Smut is an important disease of sugarcane in South Africa which results in significant losses in cane yield and juice purity. Smut-infected stools release billions of spores that can be blown across to surrounding fields and farms. SASRI advocates an integrated management approach for controlling smut, details of which appear below. A recently produced SASRI video, “Chemical Roguing for Smut” is available at the bottom of this article.
Symptoms and spread
Smut is characterised by long, black whip-like structures emerging from infected stalks (Fig 1a). These whips produce billions of infectious spores (Fig 1b) that are well adapted to wind dispersal but can also be spread in rain and irrigation water. Spores can infect germinating buds and emerging shoots after planting and harvest. Smut is also spread by planting infected seedcane.
Figure 1a) Smut whip emerging from an infected stool with b) smut spores deposited at the base of the leaf.
Integrated management strategy for smut
- Variety choice. Discuss variety options with your Extension Specialist and Biosecurity Officer. Resistant varieties are a key part of the integrated smut management strategy, but it is important to note that these may also become infected over time, particularly when conditions are favourable and smut pressure is high. It is essential to adopt additional strategies for the effective management of smut.
- Seedcane. Only use certified or approved seedcane for planting commercial fields.
- Field preparation. Make sure that crop eradication has been effective before fields are replanted so that smut is not carried over in volunteers into the newly planted crop. Leave commercial fields fallow for a minimum of 3 months after the last volunteer has been removed (usually 6-9 months in total) before replanting with healthy seedcane. If the previous crop was infected with smut, it is advisable to plant a low-growing broadleaf cover crop during this break from cane. This ensures that volunteers can be easily identified and removed. Smut spores can survive in dry soil for at least 6 months. Where possible, irrigate previously infected fields two weeks before replanting to encourage the germination of smut spores in the soil. Without sugarcane setts in the furrow, the young fungal growth will die, and the viable smut spore load will be reduced.
- Planting time. Where possible, avoid planting or harvesting smut-prone varieties in spring or early summer. This is when the risk of infection is greatest, especially after a warm, dry winter which promotes the build-up of viable spores in the environment.
- Crop management. The Biosecurity teams do not have capacity to inspect every field in their area, so it is important to do your own scouting for pests and diseases.
Figure 2. Incipient whip developing from a smut-infected stool.
Rogue infected fields routinely. In most situations, smut can be contained in smut prone varieties by intensive roguing. Ideally, roguing should start when stools are in the incipient whip stage (Figure 2) and before whips begin to emerge. Roguing operations should be repeated every 4 to 8 weeks. Consider chemical roguing instead of physically removing smut-infected stools. Eradicate severely infected fields.
Figure 3. Spray bottle used in chemical roguing.
Roguing. Digging out infected stools is a common method of roguing, but this is often difficult and time-consuming, particularly when the soil is hard. In many cases, the stool is not removed completely and much of the infected stool is left in the ground to re-grow, providing a source of infection later.
Chemical roguing is a quick and efficient method and increases the likelihood of completely killing infected stools. With this method, incipient and emerging whips are removed – cut about halfway up the shoot to ensure the entire whip is removed, but leaving enough leaf material for the effective uptake of glyphosate. These are placed in bags and later burnt, away from the field. A 10% solution of glyphosate is then sprayed over as much of the infected stool as possible without spraying the surrounding stools. A handheld spray bottle is ideal for this purpose (Figure 3). Knapsacks should not be used for chemical roguing as the risk of overspray is high. Sprayed stools will die off after 4-8 weeks.
Figure 3. Spray bottle used in chemical roguing.
Advantages of chemical roguing
- More efficient than physically removing infected stools, reducing labour requirements.
- More effective, with an increased likelihood of killing the infected stool.
- Reduced transport costs since only the whips and leaf material are transported from the field for disposal.
Figure 4. Chemical roguing.
Notes on using glyphosate for roguing:
- Follow label recommendations regarding surfactants and health and safety requirements.
- Rain or irrigation within 4 – 6 hours of application can reduce the efficacy of glyphosate.
- Wind speeds of over 10 km / hour will increase the risk of drift to neighbouring stools.
- High quality ammonium sulphate (2%) should be used to treat salts in the water. A buffer is not necessary if ammonium sulphate is added to the tank mixture.
For more information, read the full SASRI Smut and roguing Information Sheets here.