Sorghum - disease management

Sorghum ergot

Topics on this page:

• Ergot
• Other diseases

Ergot (Claviceps africana)

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
• crops in which pollination has been affected by a cold snap before or at flowering.

Ergot survives all year 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, and 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.

Take home message: Agronomic practices can minimise sorghum ergot risk.

Ergot, caused by the fungus Claviceps africana, is a fungal disease which infects sorghum heads during flowering. Infection is favoured when pollination and fertilisation is hindered by cool, humid or rainy weather.

Outbreaks in southern Queensland in the 2000-01 season occurred when there were 2-3 days of mild (maximum daily temperature less than 28ºC = mean hourly temperature less than 21ºC), rainy weather during flowering. Ergot is a problem for two reasons: the sclerotes have been shown to be toxic to beef cattle and sows, and the sticky honeydew can cause harvesting problems.

Only agronomic practices are available to reduce the losses from sorghum ergot. Planting in southern Queensland should not extend beyond late January to reduce the risk of flowering during cool weather. Planting practices which assist in producing an even-flowering stand must be followed. A hybrid with good pollen production, and preferably low tillering ability, should be used if planting late.

If rain falls during flowering, crops should be checked 10-14 days later for evidence of ergot infection, particularly around the edges and at the end of the crop facing the prevailing winds. If ergot is found, an estimation of the level of infection is useful. At present there is stockfeed limit of 0.3% sclerotes by weight for sorghum grain, which is equivalent to about 1% by number (1 sclerote in 100 seeds).

Even if there is no ergot in the main heads, late-flowering tillers are often infected, causing harvesting problems. Spraying such plants with herbicides, such as glyphosate, will stop further honeydew development, but should only be applied after grain fill (black layer formation). Research has shown that up to 70% of sclerotes can be blown out of the header during harvesting by increasing the fan speed, but this percentage will vary, depending on seed size and header.

If a particular part of the paddock is heavily infested, consider segregating it from the rest after harvest. Contaminated loads can be mixed with ergot-free loads to lower the sclerote level below 0.3%.

Many diseases are controlled by planting resistant varieties. The first resistance to sorghum ergot was reported in 1997, in an African sorghum line IS8525. Field trials in Queensland in 2000 have confirmed its resistance, and have also identified five other lines with good resistance. Early indications are that IS8525 produces highly viable pollen, even under cold conditions. Work is underway to identify the mechanisms of resistance of this line, and to determine if the resistance can be transferred to lines with high yield and other desirable traits. If the transfer is successful, the use of ergot-resistant lines will not only significantly reduce the risk from ergot, but also allow the broadening of planting windows.

• Crops should be inspected for ergot 10-14 days after flowering during cool, wet weather, particularly around the edges.
• Estimate infection levels of about 100 heads (a level of less than 1% infected spikelet should be safe).
• Spray out plants if late tillers are infected.
• Increase fan speed of headers to maximise sclerote removal during harvesting.
• Harvest heavily-contaminated areas of the crop separately.
• Consider mixing ergot-contaminated sorghum with clean seed to reduce ergot levels to less than 0.3%.

Other diseases in sorghum

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.

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.

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.

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.