- Fusarium stalk rot - red internal discoloration of sorghum stalks infected by Fusarium species
During a La Nina summer, sorghum crops are at an increased risk from fusarium stalk rot. The mild wet weather is conducive to root infection from the soil borne Fusarium species, which in turn can result in increased stalk rot. Climatic stress during late grain fill, or after pre-harvest spraying, can lead to rapid development of stalk rot and may result in lodging.
Fusarium species and Macrophomina phaseolina are the main causes of sorghum stalk rot. In Queensland, fusarium stalk rot is caused predominantly by two species, Fusarium thapsinum and Fusarium andiyazi. These species do not cause fusarium wilt of cotton, fusarium head blight of winter cereals, or fusarium cob rot of maize.
Both F. thapsinum and F. andiyazi survive in infected sorghum residues. Infection normally occurs during the early stages of plant growth. The pathogen enters through the roots, and can cause significant root death during prolonged mild, wet weather.
Other root rotting fungi such as Pythium species, also common during this weather, can exacerbate the damage. The Fusarium fungus then invades the crown, where it remains dormant until a period of stress after flowering. Invasion of the stalk may then follow.
Lodging is often the first obvious sign of fusarium stalk rot in sorghum plants, but the diagnostic symptoms of the disease are usually not evident until the plants are stressed. When a stalk infected by Fusarium is split lengthwise, a pink-red discoloration is evident from ground level up the stem.
Stalks can be infected by Fusarium but not lodge - this depends on the strength of the stalk, and on the speed at which Fusarium invades the stalk. The latter is influenced by the severity of the stress and perhaps by the tolerance of the hybrid. Fusarium or other stalk rotting pathogens may not be the sole cause of crop lodging; physiological (non-biotic) stress factors can often be the cause.
The prolonged wet weather over much of southern and Central Queensland and flooding in many localities has provided ideal conditions for the development of fusarium root rot in grain sorghum. Consequently, the risk of sorghum stalk rot developing in these crops is likely to be higher in 2011 crops than in other years.
Pre-harvest spraying of grain sorghum crops with glyphosate to assist in harvesting and grain dry down can result in the rapid development of fusarium stalk rot. Whether or not infected plants then lodge depends mainly on the weather conditions and on the time to harvest.
In general, crops sprayed before harvest with glyphosate are at greater risk from fusarium stalk rot and lodging than those which are not sprayed. The predicted persistence of the La Nina weather pattern until Autumn means that pre-harvest desiccation of sorghum crops is likely to be a common practice.
Unfortunately there is nothing that can be done after planting to control fusarium stalk rot. Plants may or may not display symptoms of fusarium stalk rot inside the stem at the time of a pre-harvest glyphosate spray, so it is difficult to determine the potential risk of lodging in these situations. Timely harvest of crops at grain maturity and particularly after desiccation by glyphosate can reduce losses from lodging.