Sorghum disease management

Sorghum ergot was first recorded in Australia in April, 1996, and is now endemic in Queensland. Losses in commercial grain crops have been minimal, with tillers flowering early or late in the growing season most affected.  Ergot can occur at any time during the growing season.  Only a short period of unseasonally cool wet weather during flowering, when pollination and fertilisation are adversely affected, is needed for infection. 

Ergot is readily identified by the 'honeydew' oozing from sorghum flowers.  This 'honeydew' is not as sticky as that produced by Aphids.  It later dries into a white powder which is often seen on the leaves and on the soil under affected plants.  Infection occurs at flowering during cool humid weather when spores land on exposed flowers.  The developing seed is replaced by a fungal growth which eventually hardens to a creamy-coloured sclerote (ergot).

Ergot spores compete with pollen for a site on the sorghum flower.  Well pollinated crops may have some ergot, but are unlikely to suffer economic damage.  Crops with impaired pollination are at risk.  Consequently, crops at greatest risk are: forage sorghums, heads on ratoon crops flowering late in the season, tillers on late flowering plants, and crops in which pollination has been affected by a cold snap before or at flowering.

Ergot survives all year round on Johnson grass, Sorghum almum, and volunteer grain and forage sorghum.  The spores are spread mainly by wind but can also be spread by insects, animals, humans, and machinery.  Tillage, stubble burning and crop rotation have little effect on survival.

Ergot can reduce crop yield, cause harvest problems with sticky honeydew, contaminate the grain sample and cause toxicity problems in livestock feeding on heavily contaminated grain.

To minimise the risk of ergot contamination, growers should:

• Sow at recommended times.
• Ensure even flowering (use press wheels, high germination seed, accurate seed depth control, good crop nutrition, wide row spacing (90 - 100 cm) especially with hybrids that tiller heavily, consider pre-harvest kill-off of tillers using glyphosate herbicide.
• Graze forage sorghum heavily to delay flowering until after grain sorghum has flowered.  This reduces inoculum levels and the potential for ergot infection of grain sorghum.

Latest information on ergot can be found here.

Rust (Puccinia purpurea)

Symptoms: early symptoms on leaves are small purple red or tan spots. These enlarge to produce elongated raised pustules that break open to release brown, powdery masses of spores.

Sorghum rust is more serious in late sown crops or susceptible hybrids in humid areas. If the disease is serious, leaves are destroyed and pinching of the grain results. Select hybrids with resistance for late planting.

Johnson grass mosaic virus

Symptoms: Mosaic (light and dark green lines on veins), Red Leaf (severe leaf reddening, followed by formation of red spots or large areas of dead tissue, and Red Stripe (red or tan stripes parallel to the veins).

Hybrids developing red leaf or red stripe reaction should not be sown late as the disease can be serious, causing stunting and death in some plants. The virus is spread from plant to plant by aphids.

Control by planting resistant hybrids. A strain of the virus exists in south and central Queensland that can infect resistant hybrids.

Head smut (Sporisorium reilianum)

Symptoms: appear at the booting stage when the head is replaced by a mass of black spores enclosed in a white fungal membrane. This membrane ruptures on emergence of the head and releases the spores. Partially affected heads are sterile.

Head smut is a soil borne disease favoured by cool weather. It may also be introduced on the seed. Control by sowing resistant hybrids. Also avoid sowing susceptible hybrids in cool weather.

Leaf blight (Exserohilum turcicum)

Symptoms: large elliptical spots up to 20 mm wide and 100 mm long, initially water soaked, but drying to straw-coloured spots with red, purple or tan margins depending on the hybrid. Spores produced on leaf spots during moist weather are spread by wind. The fungus survives on undecomposed sorghum residues, volunteer sorghum plants and Johnson grass. Severe disease can cause pinched grain and lower yields. The disease may be serious on susceptible hybrids under humid conditions and in coastal areas.

Control by sowing resistant hybrids where the disease may be a problem.

Last reviewed 13 December 2005