During warm summer months, reports of bacterial soft-rot caused by Erwinia are especially acute throughout the greenhouse industry. Soft rots are common in greenhouse vegetable liner production and are known for affecting cauliflower, cabbage, spinach, cucumber, and tomato. In the production of herbs, the disease may have limited expression during crop growth, but the bacteria can rapidly colonize wounds that may have occurred during handling and shipping.
Ornamental tropical foliage crops such as dieffenbachia, aglaonema, syngonium, and cactus are very susceptible to soft rot; potted flowering plants and cut flowers frequently have their shelf-life shortened, but landscape plants are less susceptible once they have adapted to soils containing populations of Erwinia. However, damage can be extensive when landscape plant cuttings are being rooted.
Movement of Erwinia in irrigation water
Clean water shortages are reaching critical levels in many areas of the country. Most aquifer and well systems are utilized by municipalities and agriculture. Competition for a limited supply of clean water has led water management districts to encourage alternative water sources for agricultural users. Recycled irrigation, collected storm water runoff, and retention ponds have all been proposed. Many of these alternatives have found successful application in the timber industry and field-grown crop production.
However, in greenhouse growing systems, any introduction of untreated surface water, or recycled nursery water, could potentially introduce plant diseases into container-grown plants. This is especially true of bacteria, such as Erwinia soft rot. Erwinia populations are naturally prevalent in native soils, and any water collection system will contain, and concentrate, the bacteria.
We conducted a study to determine population levels of Erwinia found in retention ponds and lakes. We examined eight nursery retention ponds and four large hypereutrophic lakes in Central Florida for Erwinia bacteria populations. Nurseries participating in this study produced herbs, perennials, annuals, bedding plants, and woody landscape plants. Within these crops, Erwinia is considered to be problematic during propagation, trimming, or production under environmentally hot and wet periods.
To conduct the study, water samples were taken from one dozen surface-water sources once a month, for a year. Bacteria were isolated onto test tube media that are designed for detecting Erwinia. Sizes of Erwinia populations were statistically compared between lakes, retention ponds, and between retention ponds that were being used to collect and recycle water within a nursery.
From the study we found that nurseries, which were actively reutilizing water in retention ponds had significantly higher populations of Erwinia. Average concentrations of bacterial cells were 34, 11, and 11 per liter in recycled nursery water, nonrecycled, and lake water, respectively. In extreme cases, populations of Erwinia in recycled water were as high as 290 cells per liter of water. Ninety-nine percent of the Erwinia, from sources isolated in this study, were found to be pathogenic when placed onto a susceptible host plant. As waste water reuse becomes an alternative resource for the nursery industry, water-purification systems will be needed for disease control. Acquiring such a system will be especially critical for growers of susceptible crops.
Identifying and controlling soft rot
Most growers are familiar with the fishy smell that’s released as Erwinia breaks down plant tissue. If this diagnostic odor is not evident, an individual plant can be placed in a plastic bag for a few hours; if Erwinia is present, the smell can easily be noted when the bag is opened. Once the first symptoms of soft rot occur in a production facility, disease spread can be rapid, and crop losses extensive. Symptoms may appear anywhere on the plant, with water-soaked lesions appearing on leaves, and soft rots on stems and plant crowns.
Erwinia cells have flagella (tails) scattered over their surface membranes. These flagella are used by bacteria to swim through wet soil and over plant surfaces. However, the primary mechanisms of movement within a greenhouse facility are splashing water, contaminated tools, and soil and insects. When bacteria come in contact with a susceptible plant, they enter through small wounds caused by handling, or by insects. For Erwinia to digest the plant host, they must exude both pectolytic and celluloytic enzymes. Pectolytic enzymes break down the pectin substances that hold plant cells together, while celluloytic enzymes break down the cellulose of the plant cell walls. Once plant cells are broken down, the bacteria easily absorb the host nutrients.
If no wounds are present for entry, Erwinia can survive on plant surfaces without the plants expressing symptoms. High humidity and moisture on the leaf trigger the release of bacterial digestive enzymes that can break down plant tissue. Greenhouse production managers will note that the most severe soft rot outbreaks occur during warm summer months, when both temperature and humidity are high. But outbreaks are also common in the middle of winter, when production facilities are airtight and humidity is high.
In summer, shade cloth or white paint can be used to reduce greenhouse temperatures. In addition, watering should be done in the morning to limit the amount of time foliage remains wet.
Erwinia bacteria also readily colonize and infect damaged roots. Improper fertilization and high salt levels can cause initial damage to roots, which bacteria may exploit.
Free draining soil or media is vital to ensuring water and nutrients flow through the root zone, and total soluble salts should be monitored. Symptoms of plant wilt, and death, will be similar to those seen with the fungal rot diseases caused by Phytophthora, Fusarium, or Rhizoctonia.
There are no compounds that can rid infected plants of Erwinia; although, bactericides containing copper can help slow the spread of the pathogen. Antibiotics have been recommended, but the bacteria adapt and populations become resistant within a matter of weeks.
Control of Erwinia is based almost exclusively on sanitation and cultural recommendations.
In areas where water shortages have been more critical, and recycled or reclaimed water is being applied, plant pathogens have been controlled by using sand filtration, ozone, UV sterilization, or chemical treatment, such as chlorine and bromine.
The following sanitation and cultural guidelines will help control Erwinia within a production facility:
- Lower inoculum levels. Leaves showing symptoms, and infected plants, must be removed and discarded immediately to lower the bacterial count and amount of Erwinia present in a production facility.
- Use surface disinfectants. Disinfectants such as bleach or quaternary ammonium compounds should be used on clipping shears, tools, and benches between plantings.
- Avoid soil additives. Erwinia can grow and persist within soil; therefore, nonsterlized native soil should not be added to the planting media.
- Lower humidity levels. Increasing both ventilation and plant spacing can lower humidity and help decrease the spread and severity of infestations.
- Decrease watering. Lowering the frequency and amount of watering will help slow the spread of Erwinia. If economically possible, drip irrigation would further help by eliminating leaf surface moisture and lowering humidity.
Knowing how Erwinia can gain access, persist, and move about in a facility is important to its control.
David Norman and Gul Shad Ali are plant pathologists at the Institute of Food and Agricultural Sciences at the University of Florida.