effect of host, nonhost, and fallow soil on populations of thielaviopsis basicola ...

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This article was downloaded by: [McGill University Library] On: 20 November 2014, At: 12:41 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Canadian Journal of Plant Pathology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tcjp20 Effect of Host, Nonhost, and Fallow Soil on Populations of Thielaviopsis Basicola and Severity of Black Root Rot M.S. Reddy a & Z.A. Patrick a a Department of Botany , University of Toronto , Toronto, Ontario, M5S 1A1 Published online: 14 Jan 2010. To cite this article: M.S. Reddy & Z.A. Patrick (1989) Effect of Host, Nonhost, and Fallow Soil on Populations of Thielaviopsis Basicola and Severity of Black Root Rot, Canadian Journal of Plant Pathology, 11:1, 68-74, DOI: 10.1080/07060668909501150 To link to this article: http://dx.doi.org/10.1080/07060668909501150 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

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Page 1: Effect of Host, Nonhost, and Fallow Soil on Populations of               Thielaviopsis Basicola               and Severity of Black Root Rot

This article was downloaded by: [McGill University Library]On: 20 November 2014, At: 12:41Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office:Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Canadian Journal of Plant PathologyPublication details, including instructions for authors and subscriptioninformation:http://www.tandfonline.com/loi/tcjp20

Effect of Host, Nonhost, and Fallow Soil onPopulations of Thielaviopsis Basicola andSeverity of Black Root RotM.S. Reddy a & Z.A. Patrick aa Department of Botany , University of Toronto , Toronto, Ontario, M5S1A1Published online: 14 Jan 2010.

To cite this article: M.S. Reddy & Z.A. Patrick (1989) Effect of Host, Nonhost, and Fallow Soil onPopulations of Thielaviopsis Basicola and Severity of Black Root Rot, Canadian Journal of Plant Pathology,11:1, 68-74, DOI: 10.1080/07060668909501150

To link to this article: http://dx.doi.org/10.1080/07060668909501150

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the“Content”) contained in the publications on our platform. However, Taylor & Francis, ouragents, and our licensors make no representations or warranties whatsoever as to theaccuracy, completeness, or suitability for any purpose of the Content. Any opinions and viewsexpressed in this publication are the opinions and views of the authors, and are not the viewsof or endorsed by Taylor & Francis. The accuracy of the Content should not be relied uponand should be independently verified with primary sources of information. Taylor and Francisshall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses,damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly inconnection with, in relation to or arising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Any substantialor systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, ordistribution in any form to anyone is expressly forbidden. Terms & Conditions of access anduse can be found at http://www.tandfonline.com/page/terms-and-conditions

Page 2: Effect of Host, Nonhost, and Fallow Soil on Populations of               Thielaviopsis Basicola               and Severity of Black Root Rot

CANADIAN JOURNAL OF PLANT PATHOLOGY 11:68-74, 1989

Effect of host, nonhost, and fallow soil on populations of Thielaviopsis basicola and severity of black root rot

M.S. Reddy and Z.A. Patrick

Department of Botany, University of Toronto, Toronto, Ontario M5S IA1. Accepted for publication 1988 11 16

Populations of Thielaviopsis basicola decreased during the first 2 weeks following introduction in soils planted to bean (host), rye (nonhost), and in fallow soil, then began to increase in soils planted to beans. The increase in numbers was associated with increase in lesions on the surface of bean roots. Pathogen populations in soils planted to rye decreased as compared to fallow soil. Extracts from soil in which rye had been decomposing for 30 days or longer inhibited the growth of T. basicola. In soil containing decomposing rye residues the severity of root rot caused by T. basicola was reduced in subsequently planted highly susceptible tobacco cv. Coker. Indigenous soil bacteria, of which 68% were antagonistic to T. basicola, increased during decomposition of rye residues. It appears that the disease suppressing effects of rye occur during its growth and during its decomposition and is associated with microbial antagonisms and antibiotic production with rye as substrate.

Reddy, M.S., and Z.A. Patrick. 1989. Effect of host, nonhost, and fallow soil on populations of Thielaviopsis basicola and severity of black root rot. Can. J. Plant Pathol. 11: 68-74.

Des populations de Thielaviopsis basicola ont diminué au cours des 2 semaines suivant leur introduction dans des sols plantés en haricot (hôte), en seigle (non hôte), et dans un sol en jachère, puis ont commencé à accroître dans les sols plantés en haricot. L'augmentation en nombre fut associée à l'augmentation des lésions en surface des racines du haricot. Les populations du champignon pathogène dans les sols plantés en seigle ont décru comparativement aux sols en jachère. Des extraits du sol dans lequel le seigle s'est décomposé durant 30 jours ou plus a inhibé la croissance du T. basicola. Dans le sol contenant des résidus de seigle en décomposition, la gravité de la pourriture racinaire causée par le T. basicola fut réduite chez le très sensible tabac cv. Coker subséquemment planté. Les bactéries telluriques indigènes, dont 68% étaient antagonistes au T. basicola ont augmenté durant la décomposition des résidus de seigle. Il semble que les effets suppressifs de la maladie par le seigle se produisent durant sa croissance et durant sa décomposition, et sont associés à des antagonismes microbiens et à une production antibiotique ayant le seigle comme matière première.

The fungus Chalara elegans (Nag Raj & Kendrick 1975), better known as Thielaviopsis basicola (Berk. & Br.) Ferraris, is a soil-borne root pathogen that attacks a wide variety of cultivated and wild plants in the field and greenhouse. Some 137 species of plants, principally in the Solanaceae, Leguminosae, and Cucurbitaceae, have been reported as hosts (Lucas 1975). It causes one of the major diseases of tobacco in Canada and elsewhere (Lucas 1975), and is important on bean (Christou 1962), cotton (Johnson & Doyle 1986), and citrus (Tsao & Van Gundy 1962); it also causes replant failure of cherry and plum (Sewell & Wilson 1975) and the blackhull disease of peanuts (Hsi 1978). Most of the Gramineae appear to be resistant and are regarded as nonhosts of the pathogen (Lucas 1975). The fungus is capable of indefinite survival in soil, even in the absence of host plants (Lucas 1975). It has been found in cultivated and virgin soils throughout the world (Lucas 1975). Various investigators observed that disease severity varied from year to year in the same soil and with the same plant species (Smith 1960), and at times T. basicola was isolated in the apparent absence of disease (Yarwood 1981). The variability in pathogenicity of T. basicola has been ascribed to many biological

and abiotic environmental factors acting on the host plant or the parasite, but conclusive information is sparse or controversial (Hsi 1978, Smith 1960). Soils that appear to be naturally conducive or suppressive to black root rot have been described, but the specific factors involved are also mostly undefined (Hsi 1978, Stutz et al. 1986).

In southwestern Ontario T. basicola is an aggressive pathogen of tobacco and to a lesser extent of legumes and other crops; accordingly, it is a serious potential threat to some of the new crops proposed as alternatives to tobacco. Effective control of tobacco black root rot caused by T. basicola is obtained by the use of resistant cultivars. Rotating with nonhost plants also appears to reduce disease severity to some extent (Hsi 1978, Smith 1960). For example, tobacco is usually rotated with rye (Secale cereale), or wheat (Triticum aestivwn), but the precise role of the nonhost plants in the suppression of disease is still not clear. Various investigators showed that nonhost plants, such as wheat, corn, or sorghum, had little influence on populations of T. basicola in field soils (Bateman 1963, Hsi 1978), and that disease severity did not appear to be directly related to pathogen populations in the soils (Bateman

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REDDY, PATRICK: THIELAVIOPSIS/SOIL POPULATIONS 69

1963). Whether rye or other nonhost plants modify disease incidence and severity indirectly through their influence on the microbial soil flora antagonistic to T. basicola or through the production of compounds inhibitory to the pathogen has received little attention.

The objectives of this research were to study the effect of host (bean), and nonhost (rye) and of fallow soil on population levels of T. basicola and on populations of associated indigenous bacteria, Additionally, the effect of decomposition products from rye on vegetative growth, population levels, and pathogenicity of the fungus was determined.

Materials and methods Source of fungal pathogen and production of

inoculum. The isolate of T. basicola used in this study was obtained from naturally infested soil from the Delhi area, Ontario, Canada, using the carrot disc method described by Yarwood (1946). It was maintained at 25° C on V-8 agar. Chlamydos­pores of T. basicola were obtained from cultures grown for approximately 21 days at 25° C on V-8 agar in Roux culture bottles (56 * 122 * 268 mm). Most of the endoconidia were removed from the culture by flooding the agar surface with sterilized distilled water (SDW) for several minutes and discarding the supernatant. Chlamydospores were dislodged from the culture by scraping the fungal mat with a sterile spatula. The resulting suspension (mostly chlamydospores but with some conidia and hyphal fragments) was dispersed using a high speed blender for 2 min to break up clumps of mycelium and chlamydospores. This suspension was then filtered through lens paper which trapped the chlamydospores and allowed most of hyphal fragments to pass through. This procedure provided chlamydospores reasonably free of endoconidia and hyphae.

Influence of host, nonhost plants, and fallow soil on populations of T. basicola and indigenous bacteria in rhizosphere and root zone soils. Bean {Phaseolus vulgaris) cv. California Red Kidney (Stokes Seeds Ltd., St. Catharines, Ontario) was used as the host plant and winter rye {Secale cereale) as the nonhost for T. basicola. Experi­ments were conducted in sterilized and unsterilized sandy loam soils (pH 6.8) collected from tobacco fields from two locations near Delhi, Ontario. In all cases composite samples were collected from the top 15 cm, and the soils were used shortly after being collected. The moisture content of the soil was adjusted and maintained at approximately 60% throughout the experiments by the method described by Johnson and Curl (1972). In experiments requiring sterilized soil, samples of the

above soil were placed in polypropylene trays (30 * 20 x 13 cm), covered with aluminum foil, and autoclaved for 1 h on each of 3 consecutive days. No microorganisms were detected when the autoclaved soil was plated onto potato dextrose agar (PDA). Both sterilized and unsterilized soils were free of T. basicola prior to use and this was confirmed by plating soil samples on VDYA-PCNB medium (V-8 juice-dextrose-yeast extract-pentachloronitrobenzene) (Papavizas 1964). The chlamydospores were added to both sterilized and unsterilized soils within polyethylene bags and mixed thoroughly. This procedure resulted in an initial concentration of log 6.80 chlamydospores/g dry weight (DWT) in each soil sample. The soils were placed in 15 cm diameter plastic pots. Each pot received approximately 850 g of the appro­priate soil. Bean or rye seeds were planted at the rate of 3 and 5 seeds/pot, respectively. The treatments were soil with T. basicola planted to beans, soil with T. basicola planted to rye, fallow soil without T. basicola, and fallow soil with T. basicola. The treatments were repeated for the sterilized and unsterilized soils. There were four replicates for each treatment. The pots were arranged in a randomized complete block design on a plant growth bench in the greenhouse at 23-25° C. The soil moisture in the greenhouse studies was maintained in the range of 60-80% of field capacity throughout the experiments by adding sterile distilled water (SDW) periodically on a weight basis to the pots to replace moisture lost (Johnson &Curl 1972). Population estimates of the pathogen present in the rhizophere and nonrhizophere soil of bean and rye plants and in fallow soil were made at 0, 2, 4, 6, 8, and 10 weeks after seeding, using the dilution plating method on VDYA—PCNB medium (Papavizas 1964). Estimates of T. basicola in the rhizophere of bean and rye plants were done as follows. One plant per treatment per replication was dug from each pot and shaken to remove excess soil. The entire root system, and the soil adhering after the gentle shaking, was detached from the shoot and designated as rhizophere. This root sample was cut into small segments, placed in 250 mL flasks containing 100 mL of SDW, and washed for 30 min by gentle stirring, using a magnet. Serial dilutions of the wash suspension were prepared and I mL aliquots from the appropriate dilutions were plated onto VDYA-PCNB media in triplicate. Samples designated as nonrhizosphere soil were obtained from the root zone area away from the rhizosphere from each appropriate treatment. They were mixed in a polyethylene bag and subsamples (10 g) taken. These were placed into 500 mL flasks containing 100 mL SDW, and serial dilutions

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70 CANADIAN JOURNAL OF PLANT PATHOLOGY, VOLUME 11, 1989

similar to the procedure described above were prepared. Plates were incubated at 25°C, and colony-forming units (CFU) of T. basicola were counted on the 6th or 7th day. Population levels of T. basicola were reported as CFU/g DWT of soil.

Total indigenous bacterial populations asso­ciated with the rhizospheres of bean and rye plants and in fallow soils with and without T. basicola were estimated at the sampling intervals described above. Aliquots of the same serial dilution suspensions of the rhizosphere and of fallow soil used earlier were plated onto Thornton's agar (TA) (Johnson & Curl 1972) in triplicate. The plates were incubated at 25° C and colony counts were recorded after 3 days and estimated as CFU/g DWT of soil,

Effect of aqueous extracts of soil containing decomposing rye residues on radial growth of T. basicola. Growing rye plants and soil were collected at the end of September from fields near Delhi, Ontario. The rye plants, which had grown to a height of approximately 20 to 25 cm, were cut into pieces approximately 2 cm long and were used immediately or stored at 4°C until use in plastic bags to retard desiccation. The rye plant material was incorporated into the soil at a ratio of 1:4 (w/ w) and placed into 30 cm diameter clay pots

(approximately 2 kg of rye and soil mix/ pot). Enough tap water was added to each of the pots to attain 60% moisture content of the soil. Soil without rye material was placed in similar pots and treated as controls. There were five replications for each treatment. The pots were placed outdoors for 30 days. The outdoor temperatures during this period fluctuated between 10°C at night and 20° C during the day. After 0, 15, and 30-days decomposition periods aqueous extracts were prepared from each treatment and their effects on radial growth of T. basicola were tested. The aqueous extracts were prepared at each decomposi­tion period using 500 g samples of the soil-plant mixture from each treatment. The samples were placed in flasks containing 1000 mL of SDW, stirred for 30 min and allowed to steep and settle overnight at 4°C. The liquid was decanted and centrifuged at 2500 rpm for 30 min. The supernatant was filtered through two layers of cheesecloth and then through Whatman No. 1 filter paper. The extracts were finally filtered using 0.22 nm Millipore filters. The filtrates were added to water agar (WA) to attain concentrations of 20% and 50% (v/ v) and poured into petri plates ( 18 mL/ plate). The agar surface on these plates was allowed

o m \-□ CD \ D Li_ U

CD O

10- -

10 0 2 TIME (WEEKS!

8 10

Figure 1. Effect of growing rye or bean plants on subsequent populations of Thielaviopsis basicola compared to fallow soil. A, fallow soil; B, bean rhizosphere; C, bean nonrhizosphere soil; D, rye nonrhizosphere soil; E, rye rhizosphere. The data are the means of four replications per treatment and were analyzed by analysis of variance and Student Newman-Keul's test (P = 0.05).

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REDDY, PATRICK: THIELAVIOPSIS/SOIL POPULATIONS 71

caused by T. basicola. To determine the effect of decomposing rye residues on disease severity and survival of T. basicola in soil, chlamydospores were added to soils with and without rye residues at a rate of log 7.89 chlamydospores/g DWT of soil. Survival of the pathogen and populations of indigenous bacteria after 0, 10, 20, and 30 days decomposition periods outdoors in the various treatments were estimated using VDYA-PCNB media, and TA, respectively. Random isolates of bacteria from representative colony types present in greatest numbers on the dilution plates were streaked onto PDA, incubated at 25° C and checked for purity and for antagonism against to T. basicola. The test bacterial isolates were streaked approximately 2.5 cm from the edge of a 9 cm diameter petri plate containing PDA and allowed to grow for 24 h. A 5 mm diameter disc taken from the edge of an actively growing colony of T. basicola on V-8 was then placed into the dish 2.5 cm from the edge on the opposite side. The plates were incubated in the dark for up to 15 days at 25° C and evaluated subjectively for differences in the nature and magnitude of the inhibition zone between the bacterial isolate and T. basicola.

Thirty-day-old tobacco seedlings of cv. Coker 319, which is highly susceptible to black root rot, were planted into soil used in the experiments described above in which rye had been decompos­ing outdoors for 30 days, with or without T. basicola. There were four replicates for each treatment arranged in a randomized complete block design on a plant growth bench in the greenhouse maintained at 23-25° C. The moisture content of the soil was maintained at 60% as described above. After 4 weeks the seedlings were rated for disease severity on a numerical scale of 0-5, where 0 denoted no symptoms and 5 denoted severe symptoms of black root rot. A disease index

Table 1. Effect of aqueous extracts of soils containing decomposing rye residues on radial growth of Thielaviopsis basicola in vitro

Aqueous extract2

R F WA

20%

6.45 a3

7.16b 7.08 b

Colony diameter (cm)1 in following decompositior

0 50%

5.12 a 6.92 b 7.08 b

20% 5.92 a 7.36 b 7.12 b

20% and 50% i periods of 0,

15 50%

4.00 a 6.34 b 7.12 b

aq 15,

jeous extracts and 30 days

30 20%

2.00 a 7.72 b 7.76 b

50% 1.58 a 7.66 b 7.76 b

'Mean of combined data from two experiments each having 5 replications. 2R = soil in which rye residues had decomposed; F = fallow soil (control); WA = water agar. 'Means within a column followed by the same letter do not differ significantly according to Student Newman-Keul's test (P = 0.05).

TIME (WEEKS)

Figure 2. Effect of rye or bean plants or of fallow soil with and without Thielaviopsis basicola on indigenous bacterial populations. A, fallow soil without T. basicola; B, fallow soil with T. basicola; C, bean rhizosphere without T. basicola; D, bean rhizosphere with T. basicola; E, rye rhizosphere without T. basicola; F, rye rhizosphere with T. basicola. The data are the means of four replications per treatment and were analyzed by analysis of variance and Student Newman-Keul's test (P = 0.05).

to dry for 48 h at 25° C and 5 mm V-8 agar discs, cut from the advancing edges of actively growing 10 day cultures of T. basicola, were placed at the centre of each plate. There were five plates for each concentration and for each treatment per replication. The plates were incubated at 25° C, and radial growth was measured after 10 days. The experiments were conducted twice.

Effects of decomposing rye residues on populations of indigenous soil bacteria and on subsequent severity of black root rot of tobacco

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72 CANADIAN JOURNAL OF PLANT PATHOLOGY, VOLUME 11, 1989

TIME (DAYS) Figure 3. Effect of rye residues decomposing in soil on subsequent populations of Thielaviopsis basicola. R, soil containing decomposing rye residues; F, fallow soil (control). The data are the means of two experiments, and there were four replications per treatment in each experiment and were analyzed by analysis of variance and Student Newman-Keul's test (P = 0.05).

a in

□ CD \ U_ CJ

CD O

TIME (DAYS) Figure 4. Effect of rye residues decomposing in soil on subsequent populations of indigenous bacteria. R, soil containing decomposing rye residues; F, Fallow soil (control). The data are the means of two experiments and there were four replications per treatment and were analyzed by analysis of variance and Student Newman-Keul's test (P = 0.05).

was calculated by averaging the rate of infection of four replications involving approximately 20 plants. Populations of T. basicola in the rhizosphere and nonrhizosphere soils of tobacco were estimated using VDYA-PCNB medium (Papavizas 1964), as described earlier. The experiments were conducted twice.

The data of all experiments were analyzed by analysis of variance and Student Newman-Keul's test at 5% level of significance.

Results Influence of host and nonhost plants and of

fallow soil on populations of T. basicola and indigenous bacteria in rhizosphere and root zone soils. Population levels of T. basicola added to unsterilized and sterilized field soil were monitored for 10 weeks following seeding with bean (host) or rye (nonhost) (Fig. 1). Pathogen numbers decreased in all treatments during the first 2 weeks. The pathogen populations then appeared to stabilize and remained relatively constant in both the sterilized and unsterilized fallow soils. After the 2 week period, populations of T. basicola began to increase in the rhizosphere and nonrhizosphere soils planted with beans. The increase appeared to

be significantly higher in the rhizosphere than in the nonrhizosphere soils. The level of increase in pathogen population was associated with the appearance of lesions on the surface of bean roots and continued to increase with increased root rot severity. At the termination of the experiment bean root rot severity was rated as 4.5 in soil that was initially sterilized and 4.0 in the unsterilized soil. Most of the bean root surface was diseased. At the end the 10-week period the pathogen populations in the bean rhizosphere had reached an average of log 7.79 CFU/g DWT of soil in the treatments that were initially sterilized and log 7.62 in the unsterilized soil. However, in soils planted to rye there was a significant reduction in the pathogen populations in the rhizosphere and nonrhizosphere soils during the 10 week period. The population decrease in the presence of rye was also significantly greater than that which occurred in the fallow soil.

The numbers of bacteria in fallow soil were somewhat lower than in soils in which bean plants were growing (Fig. 2). Bacterial populations also tended to be somewhat higher in soils to which T. basicola had been added. This was especially striking in fallow soil 6 to 10 weeks after the fungus had been added. The highest numbers of bacteria

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REDDY, PATRICK; THIELAVIOPSIS/SOIL POPULATIONS 73

Table 2. Relative percentage of indigenous bacteria isolated from soil in which rye residues had been decomposing for 30 days that were antagonistic to Thielaviopsis basicola on agar plates compared to isolates from fallow soil

Soil1

Total bacteria Number of recovered isolates

(Log cfu/ g D WT of soil)2 tested

Total isolates antagonistic to T. basicola{%)

9.60 b3

6.70 a 85 71

68 b3

23 a 'R = soil containing decomposing rye residues; F = fallow soil (control). 2Means of combined data from two experiments each having four replications per treatment 'Means within a column followed by the same letter do not differ significantly according to Student Newman-Keul's test (P = 0.05).

were associated with the host rhizosphere without T. basicola. The numbers of bacteria associated with rye rhizosphere with or without T. basicola were also relatively high.

Effect of aqueous extracts of soil containing decomposing rye residues on growth and on populations of T. basicola. Aqueous extracts obtained from soils in which rye residues had been decomposing for varying periods significantly inhibited radial growth of T. basicola (Table 1). The inhibition of radial growth increased with the decomposition period and was highest in extracts obtained from soils in which rye had been decomposing for 30 days. No inhibition was observed when soil extracts that did not contain rye residues were added to WA relative to growth observed on WA alone (Table 1). The inhibition by the decomposing rye extracts appeared to be fungistatic and normal fungal growth resumed when the advancing edges of the inhibited colonies were transferred to WA.

Populations of T. basicola were significantly reduced in soil containing decomposing rye residues (Fig. 3).

Effect of decomposing rye residues on popula­tions of indigenous soil bacteria and on severity of black root rot of tobacco. Populations of indigenous bacteria in soil with decomposing rye residues were significantly higher than in control soils without the rye substrate (Fig. 4). In addition, a significantly higher percentage of bacteria isolated from soil containing decomposing rye residues were antagonistic to T. basicola compared to isolates from fallow soil (Table 2).

Soils in which rye residue had been decomposing outdoors for 30 days were subsequently used to grow the highly susceptible tobacco cv. Coker 319. Disease severity was evaluated 30 days after planting. The inoculum levels of T. basicola at planting time were log 3.19 CFU/g in soil containing the decomposing rye residues and log 4.89 CFU/g DWT soil in the controls without the rye. Disease severity in soils in which rye residues had been decomposing for 30 days before tobacco was planted was significantly lower than that in controls that did not contain decomposing rye (Table 3). Disease severity and populations of the pathogen were directly related and both were

Table 3. Effect of rye residue decomposition in soil for 30 days on subsequent severity of black root-rot of tobacco (cv. Coker 319) caused by Thielaviopsis basicola and on pathogen populations

Populations of T. basicola from tobacco seedlings 30 days after planting

(Logcfu/gDWTofsoil)1

Treatment2 Disease index3 Rhizosphere Nonrhizosphere soil 1.96 a" 3.98 b

2.30 a4

5.67 b 1.75 a" 3.68 b

'Mean of combined data from two experiments each having four replications per treatment. 2R = soil in which rye residues had been decomposed for 30 days; F = fallow soil (control). 3The numerical disease index is the means of combined data from two experiments. There were four replications per treatment in each experiment, five plants per replication.

4Means within a column followed by the same letter do not differ significantly according to Student Newman-Keul's test (P = 0.05).

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74 CANADIAN JOURNAL OF PLANT PATHOLOGY, VOLUME 11, 1989

significantly reduced during the decomposition of rye residues in the soil.

Discussion The benefits of rye as a green-manure cover crop are well recognized and are reflected in its widespread use in southwestern Ontario. In this study rye reduced the incidence and severity of black root rot of beans caused by T. basicola by mechanisms that affected the pathogen directly and indirectly. During the time that rye, a nonhost of T. basicola, was growing, infective propagules of the pathogen were not produced in the soil. Also, our results showed some reduction in populations of the pathogen during the decomposition of rye residues in the soil and a simultaneous increase in the indigenous soil bacterial flora. Substances fungistatic to T. basicola were also produced during decomposition of rye residues in soil. Although the above mechanisms suppress T, basicola, they do not eliminate the pathogen from the soil. Some viable pathogen propagules remain. This may partly explain the results of other investigators (Bateman 1963, Hsi 1978) who concluded that nonhost plants such as sorghum, wheat, or corn had little influence on populations of T. basicola in field soil. Other studies have shown (Patrick & Toussoun 1965) that substances with phytotoxic properties may be produced during decomposition of rye and other types of plant materials in soil which increase the severity of black root rot caused by T. basicola. It appears, therefore, that in some instances there may be plant injury and increased pathogenesis following the rye cover crop and, as shown in the present studies, there could also be adverse effects exerted on the pathogen which results in disease amelioration. Although both the phytotoxic and ameliorating effects of rye on the black root rot disease caused by T. basicola have been shown to occur, more research is needed on ways to minimize the phytotoxic effects and capitalize on the disease suppressing aspects. Based on the present studies we believe that there is some reduction in populations of T. basicola and in disease incidence and severity associated with the use of rye in the

rotation. The disease suppressing effects of rye occur during both its growth and its decomposition and appear to be associated with microbial antagonisms and antibiotic production with rye as substrate.

The authors gratefully acknowledge Alice Cheung for her technical assistance. This work was supported in part by Natural Sciences and Engineering Research Council of Canada Grant No. A2384toZ.A. Patrick.

Bateman, D.F. 1963. Influence of host and nonhost plants upon populations of Thielaviopsis basicola in soil. Phytopathology 53: 1174-1177.

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