Sharon McFarlane (Senior Plant Pathologist) and Phillemon Sithole (Extension Specialist- North Coast)
Severe sour rot was identified in a certified nursery in the Midlands North area in 2024 (Figure 1). The disease was also one of the reasons for poor germination in a number of fields on the north coast (Figure 2).
Figure 1: Certified seedcane (N75) in Midlands North severely infected with sour rot.
Figure 2: Sour rot contributed to poor germination on the north coast.
Sour rot tends to infect mature (>15 months), stressed cane, and is most common after prolonged dry winters in the midlands and hinterland.
Conditions were extremely dry in much of the industry’s rainfed areas from April to September 2024. A similar trend has been observed this season. Figure 3a shows the daily estimated soil water content (SWC) for the Midlands North region for the period 1 April to 1 September 2024 and Figure 3b shows the same information over the same period in 2025.
Similar trends have been observed in other rainfed regions. The SWC is a measure of the plant-available soil water calculated for rainfed conditions for a soil with a total available water (TAW) capacity of 60 mm (typically a shallow or sandy soil) and with a full canopy cane crop growing on it.
The estimated SWC for a deeper or clayey soil, with a higher TAW, follows a similar trend, but takes a little longer to reach the stress level. The extended periods of water stress favoured sour rot development in 2024, and judging from the SWC trends, similar favourable conditions for infection will likely be repeated in 2025 in the rainfed areas should the dry conditions persist. Based on the latest seedcane inspections in the Zululand South area, sour rot is starting to develop although incidence is currently low.
Figure 3a: Midlands 2024.
Figure 3b: Midlands 2025.
Figure 3a and b: Daily reference available soil water content (SWC) for Midlands North region for 1 April to 1 September 2024 (3a) for the same period in 2025 (3b). The broken orange line represents the 50% depletion level, below which crop water stress is assumed to commence.
Typical symptoms of sour rot and disease progression (refer to Information Sheet 9.5: Sour Rot for further details.)
- Newly infected internal stalk tissue has a water-soaked appearance (1).
- As the disease progresses, the stalk tissue turns orange (2,3).
- The rind of infected stalks may have a mild ‘blush’ (4).
- When infections are severe, the rind of the stalk may blister (5) and black spore masses may eventually be seen (6).
- Infected stalks have a characteristic sour odour.
Management options for sour rot
Where possible, avoid exceeding the optimum harvest age of each variety, especially when grown on marginal soils.
Flag affected fields for early harvest.
Do not ripen drought-stressed or sour rot-infected cane (Refer to Information Sheet 4.6: Principles underlying chemical ripening and late-season quality maintenance), because this will place additional stress on the crop and is likely to result in increased sour rot incidence and severity.
If sour rot is detected in seedcane, preferably find an alternate source as severely infected stalks such as those in Figures 1 and 2 will not germinate. Should this not be possible and if sour rot incidence is less than 10% in the field, infected stalks can be identified (based on external symptoms) by trained staff members and discarded. Contact your Extension Specialist or Biosecurity Officer for advice.
When harvesting commercial fields, remove obviously infected stalks. This will improve juice quality when the cane is crushed, which is often sufficient to warrant the extra labour cost of this operation (and avoids the risk of rejection of the entire field).