plant loss (less rooting) had occurred on the compacted plots (Table 1). The index range from 2 to 2.7 suggests that 15 to 40% of the roots were infected. Fresh vine weight was reduced (p = 0.05) by compaction and increased by prior oat crop (p -- 0.01) ; there was no overall interaction effect (Table 2). There were no interactions of site (dupli- cate plots with the same treatment in 1988) with com- paction or prior oat crop affecting root disease index, but there were interactions affecting vine weight (data not shown). Compaction had a greater impact on fresh vine weight in plots 1 through 4 while a prior oat crop (1988 crop) had a smaller impact in plots 1 through 4. These interaction effects were significant at p = 0.05, and suggest a less adverse effect of compaction in a bet- ter drained site and also the possibility for carryover from the 1987 oats.

At harvest both measured parameters of growth (Table 3) showed prior oat vs pea history and com- paction effects similar to those measured at anthesis. Both fresh vine weight and adjusted pea yield were reduced by soil compaction and increased by a pri- or oat crop (Table 3). The largest vine and pea yield occurred following an oat crop in 1988 plus avoid- ance of soil compaction. Either compaction or prior peas reduced growth, and compaction plus prior peas produced the greatest yield reduction.

Vine weight and pea yield both increased as growth and plant density increased, as expected in a typical root diseased crop. Predictions of vine weight (Y) and pea yield (y1) using vine height/length and total num- ber of plants were as follows:

Y (kg ha -1) = 3 0 , 2 6 3 + 8 9 3 Xl + 118 X2 R 2 : 0.93

y1 (kg ha - l ) = - 4 8 0 6 + 145 X1 + 8.5 X2

R 2 : 0.91

where X 1 is vine height/lengthin cm, and X2 is number of plants per 3 m 2. Variables X, and X2 were each measured three times within each plot (Fig. 1) outside the area used for harvest. These regressions predicted yields of vine and peas within 20% of those observed directly in Table 3 and the rank of yields remained the same. The ratio of living to total pea plants averaged 0.58 in the absence of soil compaction, 0.38 with a prior oat crop plus soil compaction, and 0.18 without a prior oat crop but with soil compaction.

Table 3. Fresh vine weight and yield of peas at harvest in 1989 as related to treatments of soil compaction and prior crop of oats

Previous Soil Fresh Yield c r o p compaction vine weight of peas ~

. . . . . kg ha -1 . . . . .

O~s 19380 2590 O~s + 10590 1290 Peas 17500 2160 Peas + 5870 460 Std. e~or b 2110 380

a Yield adjusted to tenderometer of 90 using relation: adj. yield = observed yield + factor; factor = [(1.55 × tenderometer)- 37)]/100. b Derived from regressions between harvest weight and auxiliary measurements of vine height/length and plant density. Each has 5 degrees of freedom.

Soil properties related to compaction

Profiles of soil bulk density taken after compaction and planting in 1988 and again after repeat of the same treatment in 1989 show a similar intensity and depth of compaction in both year (Fig. 3). The maximum density in the upper 30 cm occurred at about 10 cm which coincides with the maximum operating depth of the spring-tooth cultivator used for secondary tillage; otherwise this maximum would have extended closer to the surface. Profiles for the control (noncompacted) and the compacted soil differed markedly above 15 cm and coincided at about 20 cm in plots 1 through 4 and 25 cm in plots 5 through 8. All four profiles show a thin layer (about 4 to 6 cm) of high bulk density at about 30 cm; this is a characteristic moldboard plow-pan created by one or more of the fall moldboard tillages in the period 1982 to 1986. In all profiles the subsoil below 30 cm had a lower bulk density than in the I0 to 30- cm layer. The greater variability of bulk density below 30 cm in plots 1 through 4 than in plots 5 through 8 could be due to greater thickness of mollic epipedon in plots 5 and 8. In these profiles, therefore, there are two depths (at about 10 cm and 30 cm) where compaction may have significantly influenced both soil drainage and mechanical impedance to rooting.

While sampling, it was noted that oat residue occurred only within the upper 10 cm in 1988 and 1989. These residues were incorporated during the fall chis- eling in 1987 and 1988. Ksat measured at three depths in each of the two compaction treatments (Table 4)

241

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lO

20

3o

40

E 50

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0 C3 0 ~

10

20

30

40

5O

0.5

i 0 •

0 o •

O 0 • ~ ' 0

,.'• 1988

I

I i

O • o ° •,k %0o?

0 ~0 • •

":"t 1988 •

1,0 1.5

Plot

o Conlrol 21 4

• Packed 1, 3

t Composlle subsoil

I I

Plot

o Confrol 5, 7 • Pocked 6, 8

• Composlle subso;I

I I 0,5

I J ~,0 0 •

0 0 0!0 ~A•A

• @• ~

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I 1989

I

i

° AOoo, . 0 •

0 0 ~ • 0 0 0 ~ •

°o i °It

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1989

1'.0 1.5

Bulk density (g c m 3)

Fig, 3. Profiles of soil bulk density produced by the noncompacted (control) and compacted (packed) treatments imposed in 1988 and 1989 by surface traffic before secondary tillage (Points above 29 cm have a standard error = 0.04 g cm-3; those below 29 cm have a standard error = 0.06 g cm-3).

Table4, Saturated hydraulic conductivity (Ksat) of the Web- ster clay loam as related to compaction during secondary tillage

Soil Ks,~t (/zm S - l ) at indicated soil depth a

compaction 10 cm 25 cm 40 cm

0.88 0.49 110.3

+ 0,08 0.85 41.3

a Measured 10 August 1989. Each reported mean is the geometric mean of measurements made in each of six undisturbed cores.

shows the potential impact of compaction on internal soil drainage during a major infiltration event. Depths for measured Ksat were suggested by bulk density pro- files (Fig. 3). The subsoil Ks at (40 cm) was significant- ly (p = 0.01) larger than all values in the upper 28 cm, and traffic compaction significantly reduced Ksa~ in the depth range of 7 to 13 cm. The noncompacted treat- ment mean at 10 cm and both means at 25 cm were not different, statistically (p = 0.05). Only the subsoil Ksat values (Table 4) are large enough to expect a sig- nificant macropore component (Logsdon et al., 1990).

Significance of the means in Table 4 was tested assum- ing that K8 at is lognormally distributed; and, therefore, no single standard error can be given.

During the 1988 and 1989 seasons soil water con- tent/soil matric potential were measured intermittently. The first evidence of a changed internal drainage due to the compaction treatment was an accelerated sur- face seal and more surface water movement during the irrigations. This obvious change in soil water contents at shallow depths occurred as early as during the first irrigation, within 18 and 23 days after planting in 1988 and 1989, respectively. Respective rainfall accumula- tions in these periods were 2.4 and 2.2 cm.

Tensiometers manually read in 1988 showed mean soil water potentials in the range of -3 to -15 kPa at the 25-cm depth and -2 to -3.5 kPa at the 40-cm depth; there were no differences (p = 0.05) between com- pacted and noncompacted treatments. Measured bulk density (Fig. 3) and the related soil water characteris- tic (Wu et al., 1992) indicate that the air-filled porosity at these depths ranged from 0.05 to 0.12 cm 3 cm -3. Soil samples taken on 7 June indicated a higher water content at the 30 to 50-cm depth under the compacted plots.

242

Water potential measured continuously after 1 June until 1 July 1989, using Watermark 1 sensors, indicat- ed drainage differences between the compacted and noncompacted treatments. Sensors at the 10-cm depth responded to rainfall or irrigation during the first 40 days after planting. They showed a range of water potential from -2 to -100 kPa in the noncompacted plots, but the range in the compacted plots was -2 to -20 kPa. Sensors at the 25-cm depth ranged from -2 to -50 kPa in the noncompacted plots but only from -2 to -10 kPa in the compacted plots. Sensors at the 40 and 60-cm depths in the compacted and noncompacted plots showed no sensitivity to rainfall or irrigation; they remained in the range of -2 to -5 kPa. These measure- ments agree with those in 1988 showing a troublesome low air-filled porosity.

Discussion

This field study in an A. euteiches infested nurs- ery located on a moderately to poorly-drained/fine- textured soil in a humid climate demonstrated that a prior crop of oats can reduce common root rot poten- tial, and that soil compaction associated with sec- ondary tillage and planting can increase disease severi- ty in a moderately susceptible pea variety (cv. Bolero). Increased disease severity was demonstrated in each of two years following compaction; whereas disease severity was reduced due to a prior oat crop grown as a full-season (1988) or summer crop (1989). A chisel plow operation 15-cm deep in the fall of 1988 relieved most of the compaction produced during the 1988 sec- ondary tillage preceding planting (Fig. 3). Supplemen- tal irrigation was used to assure disease development, but 1988 was an abnormally dry period for pea pro- duction, while 1989 had about 20% less precipitation than in a normal season (Fig. 2). Results in 1989 indi- cated that the potentially greater disease severity due to soil compaction was reduced by a prior oat crop and increased by a prior pea crop.

Chisel plowing as the primary tillage may have been an important element in reducing root rot with prior oat crops in our field experiments and those of Tu and Findlay (1986). Primary tillage with a chisel plow should have retained nearly all root and shoot tissue of oats in the upper 15 cm, while moldboard plowing would have deposited only 10% of the residue in the upper 15 cm (Staricka et al., 1991). Observations dur- ing 1988 and 1989 after fall chiseling confirmed that oat residue remained in the upper 10 cm. The likelihood

that chisel plowing retains the oat residue where it can influence infection by A. euteiches is suggested by the studies of Burke et al. (1969b). Release of compounds, such as saponins, from decomposing oat residue in the upper 10 cm would be available to reduce infection of young pea roots; such would not be the case with oat residue incorporated with a moldboard plow (Staricka et al., 1991). Our results did not show a significant oat effect beyond one subsequent pea crop; Tu and Find- lay (1986) suggest that frequent summer oats can have an accumulative effect when chisel plowed. Chan and Close (1988) have reported that A. euteiches inoculum can be sustained on nonhost roots. Because the species range of sustaining and nonsusceptible host plants may be diverse, an oat crop the prior growing season with strict weed control and chisel plowing may be neces- sary components of cultural control.

Similarly, the soil bulk density profiles in the 20 to 25-cm depth and the thicker zone of high bulk den- sity in compacted plots (i.e. 10 to 25 cm deep), are indicators of soil strain from surface traffic (Fig. 3). These observations agree with those of Kinney et al. (1992), which showed strain to 30 cm from passage of a somewhat larger tractor that was not drawbar loaded. The soil compaction treatment in this study represents typical compaction produced by random wheel traffic after primary tillage. It consistently increased disease symptoms and reduced pea growth. Compaction was detected by high bulk density in the depth range of 5 to 15-cm with a Ksa~ value low enough to anticipate nearly saturated soil and anaerobic conditions in the upper 15 cm during a moderate rain or irrigation. Con- sequently, some of the adverse growth response could have been a direct effect of poor aeration. Peas are especially sensitive to poor aeration (Allmaras et al., 1988a).

Compaction is also commonly produced by heavy axle loads (Voorhees et al., 1986) and by the shear of a moldboard plow. Such plow pans can be located 10 years after conversion to a ridge-till or no-till in a Nicollet-Webster soil (Logsdon et al., 1990). It is com- mon to find a Ksat of 1/~m s- l in the 25 to 40-cm layer, and in most soils this low Ksat persists throughout the year. A Ksat in the range of 20 to 50 #m s- 1 is desired for good internal drainage in this soil. Soil water poten- tials at the 40 to 60-cm depth were insensitive to irri- gation/rainfall, which could have been caused by the low Ksat at 25 cm (Table 4). It is also likely that there was not sufficient transpiration by the peas to deplete water from depths greater than 30 cm. Burke et al. (1969b) indicated that A. euteiches infection in

pot studies may occur when the soil is at "field capac- ity" and that infection increases markedly when soil is water saturated for 24 hr. Pea seedlings removed from Wisconsin fields within 25 days of planting showed <10% disease symptoms but from 40 to 90% latent infection (Burke et al., 1969b). The higher soil-water potential (i.e. near saturation) in the compacted com- pared to the noncompacted treatment is strong evi- dence that infection could have occurred earlier in the compacted treatment. This serious damage from traffic after primary tillage indicates a need to control traffic; such control would be facilitated with a once-over com- bined secondary tillage and planting. Another option is to use special shovels directly behind the tractor tire to reduce the wheel traffic compaction in the upper 15 c m .

It was striking that even the noncompacted plots remained wet below the 25-cm depth, and transpira- tion was not sufficient to remove more water than that drained by gravity. In a humid climate, root infection by A. euteiches could be delayed by a drier soil early in the spring. Our research demonstrated the value of oats for disease control and others have shown the value of rye (Secale cereale L.) as a winter crop to transpire water from the upper 40 cm (Power and Biederbeck, 1991). A summer oat crop followed by fall seeded rye that is killed just before pea planting may be helpful in suppressingA, euteiches in a humid climate on soils with moderate to poor drainage.

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Section editor: R Rodriguez Kabana