Rhizoctonia root rot

Rhizoctonia is caused by the Rhizoctonia solani fungi, (AG8 strain in cereals and the AG1, AG2, AG3, AG4, AG5, AG6, AG7, AG9, AG10, AG11, AG12, AG13, AGBI, ZG1, ZG3, ZG4 and ZG6 strains in vegetables, pulses, oilseeds, pasture legumes and other plant species).

Rhizoctonia root rot is a fungus which inhabits the soil and was identified as a major disease (world wide) of plants over 100 years ago. In 1934 Rhizoctonia solani AG-8 was identified as the cause of "purple patch" or "bare patch" in cereals on light sandy soils in South Australia. Rhizoctonia is a strongly competitive fungus which survives on plant residues as well as living roots and attacks a wide range of plant species. Rhizoctonia occurs in winter rainfall areas affecting cereals, pulses, oilseeds and pasture legumes.

Rhizoctonia bare patch is a disease of cereals that infects the plants roots, rotting the outer sheath of the plants roots resulting in the typical "spear tip" symptoms of the disease. The loss of root mass by Rhizoctonia results in reduced uptake of water and nutrients by the plant. Bare patches first appear in crops from an early stage 3 to 5 leaf, (4 to 6 weeks after sowing) and form sharply defined areas of stunted plants (bare patches). These stunted plants form circular bare patches of varying sizes from less than a meter to several meters across.

Yield losses are closely related to the incidence of Rhizoctonia bare patches in the crop, in these patches losses can range from 5% to well over 50%, although low losses of 0-10% can occur outside these patches where crop and plant symptoms are less apparent.

Rhizoctonia is estimated to cost Australian wheat and barley growers $77 million annually.

Life cycle

Rhizoctonia survives between crops in the soil mainly fine fungal threads in particles of plant residues and organic matter. The Rhizoctonia fungus is a very competitive fungus and will spread through moist soil after the opening autumn rains, when the Rhizoctonia fungus grows out of the plant residues and organic matter and spreads rapidly through the soil to infect the roots of young or germinating plants.. The Rhizoctonia fungus is most active between the temperatures of 10°C and 15°C. Once inside the plant roots, the Rhizoctonia fungus spreads rapidly throughout the root system rotting the outer epidermis and cortex of the root, resulting in this part of the roots disappearing and leaving the inner endodermis structure of the root intact. This gives the roots their typical spear tipped appearance.

Control

Rhizoctonia attacks many plant species and Rhizoctonia infected residues from the previous year can result in serious damage to the current years crops and as such it is very difficult to control through rotations.

A disease management plan incorporating the following is recommended.  

1. Conduct a Predicta B Root Disease Test in February/March to determine levels present in soil.
2. Conduct a nutrition soil test to determine nutrition needs.
3. Ensure adequate nutrition of crop especially Nitrogen, Phosphorus and Zinc.
   • Apply adequate starter nitrogen 10 to 25 kg nitrogen/ha depending on row spacing.
   • Apply 4 to 6 kg phosphorus/T expected yield depending soil type and phosphorus levels
   • For high pH, or calcareous soil or soils with poor zinc history apply 0.5 to 1 kg zinc/ha.
   • Apply a multi nutrient foliar spray at the 5 to 8 leaf stage to improve the plants health and its ability to resist attack by diseases.
4. Control summer and autumn weeds early.
5. Have a 4 week weed free break prior to sowing (minimum of 2 weeks).
6. Cultivate 2.5 to 5 cm below seeding depth (up to 10 cm can be tried).
7. Retain stubble and organic matter to improve health of the soil.
8. Apply a zinc seed treatment nutrient such as Zincflo® Plus to improve early plant health and the plants ability to resist attack by disease.
9. Avoid use of sulphonylurea herbicides or those herbicides that restrict root growth.
10. Aim to improve soil fertility by increasing organic matter and nutrient levels.
11. Look at applying a multi nutrient foliar spray to the crop to improve plant health and nutrition (product should contain at least nitrogen, phosphorus, sulphur, zinc, copper and manganese).
12. Aim to control any grass weeds early in pasture year prior to cropping year.

Combine the above management measures and treat seed with Rancona^® Dimension at 3.2 L/T.