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Survey of soil-borne disease suppression to Rhizoctonia solani in low rainfall farming systems on upper Eyre Peninsula, South Australia.

Amanda Cook1, Nigel Wilhelm1 and Chris Dyson2.

1SARDI, Minnipa Agricultural Centre, PO Box 31, Minnipa, South Australia 5654. Email cook.amanda@saugov.sa.gov.au, wilhelm.nigel@saugov.sa.gov.au

2SARDI, GPO Box 397, Adelaide, South Australia, 5001. Email dyson.chris@saugov.sa.gov.au

Abstract

Biological soil-borne disease suppression is defined as the ability of soil biota to compete with and inhibit a plant pathogen, thus decreasing disease incidence and severity. Rhizoctonia solani AG8 is a major wheat root pathogen in many Eyre Peninsula (EP) soils. An increased understanding of disease suppression is required to improve control of this pathogen within low rainfall farming systems.

During 2006 and 2007 a survey investigated the occurrence and extent of disease suppression in a range of soils across upper EP. Potential disease suppression was estimated using a bioassay with young wheat seedlings. The assay estimates the potential of the microbial biota in the soil to compete with Rhizoctonia under ideal conditions. Paddock management history for the last ten years was collected from each location. In 2007 fifty of the survey paddocks were sown with a cereal crop. These locations were visually scored for Rhizoctonia patches early in the growing season to compare potential suppression estimated in the bioassay with actual disease expression in the field.

Key wordsj

Bioassay, soil biota.

Introduction

The importance of soil biology in current dryland farming systems is not well understood due to the complexity of these systems which involve the cycling of nutrients and microbial interactions, and lack of current research tools. The soil-borne fungus Rhizoctonia solani is one of the most important plant pathogenic fungi, with a wide range of plant hosts and world wide distribution (O’Brien and Zamani, 2003). Rhizoctonia solani AG8 is a major disease agent in cereal based farming systems, despite advances in control and management of other disease pathogens through plant breeding and better understanding of disease cycles.

The upper EP is a semi arid Mediterranean environment which on average receives less than 350 mm of annual rainfall and produces 24% of SA’s cereal production (Adcock, 2005). On upper Eyre Peninsula, rhizoctonia is estimated to reduce profitability by around $65 million dollars per year (N Cordon, Rural Solutions SA, pers comm.). The decline of R. solani disease symptoms and the development of biological disease suppression in a dryland cereal system were first observed in a tillage and rotation trial at Avon, SA. In 1983 the severity of rhizoctonia resulted in poor plant growth in 46% of the crop area, but this declined to negligible levels by 1990. The soil was an alkaline calcareous sandy loam, pH 8.2, organic carbon of 1.6%, total N 0.15%, CaCO3 8% (Roget, 1995, Wiseman, et al., 1996).

Such 'disease suppression' offers hope for substantially reducing the impact of rhizoctonia in upper EP farming systems. A survey of eighty one sites was conducted across the upper EP during 2006 and 2007 to estimate the extent of disease suppression in current farming systems. Management histories were collected from each location in an attempt to identify which factors were important to the development of disease suppression.

Methods

Pot Bioassay

A pot bioassay was used to assess potential disease suppression (Roget et al., 1999) with some bioassays carried out in summer, with no plants actively growing in the sampled soil, and others sampled during crop growth. Growing plants have the ability to change the soil microbial population especially within the plant root rhizosphere. Approximately 5 kg of 0-10 cm soil was collected from each paddock, two to four weeks before each bioassay, along a 500 m transect. The soil was air dried and stored at 4oC. A sample from a paddock on the Minnipa Agricultural Centre (MAC), N12 was included within each pot bioassay as a benchmark soil. This is a continuously cropped non-grazed paddock which has shown accumulation of soil organic carbon over twenty two years and reduced visual symptoms of rhizoctonia over this time (B Holloway, University of Adelaide, pers comm.). MAC N12 showed potential disease suppression in a soil bioassay of commercial farm paddocks in southern Australia but at a lower level than Avon (Roget and Gupta, 1999), but was used as the benchmark soil for the Eyre Peninsula survey.

Pots were 300 mL and contained 300 g of sieved air-dried soil. Pot soils were untreated (nil (N)), or had Rhizoctonia inoculum (R) added, or Rhizoctonia inoculum plus sucrose (RC) added. One gram of sucrose was thoroughly mixed through the dry soil of the sucrose treatments. All pots inoculated with Rhizoctonia had 100g of soil removed, and one 8 mm plug from the edge of a R. solani AG-8 colony growing on quarter strength PDA placed centrally and the soil replaced. The pots were watered to 10% of weight and kept at 10-12oC, with a 12-hour light/dark regime for two weeks. Five wheat seedlings per pot (cv. Wyalkatchem, sterilised) were then planted, watered weekly, and grown for four weeks, after which time the roots were washed and scored for disease using the root scoring method described by McDonald and Rovira (1983).

This bioassay estimates the potential of the microbial population in the soil to compete with the disease pathogen. Potential suppression is expressed as score at R – less the score at RC (absolute value). Microbial competition lowers the level of rhizoctonia disease on the seedling roots in a potentially suppressive soil. The pot bioassay was developed using soil from the long-term farming system experiment at Avon using carbon (sucrose) substrate amendments, and 1g of sucrose showed the greatest difference in disease suppression and decline of rhizoctonia. Sucrose (carbon) is added to ensure that there is adequate energy for microbial growth and expression of disease suppression.

All soil samples were analysed for fertility, pathogen DNA and chemical characteristics. Farm paddock management history for the previous ten years were also collected and included rotations, cereal yields, tillage practices, fertiliser management, herbicide use and frequency of stubble burning.

Detailed Field Survey

Fifty of the sampled paddocks were sown to a cereal crop in the 2007 season and these were surveyed for rhizoctonia disease early in that growing season. A 200 metre transect, following the same transect where soil was collected for the pot bioassay, was scored every two metres for visual patch symptoms of Rhizoctonia. Two plants were removed at random every ten metres along the transect, and the roots were washed and scored for rhizoctonia using the McDonald and Rovira method (1983). Leaf material was dried and analysed for nutritional status.

Statistical analysis

Soil nitrogen, calcium carbonate, disease pathogen levels (DNA g/soil), soil type, % cereal crop, % medic pasture, P and N fertiliser inputs (units/year) and leaf nutrient levels were analysed using GENSTAT 10.1 by multiple regression analysis to relate measured parameters, and interactions, to both current rhizoctonia status (bioassay and paddock survey) and long term wheat yield. The MAC N12 results were limited to four samples displaying wide variation to avoid unwarranted replication. Soils were classified according to collection time, either over summer (Dec-April) or in crop (May-Nov), and soil type, either grey calcareous sandy loam, red sandy loam or sand.

Results

Bioassay

Many locations in the survey had suppression to rhizoctonia in the pot bioassay higher than MAC N12 (Fig 1). Mean potential suppression was higher for the sand soils than for other soil types (Table 1).

Potential disease suppression at eighty-five locations was negatively correlated with average grain yield over ten years (P<0.01, negative) and positively correlated with soil collection time (P<0.01, higher in summer) (R2 =27%; rsd=0.82). The interaction with long term yield shows potential disease suppression in this survey was strongest in paddocks yielding approximately 1.0 - 1.5 t/ha (Fig 2).

Figure 1. Distribution of potential disease suppression of Rhizoctonia in 108 EP soils using the pot bioassay.

Figure 2. The relationship between potential disease suppression using the bioassay and long term paddock wheat yield on upper EP soils (yield data from farmer records).

Table 1. Soil properties, management history and potential disease suppression on several soil types of upper EP. (*Average over a ten year period from farm management history, other measurements taken at time of survey.)

 

Grey calcareous
sandy loam

Red sandy
loam

Sand

Factor

Mean

Min

Max

Mean

Min

Max

Mean

Min

Max

% CaCO3

55

16

82

9

0

65

n/a

    

% Cereal*

60

0

100

68

10

100

67

50

90

Ave P input (kg/ha)*

12

0

20

11

0

16

11

8

13

Potential Suppression

0.8

-1.1

3.5

0.8

-0.8

3.5

1.6

-0.7

3.3

Rhizoctonia DNA g/soil

0.6

0.1

1.8

0.6

0

1.9

0.6

0.1

2.6

Yield (t/ha)*

1.1

0.3

2.2

1.5

0.7

2.8

1.7

0.9

2.5

Total N %

0.15

0.07

0.23

0.11

0.05

0.38

n/a

    

A correlation analysis of wheat yield over the 85 EP soils showed correlations (R2 = 38%; rsd = 0.41) with: average phosphorus fertiliser inputs over ten year period) (P<0.01, positive), potential suppression (P<0.01, negative), calcium carbonate content (P< 0.01, negative), the interaction between potential suppression and calcium carbonate (P<0.05, positive) and soil type (P<0.1).

Survey

The correlation analysis of rhizoctonia disease score from fifty paddocks in 2007 showed P inputs (P<0.05, negative), average cereal grain yield over ten years (P<0.05, negative) and calcium carbonate content of the soil (P<0.01, positive) were correlated to the incidence of disease in crop (R2 = 42; rsd = 0.15). There were no interactions between these with potential disease suppression. The addition of P reduced disease symptoms. This analysis identified that current rhizoctonia disease status reflected the long term yield achieved. Leaf tissue nutrient analysis of the surveyed paddocks showed zinc and phosphorus deficiency are still an issue on Eyre Peninsula soils with 70% of paddocks below the total critical nutrient level of 4400 mg/kg P and 50% below 16 mg/kg for Zn.

Conclusions

Potential disease suppression was higher in soils collected in the summer period than those collected during the cropping season. In this survey disease suppression was strongest with paddocks yielding 1.4 t/ha, this may be reinforced by the greater number of soils within this sample, or the amount of carbon added to the bioassay. Nitrogen is believed to be a ‘switch’ to activate disease suppression expression (Roget and Gupta, 2006) so in soils with high nitrogen, the amount of carbon added may not have been enough to allow the full expression of disease suppression.

There was no significant relationship between rhizoctonia disease in crop and potential disease suppression indicating that either; other factors are controlling disease expression or masking suppressive activity, the level suppressive activity is too low, or the potential suppression bioassay is not appropriate for these soils.

The rhizoctonia disease score from fifty paddocks scored in 2007 showed the main factors correlated with rhizoctonia disease were phosphorus (P) inputs, average cereal grain yield over ten years and the calcium carbonate content of the soil. The lower the grain yield of a paddock over the ten year period the more likely it was to have rhizoctonia present, and the level of calcium carbonate in the soil increased the level of the disease or expression of rhizoctonia. Phosphorus and zinc deficiency was an issue on some of the EP soils surveyed.

This broadscale survey of disease suppression on upper EP soils is the first of its kind Accurate soil organic C and microbial respiration measurements have yet to be finalised, and further research is required to understand the expression of biological disease suppression of Rhizoctonia in these environments.

Acknowledgements

Thank you to SAGIT for research funding and Wade Shepperd for his technical assistance. Thank you to Annie McNeill, David Roget, Gupta, Sjaan Davey, Steve Barnett and Alan McKay for discussions and advice.

References

Adcock, D.P. (2005) Soil Water and Nitrogen Dynamics of Farming Systems on the Upper Eyre Peninsula, South Australia. The University of Adelaide, PhD Thesis.

MacDonald, H.J. and Rovira, A.D. (1983) Development of Inoculation Technique for Rhizoctonia solani and Its Application to Screening Cereal Cultivars for Resistance. Ecology and Management of Soilborne Plant Pathogens, Melbourne, The American Phytopathological Society.

O’Brien, P.A. and Zamani, M. (2003) “Production of pectin enzymes by barepatch isolates of Rhizoctonia solani AG 8.” Australasian Plant Pathology, 32: 65-72.

Roget, D.K. (1995) "Decline in root rot (Rhizoctonia solani AG-8) in wheat in a tillage and rotation experiment at Avon, South Australia." Australian Journal of Experimental Agriculture 35(7): 1009 - 1013.

Roget, D.K., Coppi, J.A., Herdina and Gupta, V.V.S.R. (1999) Assessment of suppression to Rhizoctonia solani in a range of soils across SE Australia. In: Proceedings of the First Australasian SoilBorne Disease symposium. R.C. Magarey (Ed.), pp. 129-130, BSES, Brisbane, Australia.

Roget, D.K. and Gupta, V.S.R.Vadakattu, (2006). “Rhizoctonia control through the management of disease suppressive activity in soils.” 18th World Congress of Soil Science, Philadelphia, Pennsylvania, USA.

Wiseman, B. M., Neate, S.M., Keller, K.O. and Smith, S.E. (1996). "Suppression of Rhizoctonia solani anastomosis group 8 in Australia and its biological nature." Soil Biology and Biochemistry 28(6): 727-732.

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