Bacterial wilt is a disease affecting tomato plants, caused by a soil-dwelling bacterium. It is prevalent in warm, humid regions and is known for its ability to cause rapid decline and death in infected plants. This disease is challenging for gardeners because the pathogen can persist in the soil for extended periods, making future plantings vulnerable. Understanding its progression and how it spreads is important to managing its impact.
Identifying Bacterial Wilt in Tomatoes
The initial signs of bacterial wilt can be subtle and mistaken for water stress. A single leaf or a young shoot may droop during the warmest part of the day, only to appear to recover overnight as temperatures cool. This temporary recovery is a hallmark of the disease’s early stage. As the infection progresses, this wilting becomes more widespread and permanent, affecting the entire plant.
Unlike many other plant ailments, bacterial wilt causes the plant to collapse while the leaves remain green, avoiding the significant yellowing often associated with other vascular diseases. In some cases, plants might show stunted growth or develop adventitious roots along the stem. A lengthwise cut of an infected stem will reveal yellowish to brown streaks within the vascular tissue, the plant’s water-conducting channels.
A diagnostic method for home gardeners is the bacterial streaming test. By cutting a section of the lower stem from a wilted plant and suspending it in a clear glass of water, a milky, white substance will begin to stream from the cut end into the water. These streams are composed of bacterial cells, confirming their presence in the plant’s vascular system.
The Cause and Spread of the Disease
Bacterial wilt is caused by Ralstonia solanacearum, a bacterium that can survive for long periods in soil and water. This pathogen infects tomato plants by entering through wounds in the root system. These entry points are often created during transplanting, cultivation that disturbs the soil, or by the feeding of pests like nematodes. Natural openings where secondary roots emerge are also entry points.
Once inside the roots, the bacteria invade the xylem, the tissue responsible for transporting water and nutrients from the roots to the rest of the plant. Inside these vessels, the bacteria multiply. This mass of bacterial cells and the slimy substances they produce create a blockage, preventing water from reaching the stems and leaves.
The disease spreads primarily through contaminated materials. Soil carried on garden tools, shoes, or equipment transfers the bacteria to new areas, and water runoff from an infected plot can spread the pathogen to nearby plants. Purchasing and planting infected, yet symptomless, seedlings also introduces the disease to a clean garden. High soil temperatures, between 82°F and 95°F, and high moisture levels create the ideal environment for the disease to develop and thrive.
Management and Prevention Strategies
Once a tomato plant is infected with bacterial wilt, there is no chemical treatment that can cure it. Management, therefore, focuses on preventing its spread and reducing its presence in the soil. If a plant is confirmed to have the disease, it must be removed immediately. The plant, along with the soil directly surrounding its root ball, should be dug up, bagged, and disposed of in the trash to prevent further contamination; do not add it to a compost pile.
Planting tomato varieties bred for resistance to bacterial wilt is an effective strategy. Cultivars such as ‘Kewalo’ or ‘BWN-21’ can withstand infection. While resistance does not mean complete immunity, these varieties offer a better chance of producing a healthy crop in soil where the bacterium is present. Checking with local extension services can provide recommendations for resistant varieties suited for your region.
Since the bacteria thrive in wet conditions, improve soil drainage. Planting tomatoes in raised beds can help prevent waterlogged soil around the root zone. Amending heavy clay soils with organic matter can also improve their structure and drainage.
Crop rotation helps reduce the bacterial population in the soil. Avoid planting tomatoes or other susceptible plants, such as peppers, eggplants, and potatoes, in the same spot for at least three to five years. Rotating with non-susceptible crops like corn, beans, or cabbage can help break the disease cycle by depriving the bacteria of a suitable host.
Sanitation is fundamental for preventing the spread of bacterial wilt. All tools, including trowels, shovels, and stakes, should be thoroughly cleaned and sanitized, especially after working in an area where an infected plant was found. A solution of one part bleach to nine parts water is an effective disinfectant.