Bacterial Soft Rot

What is Bacterial Soft Rot?

Bacterial soft rot is a widespread and destructive plant disease caused by various bacteria species, primarily from the genera Erwinia, Pectobacterium, and Pseudomonas. It affects a vast number of plants, resulting in severe yield loss in many crops.

Host plants

Bacterial soft rot affects a wide range of plants. The bacteria responsible for soft rot are opportunistic pathogens that can infect plants through wounds or natural openings, such as stomata or lenticels. Here are some of the most commonly affected host plants:

• Potato (Solanum tuberosum): This is one of the most well-known hosts for bacterial soft rot. The disease can cause significant losses in stored potatoes.
• Carrot (Daucus carota): Especially in storage, carrots can become soft and mushy when infected.
• Onion (Allium cepa): Bulbs can rot from the inside out, and often, a foul smell accompanies the rot.
• Cabbage (Brassica oleracea): Outer leaves can become watery and slippery.
• Lettuce (Lactuca sativa): Infected parts turn slimy and decayed, often leading to the whole head rotting.
• Cucumber (Cucumis sativus): Common in post-harvest storage, especially when cucumbers are kept in conditions with high humidity.
• Tomato (Solanum lycopersicum): Fruit can become soft, watery, and eventually collapse.
• Pepper (Capsicum spp.): Fruit softening, often accompanied by a foul smell.
• Begonia (Begonia spp.): This ornamental plant can also be a host, with rot often starting at the base of the stem.
• African Violet (Saintpaulia ionantha): Another ornamental plant susceptible to bacterial soft rot, especially when overwatered.
• Aloe (Aloe spp.): A commonly affected succulent, where the rot often starts at the base.
• Orchids: Especially when they are overwatered or in conditions of high humidity.
• Hyacinth (Hyacinthus orientalis): Bulbs can rot, especially in damp conditions.

This list is not exhaustive, as bacterial soft rot affects numerous plants, including many other fruits, vegetables, ornamentals, and even some tubers and bulbs. The best defense against soft rot is prevention, through good cultural practices, proper irrigation management, and careful handling during post-harvest operations.

What are the Symptoms?

Bacterial soft rot symptoms are particularly distinctive, and they often progress rapidly under conducive conditions. Here are the major symptoms associated with bacterial soft rot:

• Initial Infection: The initial symptoms often start as water-soaked lesions. These lesions appear translucent and might be more evident when held up to the light.
• Softening and Decomposition: As the name suggests, the primary symptom of bacterial soft rot is the softening of infected tissues. Affected parts become cream to tan in color, mushy and watery.
• Foul Smell: As the bacterial rot progresses, the decomposition of the plant tissue results in a characteristic foul and putrid smell. This odor is often one of the first signs noticed by growers.
• Cavities: In tubers, bulbs, and some fruits, the rot often hollows out the interior, leaving a shell of outer tissue that appears relatively normal. When squeezed, these may exude a watery, smelly liquid.
• Black Edges: The margins of the softened areas may appear black or dark-colored, especially in some vegetables like potatoes.
• Progressive Collapse: In severely affected plants, especially those with high water content, the decay can cause the entire structure to collapse.
• Bacterial Ooze: In very wet conditions, the bacteria may produce a slimy ooze that can be seen exuding from the infected areas. This ooze can spread the bacteria when splashed by rain or irrigation.
• Above-Ground Symptoms: In some plants, an above-ground wilt or general decline may be noticed, especially if the bacterial infection is in the root system or lower stem.

These symptoms can be devastating, especially in storage where one infected produce item can lead to the spoilage of many others in close proximity. Early detection and removal of infected materials, combined with preventative measures, are essential to manage bacterial soft rot.

What Causes Bacterial Soft Rot?

Bacterial soft rot is caused by several bacteria that infect plants through wounds or natural openings, such as stomata or lenticels. Here are the primary causes and conditions that favor bacterial soft rot:

• Infectious Agents:
  • Soft rots are caused by several bacteria such as Pectobacterium carotovorum (previously called Erwinia carotovora), Dickeya dadantii (previously called Erwinia chrysanthemi), and certain species of Pseudomonas, Bacillus and Clostridium.
• Entry Points:
  • Wounds: Any damage to the plant, whether from harvesting, insect feeding, mechanical damage, or other means, can provide an entry point for the bacteria.
  • Natural Openings: Stomata, lenticels, and the bases of leaf petioles can be natural entry points.
• Environmental Conditions:
  • Moisture: Wet conditions, especially standing water or high humidity, greatly favor the development and spread of bacterial soft rot.
  • Temperature: Most Pectobacterium species prefer temperatures between 70°F to 80°F (21°C to 26°C), but they can cause disease in a broader range.
• Other Factors:
  • Poor Drainage: Saturated soils or those with poor drainage can facilitate bacterial infection.
  • Infected Plant Debris: The bacteria can survive in soil or on plant debris, acting as a source of infection for subsequent crops.
  • Storage Conditions: Inappropriate storage conditions, especially those with high humidity and lack of proper ventilation, can lead to the rapid spread of the disease in harvested produce.

Understanding these causative factors can help in devising effective preventive strategies.

How to Control and Prevent Bacterial Soft Rot?

Controlling and preventing bacterial soft rot is essential to safeguard the health of plants and the quality of harvested produce. Here are some strategies and methods for control and prevention:

• Good Sanitation Practices:
  • Clean Equipment: Regularly disinfect tools, equipment, and work surfaces.
  • Field Sanitation: Remove and destroy infected plant material to reduce the source of bacteria.
  • Storage Hygiene: Ensure that storage facilities are clean and free from any remnants of previously stored produce.
• Proper Irrigation:
  • Avoid Overhead Irrigation: Use drip irrigation or soaker hoses to minimize water splashing, which can spread bacteria.
  • Water Early: If overhead irrigation is unavoidable, water early in the day to allow foliage to dry before nightfall.
• Optimal Planting Practices:
  • Plant Health: Plant only healthy, disease-free seeds or transplants.
  • Spacing: Adequate spacing between plants ensures good air circulation, which can reduce the humidity around the plant.
  • Soil Preparation: Ensure good drainage by amending soils if necessary.
• Proper Storage Conditions:
  • Dry Before Storage: Ensure produce is dry before storing. Wet conditions favor bacterial growth.
  • Storage Temperature: Store susceptible produce at recommended temperatures to reduce the risk of bacterial growth.
  • Inspect Stored Produce: Regularly inspect and promptly remove any produce showing signs of rot.
• Crop Rotation:
  • Rotate crops with rot-resistant varieties like corn, snap beans, and beets. Avoid planting susceptible plants in the same location year after year.
• Use of Resistant Varieties:
  • Some plant varieties are less susceptible to bacterial soft rot. Whenever possible, use resistant varieties, especially in areas with a history of the disease.
• Chemical Control:
  • While bacterial soft rot is primarily managed through cultural practices, there are some bactericides that can help in specific situations. Always read and follow label directions and ensure the product is labeled for the specific crop and disease.
• Biological Control:
  • There are beneficial bacteria and fungi that can combat soft rot bacteria. Products containing these beneficial organisms can be applied to plants or soil.
• Minimize Wounding:
  • Both during planting and harvesting, care should be taken to avoid wounding plants. Bacteria often enter through wounds or natural openings.

By implementing these measures, gardeners and farmers can significantly reduce the risk of bacterial soft rot, ensuring healthy plants and high-quality produce. Regular monitoring and prompt action at the first sign of disease are essential.