Soft rot and blackleg in potatoes

Estimated reading time: 8 minutes

Soft rot and blackleg are often treated as ‘bad luck diseases’: the kind that appear late, ruin a portion of the crop, and then vanish until the next season. In reality, they behave more like a system failure, unfolding step by step across the production chain.

These diseases are rarely caused by a single mistake. They develop when multiple small risk factors accumulate: a wet spell followed by heat, a hurried harvest under marginal conditions, bruising that seems minor at the time, sanitation that is ‘good enough’ until it isn’t, and storage environments where oxygen levels drop and bacteria thrive.

Potatoes can appear fine at lifting yet break down weeks later, leaving growers and packers to deal with losses, claims, and damaged market confidence. This article focusses on practical actions to reduce risk at the points where outbreaks typically begin and where they are most likely to accelerate.

Understanding the disease

Soft rot and blackleg are caused by bacteria that thrive in warm, wet, low-oxygen conditions. In the field, the problem may begin at the seed tubers or stem base (blackleg). Later, it often manifests as tuber breakdown (soft rot) during harvest, handling, transport, curing, and storage.

The bacteria involved can be present at low levels in many production environments, including on seed and surfaces, as well as in soil and water. They are not always introduced just once and, more often, conditions develop that allow them to multiply and invade. The outcome ultimately depends on three interacting elements:

• The bacteria, which are present and ready to exploit openings.
• The potato, where stressed tubers and stems are easier to invade.
• The environment, in which excess moisture, heat, and low oxygen accelerate breakdown.

Once the system tips in the bacteria’s favour, soft rot can spread rapidly, especially in piles or bins, where a single rotting tuber becomes a source of moisture and inoculum for many others.

Where it often starts

Many soft rot and blackleg problems are seeded early, even if visible damage appears later. Seed carries a biological history: how it was grown, harvested, handled, stored, and transported. Even certified seed can carry low levels of bacteria without obvious signs. The question is whether field and handling conditions allow those bacteria to become active.

Early risk factors include:

• Warm seed placed into cool soils (or the reverse), which drives condensation and stress.
• Rough handling during loading, which creates infection points.
• Poor sanitation on conveyors, which spreads bacteria from a few tubers across many.
• Planting into waterlogged or compacted soils, which reduces oxygen at the stem base.

Blackleg is fundamentally a stem base problem. Wet, oxygen-poor conditions increase vulnerability, while heat can amplify bacterial growth and symptom expression. The mindset shift is simple yet powerful: Treat seed handling as part of disease control, not just logistics.

Water, heat, and stress

In much of South Africa, irrigation is essential and influences the outcomes of soft rot and blackleg. These bacteria thrive when soils remain wet for extended periods, particularly under warm conditions. Waterlogged soil reduces oxygen around roots and stems, creating ideal conditions for blackleg to develop. Excess moisture can also contribute to lenticel swelling and micro-cracking, which allows bacteria to enter tubers.

Practical irrigation practices that separate manageable risk from serious losses include:

• Avoiding prolonged saturation, particularly in heavier soils and low-lying areas.
• Adjusting irrigation timing during warm spells, as soils remain chronically wet under short, frequent irrigations.
• Preventing standing water after rain by keeping drainage pathways open.
• Matching irrigation to canopy development and crop evapotranspiration needs rather than following habits.

Drip and low-flow systems can reduce risk, but only if they do not result in permanently wet soils. Potatoes tolerate short periods of moisture stress far better than they tolerate long-term oxygen deprivation.

Soft rot is often most severe in seasons when the crop experiences fluctuations, such as warm weather, intermittent rainfall or heavy irrigation, and rapid growth that outpaces the plant’s ability to maintain tuber integrity. Stress increases vulnerability in two ways: stressed tubers develop weaker skins and more micro-injuries, and stressed tissue can leak nutrients that bacteria feed on.

Heat accelerates all processes, speeding bacterial multiplication and increasing respiration in storage, which raises the risk of oxygen depletion in hotspots if airflow is uneven. This is why late-season management matters: ensuring that tubers survive harvest and storage without breaking down.

Harvest is the biggest risk event

Risk most often turns into actual loss during harvest and the initial handling steps that follow. Bacteria require entry points, and bruises, cuts, or skinning injuries provide them. Moisture then allows bacteria to move and multiply.

Common harvest drivers of outbreaks are lifting in wet conditions, which causes soil to stick and tubers to remain dirty and wet, thereby increasing damage, and harvesting when tubers are warm, accelerating bacterial multiplication and tuber respiration. Other factors are excessive drop heights and sharp transitions on conveyors, which create invisible bruising that leads to breakdown later, and poor separation of damaged tubers, where minor injuries can escalate into a large storage problem.

Operational pressures often push harvest to continue under marginal conditions. The cost of delay is real, but so is the cost of a breakdown amid limited flexibility. A useful discipline is to treat harvest as a quality operation, not simply a lifting operation: manage speed, reduce drop heights, cushion transitions, and stop when conditions increase risk.

Prioritise sanitation

Soft rot bacteria cling to surfaces such as knives, bins, grading equipment, cutting lines, trailers, and storage infrastructure. Even a small fraction of infected tubers can contaminate equipment, spreading bacteria to otherwise healthy tubers.

The sanitation challenge is that many operations clean only occasionally – when time allows or after a problem has already appeared. Bacteria exploit this inconsistency.

Practical sanitation measures include cleaning and disinfecting seed-cutting equipment on a routine schedule, regularly washing and sanitising grading and handling lines (particularly after processing wetter field lots), avoiding the mud layer problem in which wet soil and plant material stuck to surfaces becomes a bacterial reservoir, and ensuring that water quality in wash or flume systems does not become an inoculum source.

Sanitation is most effective when it is built into the system. If it relies on memory or goodwill, it will be uneven, and that is exactly where outbreaks can spread.

Importance of storage

Many soft rot losses become disasters in storage because the environment allows the disease to intensify. Conditions that favour rapid development include warm tubers with high respiration entering storage, high humidity combined with free moisture on tuber surfaces, and low-oxygen pockets caused by poor airflow distribution. Additional risks arise from hotspots created by blocked ducts, uneven airflow, or insufficient fan performance.

When oxygen is limited, potatoes become stressed and break down more quickly. Bacteria thrive in these low-oxygen micro-environments, especially when moisture is present. Storage airflow is only as effective as its weakest zone.

The most effective approach to prevention is to think in terms of risk zoning:

• Where is airflow weakest?
• Where does condensation occur?
• Where do warm tubers tend to accumulate?
• Which areas have historically shown higher levels of rot?

Curing and drying

Curing and drying are sometimes rushed because the crop needs to move and storage space is limited. From a soft rot perspective, curing is the process of turning a vulnerable, freshly harvested tuber into a more sealed and resilient product.

Curing supports wound healing and skin set. When done properly, it reduces entry points for infection. When skipped or shortened, bruises and nicks remain open for longer, allowing bacteria to exploit that window. Curing requires a balance of temperatures that support healing without unnecessarily accelerating bacterial growth, high humidity levels to prevent excessive dehydration without creating free moisture, and airflow to remove surface moisture while preventing condensation.

In practice, good curing is measured by outcomes: tubers are dry on the surface, wounds have healed, and the pile remains uniform rather than developing damp pockets.

A chain-of-custody risk map

When soft rot and blackleg are viewed as a chain, the goal becomes breaking that chain at multiple points. Relying on a single, perfect intervention is risky. Four practical pressure points define most outcomes:

• Seed and early-season conditions: Careful handling and avoiding waterlogged planting environments.
• Water and stress management throughout the season: Avoiding chronic saturation, reducing stress fluctuations, and protecting tuber integrity late in the season.
• Harvest and handling discipline: Minimising bruising, managing moisture, and avoiding unnecessary heat exposure and prolonged holding times.
• Storage conditioning and airflow: Entering storage as cool and dry as possible, ensuring airflow reaches all zones, and preventing low-oxygen pockets.

Each pressure point may reduce risk by only 10 to 30%. However, when effectively combined, their impact becomes decisive.

Closing perspective

Soft rot and blackleg may feel like biology simply taking its course. Yet, in many cases, major losses are not inevitable. They result from a chain of events that can be tightened.

Prevention often comes with immediate costs, such as a slower harvest pace, more rigorous sanitation, better lot separation, and improved monitoring. The benefits appear later, when the crop holds in storage, meets grading standards, and leaves the farm with fewer surprises.

With increasing climate volatility, including hotter spells, heavy rainfall events, and sharper fluctuations, resilient farms will not be those chasing perfect seasons. Instead, they will be the ones building systems in which one difficult week does not automatically lead to a storage disaster. Soft rot and blackleg are not only plant pathology topics; they are matters of management, and management is something you can control.

What growers can do this season

Not every farm can rebuild infrastructure in a single year, but every farm can tighten practices in ways that materially reduce losses. High-impact actions that often deliver quick returns include:

• Walking fields after rain or irrigation and marking areas that remain wet the longest, as these zones often produce higher-risk tubers.
• Separating lots by risk at harvest, ensuring that wet fields and heavily damaged lots are not stored for extended periods.
• Reducing drop heights and sharp transitions on harvest and grading equipment, as small mechanical adjustments can deliver significant disease control benefits.
• Treating sanitation as a scheduled activity rather than a reaction, particularly during early seed handling.
• Confirming storage airflow performance and identifying dead zones early, recognising that ‘average good’ is not the same as ‘consistently good’.
• Prioritising rapid surface drying after harvest, especially for dirty tubers.
• Managing temperature at entry into storage, as warm tubers increase respiration, moisture movement, and the risk of hotspots.

– Lukie Pieterse, editor, Potato News Today

For more information, send an email to lukie555@gmail.com or lukie@potatonewstoday.com