Bare patches may appear in the crop from an early growth stage and form sharply defined areas of stunted plants. Bare patches may vary in size from a few centimetres to several metres in diameter, (Figure 3.11).
Diseased plants are usually stunted and sometimes appear purple in colour. If plants roots are severely infected the cortex will be eaten away and the central cylinder (stele) breaks, leaving the characteristic brown ‘spear tips’, (Figures 3.12 and 3.13).
Figure 3.11 Bare patches in a crop caused by Rhizoctonia solani (AG-8)
Figure 3.12 Characteristic spear tipping on primary and secondary roots, caused by Rhizoctonia solani (AG-8)
Figure 3.13 Characteristic spear tipping mostly on the secondary roots, caused by Rhizoctonia solani (AG-8)
Rhizoctonia solani (AG-8) is a fungus that survives between crops in particles of plant residue in the top 5-10cm of soil. The fungus grows out of this material after autumn rains and spreads rapidly through the soil forming a hyphal (cobweb) net. The roots of germinating plants are infected when they grow into the hyphal net. The fungus is most active at temperatures of between 10ºC and 15ºC.
The Rhizoctonia fungus is a weak, unspecialised pathogen that has the ability to attack immature cereal roots, but not mature plant roots or stem tissue. This means the primary or seminal roots are often attacked first, but later in the season the immature secondary or nodal roots may also be attacked as they emerge, (Figures 3.12 and 3.13).
Rhizoctonia solani (AG-8) has a saprophytic phase and a parasitic phase. However, the phases are not completely independent and both saprophytic and parasitic hyphae can be active in the soil at the same time. In this respect, Rhizoctonia differs from specialised parasites like the take-all fungus in that it can move freely through the soil and the saprophytic phase can be active over the summer, (Figure 3.14).
Stubble acts as a substrate for saprophytic growth of the fungus and provides a means for the fungus to survive over the summer. Cereal straw, a material with a high C/N ratio, favours the multiplication of Rhizoctonia solani (AG 8) in the soil. However, research has shown that the high stubble loads in conservation cropping systems also encourage the build-up in soil microbes that suppress the activity of Rhizoctonia. This suppressive activity has been shown to increase over a five to eight-year period. So while Rhizoctonia inoculum often increases during the first few years following the adoption of conservation cropping, systems that encourage high soil biological activity, through stubble retention, will build up populations of microbes that suppress the activity of Rhizoctonia and reduce the seasonal impacts of this disease.
Soil moisture plays an important role in Rhizoctonia survival. The fungus is capable of growing below the permanent wilting point (pF 4.2) and can survive dry summers. This gives it a competitive advantage and disease damage is often severe following dry summers. However, summer rainfall (more than 30mm in a week) will reduce inoculum as the fungus does not compete well with other soil microorganisms in a warm and moist environment.
Figure 3.14 Disease cycle of Rhizoctonia root rot in cereals. Illustration by Kylie Fowler
Plan ahead and prevent the fungus multiplying in the year before sowing. Control grasses and prevent seed set with selective herbicides by spray topping or cultivation.
Rhizoctonia multiplies on weed roots so it is imperative to control weeds from the first autumn flush. If weeds are established, a 3 to 4 week chemical fallow prior to sowing will reduce disease damage. Weeds must be dead for 2 to 3 weeks before sowing. In a season with a late break do not delay sowing past the optimum sowing date in order to chemical fallow for Rhizoctonia control.
Soil disturbance has, in many instances, proved effective in controlling Rhizoctonia. However, multiple cultivations before sowing are not necessary, provided weeds are controlled and there is a single deep cultivation (50 to 100 mm under the seedbed) in the fortnight prior to sowing.
In a direct drilled operation, a depth modified seeder that disturbs the soil 50 to 100 mm under the seedbed while sowing at the correct depth, will provide enough soil disturbance to reduce disease damage. This can be done by using a long narrow point or by having cultivating tynes working directly in line with, but 50 to 100 mm deeper than sowing tynes. It may be necessary to use a deflector plate with the long narrow point to prevent the seed dropping to the furrow bottom.
The PreDicta B soil test can be used to determine the amount of Rhizoctonia inoculum in the soil. Use the test before sowing to identify paddocks that are most at risk to root disease.
Rhizoctonia has a wide host range and all crops, pasture and weed species grown in rotations are susceptible. There are no resistant cereal varieties and crop rotations are not effective in controlling this disease. However, rotations that include a canola
or mustard crop will lower the level of Rhizoctonia inoculum in the soil.
Rhizoctonia root rot is more likely when:
- conditions are dry over the summer
- weed growth is not controlled before sowing
- there is no soil disturbance before sowing
- late sowing into cold soils slows down plant root growth
- the soil is compacted
- soil fertility is low, especially P, N and Zn
- Sulfonylurea herbicides are used on soils conductive to Rhizoctonia.