Source: GRDC Grainzone
 

Title	Rhizoctonia Update: A summary of current knowledge including recent developments
Description	Research Update for Advisers - Southern Region
Authors	David Roget, CSIRO Sustainable Ecosystems, Adelaide, Ph: 08 8303 8528, David.Roget@csiro.au Gupta Vadakattu, CSIRO Entomology, Adelaide
Presented	Adelaide & Bendigo

Rhizoctonia root rot of cereals (caused by Rhizoctonia solani AG8) is an ongoing concern for many farmers. This is because it is difficult to predict its occurrence and severity, the options for control are limited and the control options may only provide partial reduction in disease expression and yield loss.

The principal issues that make rhizoctonia root rot difficult to control are (i) a wide host range i.e. limited rotational controls and (ii) the pathogen has strong saprophytic ability, that is, it can survive in the soil on organic residues and does not need a growing host plant. The saprophytic ability of the pathogen can be strongly influenced by the soil environment (soil type, fertility, moisture, temperature etc) and this is why the disease can appear out of the blue in some paddocks and why some seasons are much worse than others.

Current in-season control strategies

1. Strategies to reduce inoculum
   1. Cultivation - break up of fungal hyphal networks.
   2. Removal of volunteer plant growth prior to sowing (in seasons with an early break).
2. Strategies to help roots avoid inoculum through encouraging faster early growth
   1. Adequate nutrition - particularly P, N and Zn.
   2. Soil disturbance below seeding depth in no-till systems - narrow points.
   3. Early sowing into warm soils where possible.
   4. Awareness / avoidance of SU herbicides - both in relation to pre-sowing applications and residues from the previous season. 

Long-term control through the development of increased disease suppressive activity in soils

The level of disease suppressive activity in soils against fungal diseases is a function of the population, activity and composition of the microbial community. All soils have an inherent level of suppressive activity, but this level can be significantly modified by management practices used within a farming system. At Avon , SA, disease suppression increased from a low to high level over a period of 5-10 years following a change in management practices to full stubble retention, limited grazing and higher nutrient inputs to meet crop demand (Roget 1995). The increase in suppression provided complete control of the soil borne diseases Rhizoctonia (Roget 1995) and take-all (Roget 1997). Soils with high levels of disease suppression have also been identified in commercial farms across SA and Victoria (Roget et al. 1999). The management factors consistently related to soils with improved disease suppression included intensive cropping, stubble retention, limited grazing, limited or no cultivation and above average yields (high water use efficiency). These management practices increase biologically available carbon inputs and result in changes to the composition and activity of the soil microbial community over time (Gupta and Neate, 1999). These changes result in greater competition for soil resources that, along with predation and inhibition of pathogens, lead to increased disease suppression.

More recent findings on Rhizoctonia

1. Role of soil mineral N and its management

In the short term (1-3 years), the effectiveness of the disease suppressive activity already developed in the farming system / soil can be influenced by the availability of mineral N, particularly during the summer and early autumn period. As the amount of available N (i.e. nitrate N) in the topsoil increases during this non-crop period, the disease suppression occurring in the following crop season decreases. The underlying suppressive activity may not be lost, but is not expressed effectively in the presence of high mineral N levels.

Any factors that result in the accumulation of higher available N levels for longer periods of time will tend to curb the effectiveness of the suppressive activity of the soil. Some of these factors include:

• Legume pastures
• Green manures
• Non harvested crop paddocks i.e. sprayed to control resistant weeds
• Conditions suitable for higher net N mineralisation during the non-crop season, i.e. suitable moisture and temperature and no N sink (i.e. microbial immobilization associated with crop residues with wide C:N ratios)
• Regular removal of carbon inputs by burning
• Conditions not favouring leaching of mineral N from topsoil during the early part of non-crop periods.

In soils with moderate to high disease suppressive capacity, multiple years of higher available N levels may be required to have a visible impact on rhizoctonia severity.

A good example of suppression dynamics occurred at Avon , SA. A high level of disease suppression was developed under a productive cropping with complete stubble retention from the late 1970's to the late 1980's. This suppression was very stable and provided complete control of take-all and rhizoctonia through the 1990's. In the late 1990's herbicide resistant grass weeds had become such an issue that the plots were sprayed out at anthesis to limit seed set. This practice was done for three years in succession. Following the first year of anthesis spraying a very low level of rhizoctonia was observed on the crop roots. Following the second year of crop spraying, small patches of rhizoctonia damage were observed. After the third year of spraying, the following crop (2003) was severely affected by both rhizoctonia and take-all. In 2003 the level of mineral N in the topsoil at sowing had reached 70 kg/ha. Crops were harvested is 2003 and subsequently, soil mineral N levels declined over the summer / autumn. In 2004 disease levels in crops declined substantially but were still detectable. It is likely that full suppressive activity will be restored following re-establishment of C and N turnover processes that will prevent the accumulation of high levels of mineral N during the non crop periods (i.e. summer and early autumn periods) through crop harvesting and export of N.

From the point of view of strong suppressive activity, a good farming system includes a productive intensive cropping system to provide export of N and a strong N sink through a supply of biologically available carbon (production and retention of residues) to maintain higher levels of microbial C turnover. This does not necessarily equate to a low fertility system but one in which the timing of N availability is more synchronized with the crop requirements. In such systems, early season N availability may be heavily dependent on fertiliser N with later crop requirements supplied by net N mineralisation through microbial turnover.

2. Canola as an effective break crop

Long-term rhizoctonia DNA monitoring on a grey calcareous soil on E.P. and observational evidence from the Mallee has shown that canola can substantially reduce the level of rhizoctonia inoculum with a resultant reduction in disease and higher cereal yields in the following season. At this stage it is not clear how specific this effect is to canola, as distinct from other brassicas or how soil type specific this effect may be.

3. Occurrence of Rhizoctonia in 2004

Occurrence of rhizoctonia damage was unexpectedly high in many southern regions in 2004. In general rhizoctonia incidence increases following drought years and decreases following good rainfall seasons in response to either a related decline or increase in soil microbial competition. Following a favourable season in 2003 a higher incidence of rhizoctonia occurred than was expected. This was largely due to a very dry period between January and sowing which impacted on rhizoctonia incidence and severity by a) reducing competitive microbial activity and b) severely limiting available N in intensive cropping systems.

Good yields or at least good biomass production in 2003 removed nearly all available N by harvest. The dry summer / autumn conditions in 2004 provided little opportunity for breakdown of the large stubble loads and hence there was limited or no net mineralisation of N.

Sowing was generally undertaken on marginal moisture conditions. In intensive cropping systems the incorporation of stubble in the sowing processes added to the N draw down due to immobilization and even the fertiliser N added at what would normally be regarded as adequate levels would often have been quickly immobilised with net mineralisation (N availability) only occurring 3 or 4 weeks after sowing. This has resulted in very N deficient slow growing plants.

Rhizoctonia is a very opportunistic disease. It is greatly favoured by slow growing plants. In 2004 we had a scenario of a June sowing into cold soils that were N deficient. These same conditions favoured the development of yellow leaf spot in wheat on wheat rotations, which further delayed development. This developed into a cycle of constraints that fed off each other. The low N status in the topsoil over summer / autumn would have optimised the disease suppressive activity of the soil and in paddocks where suppression levels are high, disease was not a problem (e.g. Avon , SA). However in paddocks where suppression levels are low to moderate, the other factors contributing to disease previously mentioned would have outweighed any suppressive activity.

References:

Gupta, V.V.S.R. and Neate, S.M. (1999) Root disease incidence-A simple phenomenon or a product of diverse microbial/biological interactions. In: Proceedings of the 1 st Australasian SoilBorne Disease symposium,R.C. Magarey (Ed.), pp. 3-4, BSES, Brisbane, Australia.

Roget DK, Coppi JA, Herdina, Gupta VVSR (1999) Assessment of suppression to Rhizoctonia solani in a range of soils across SE Australia . In 'Proceedings 1 st Australasian Soilborne Disease Symposium'. Gold Coast, Qld.

Roget DK (1997) Development of take-all suppressive soils in South Australia . In 'Proceedings 11th APPS Conference', Perth , W.A.

Roget DK (1995) Decline of root rot (Rhizoctonia solani AG-8) in wheat in a tillage and rotation experiment at Avon , South Australia . Australian Journal of Experimental Agriculture 35, 1009-13

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