The ABC’s Gardening Australia program on June 20th told gardeners how to identify plants affected by bacterial wilt but, unfortunately, did not tell them how to eliminate the disease.
Bacterial wilt (Ralstonia solanacearum) is a serious soil disease that can, like Fusarium and Verticillium wilts, spread throughout the garden on boots, gardening tools, and infected plant material and seeds. It grows best in temperatures of between 30 and 35° C. and the bacterium requires both heat and moisture to multiply. Consequently, it is more commonly found in areas with wet summers.
It can affect the entire tomato family, the banana family (including Heliconia), onions, papaya, ginger, mung beans, cashews and peanuts. Like the fungal wilts, it affects the water-conducting tissue of plants and causes rapid wilting. Diagnosis can be determined from a section of stem pruned from near the base of a suspect plant. Immediately after pruning the stem, suspend it in a glass of clean water for several minutes. Milky threads will begin to leak from the stem and the water will quickly become white if Bacterial wilt is present.
Remove all plants, tubers and weeds from infected beds and destroy them, or dispose of them in a sealed plastic bag. Any remaining plant material can infect future crops of susceptible varieties – do not compost this material.
Raising beds to 20 cm or more can help deter this disease. After working on affected beds, wash boots and garden tools and allow them to dry in direct sunlight.
Bacterial wilt often occurs in conjunction with root knot nematodes. These pests can be eliminated by growing a green manure bio-fumigant.
Allowing a fallow of at least 18 months will also help, especially if soil in the bed is kept dry. This can be achieved by covering the bed with clear plastic, anchored around the edges. This process is called solarisation and it works best in warmer months, as bacterial wilt pathogens cannot survive in temperatures over 41° C.
After solarisation and bio-fumigants, grow a green manure of corn or maize and dig it into the topsoil. This will restore organic matter to soil and encourage the growth of beneficial mycorrhiza fungi. Bacterial wilt is more likely to occur in soil that is low in nutrients and organic matter, and has a high pH. Before growing crops in the treated beds, add plenty of complete organic fertiliser and as much compost as you can spare. Also check that soil pH is in the 6.5-7.5 range. Avoid growing susceptible crops in the treated beds for at least 3 years after diagnosis of the disease. Maintaining organic cultivation methods and practicing an adequate crop rotation will help prevent recurrence of this disease.
Soil fungi that affect the water-carrying parts of plants cause wilt diseases that can affect a wide range of vegetables, grains, and ornamentals. Fruit trees can also be affected.
Wilt diseases are commonly caused by not practicing a proper crop rotation. Adding organic matter to soil helps to limit soil-borne diseases because the beneficial fungi in organic matter out compete the pathogens. Avoid using glyphosate because it has been shown to affect the microorganisms in soil that assist in keeping diseases under control.
To find out which fungus is affecting your plants, pull out (if possible) one of the affected plants and cut open the stem near the roots.
If it’s Fusarium wilt, the inside of the stem (in most plants) will be pink to reddish brown. In beans, the inside of the stem will be dark brown with reddish roots. According to research recently published by the US Department of Agriculture, Fusarium diseases are becoming a serious problem in GM crops that have been engineered to be glyphosate-resistant. The research found that glyphosate exuding from the roots of this type of GM crops stimulates Fusarium fungi in soil. In wheat, these fungi cause Fusarium Head Blight. Fusarium produces several toxins in plants that are not destroyed by cooking. These become a health problem when present in large quantities. One type causes vomiting. Another type causes cancer and birth defects, while a third type of toxin is lethal. It is important to act to prevent the establishment of Fusarium in garden and agricultural soils.
If it’s Verticillium wilt, the outside of the stem appears normal but the inside of the stem will be dark brown to black. This disease is more common where drainage is poor. Improve drainage and control weeds. Give any unaffected plants in the bed a drink of seaweed extract tea as potassium and trace elements in this tea assist in building resistance to disease.
TREATING FUNGAL WILT DISEASES
Remove all weeds and affected plants and burn them or dispose of them in a sealed plastic bag. Do this carefully, as spores can be spread by shoes, and gardening tools. Wear rubber boots and wash them and all tools after working in infected soil. Then dry these in direct sunlight.
When soil temperature is 14° C. or higher, grow a green manure crop of bio-fumigants such as Green Harvest’s BQ Mulch, yellow mustard, or radish. The peppery members of the Brassica family produce good quantities of glycosinolate that breaks down in wet soil to produce a gas that is effective against fungal pathogens and nematodes. Slash the green manure before it flowers, and hoe it into the topsoil. Then water the bed and cover it thickly with mulch.
Recent Spanish research has shown that, during summer, solarization is effective in treating these diseases. Place clear plastic sheets over irrigated beds and leave them in position for a minimum of 2 months.
Then grow a green manure crop of corn or maize and slash it when it is knee high and dig it into the topsoil. Wilt diseases are more common where soil is low in broken down or decomposed organic matter, and bio-fumigation will also affect beneficial mycorrhiza fungi in soil. Replacement of organic matter through green manures and as much compost as you can spare will encourage the re-establishment of mycorrhiza and other beneficial fungi and bacteria that can control soil pathogens when organic cultivation methods are used.
You will also need to practice a long crop rotation for different plant families until your soil is free of disease.
I sometimes hear garden experts say that organic fertilisers are not as high in nutrients as chemical fertilisers, so you have to use more of them. This is simply not true.
The American Association for the Advancement of Science (AAAS) reported in February, 2009 that hundreds of studies have shown that “incrementally higher levels of fertilizer negatively impact the density of certain nutrients in harvested foodstuffs.” They also reported that the complex way in which nitrogen is absorbed in organic cultivation results in more efficient assimilation of the nutrient, allowing organically grown plants more energy to produce antioxidants, and the formation of less nitrates. Nitrates in food can form carcinogenic nitrosamines in the digestive tract.
Excess use of nitrogen fertilisers (including uncomposted manures and manure teas) promotes bursts of soft, sappy growth that is much loved by chewing and sap-sucking garden pests. Overuse of a particular nutrient can block the absorption of other nutrients. Nitrogen and phosphorus compete for absorption. Overuse of chemical nitrogen fertilisers can also result in deficiency of the less mobile phosphorus.
Organic fertilisers don’t need to be as high in nutrients. Organic fertilisers made from a variety of recycled organic matter will contain a full range of major nutrients and trace elements. Organic fertilisers in the form of compost, castings from worm farms, animal manures, leaf mould, and broken down green manure crops and organic mulch add humus to soil, but chemical fertilisers do not.
Humus, the most stable form of organic matter, consists of electrically charged particles called ions. Nutrient elements also carry a weak electrical charge. Humus has a large surface area and many charged sites to hold nutrient elements through electrostatic force where they are easily accessible to plants, and regulate their absorption so that nutrients are not absorbed by plants in toxic quantities. Humus also provides a habitat for a group of beneficial fungi that assist nutrition in a wide range of perennial plant families. Some chemical fertilisers, such as superphosphate, suppress the activity of these fungi and other beneficial soil organisms.
Although clay particles in soil also carry an electrical charge and are capable of holding some nutrients, without humus in soil, phosphorus can become locked up with iron, manganese or aluminium, and unavailable to plants, and nitrogen and sulphur can leach from soil.
A suitable soil pH plays an important role in efficient absorption of a full range of nutrients. Adding extra fertiliser when soil is too acid or alkaline for particular species of plants will not help their growth. Humus in soil assists in maintaining a suitable pH. See:Changing soil pH
Although we tend to worry about plants getting enough fertiliser, fertiliser plays a relatively small, but essential, part in plant growth. The major contributors to plant energy are water and carbon dioxide. In the presence of sunlight, the green parts of plants can convert these into carbohydrates, which form the cell structure of plants. You could say, in fact, that plants are solar powered.
Green manures are an easy, cheap way to produce organic matter for soil by growing grains until they are knee high, or inoculated legumes* until they start to flower, then slashing them, and leaving them, as organic mulch, on the soil surface to break down, or slashing them into smaller pieces and digging them into the top 10 cm of soil. Suitable green manure crops for each season and climate zone are included in ‘What to grow’ that is posted on this blog each month.
For those unfamiliar with growing green manures, step-by-step guides can be found in my book, Easy Organic Gardening and Moon Planting, or in the March/April 2008 issue of Warm Earth magazine.
If planning to use the bed immediately after slashing the green manure, digging fresh, organic matter into topsoil can cause a temporary nitrogen deficiency, as soil bacteria require nitrogen to break down the materials, then nitrogen then becomes available for your crops. Apply some complete organic fertiliser to the bed so that nutrients are immediately available to your plants.
* Inoculating legume seed enables the legume to fix nitrogen efficiently by introducing the nitrogen-fixing bacteria to soil. Uninoculated legumes can be still be grown as green manures to supply organic matter. (See post on Fixing nitrogen.)
If you live in a frost area, make a note of when you sow peas, sweet peas or broad beans and when they start to flower. The foliage of these legumes is frost hardy, but the flowers are not. Yet, they do not crop well when temperatures are too warm. Peas can take from 7 to 10 weeks to produce flowers, and broad beans can take from 7 to 13 weeks to produce flowers, depending on local temperatures. Sowing too early or too late for local conditions can result in a disappointing crop. As a general rule where frosts occur, do not sow seed until 10 weeks before the usual last frosts in your area. If you have unusually late frosts, you can protect your plants with a temporary plastic canopy, if a frost is predicted.
It is too late to grow broad beans as a crop in warmer areas, but they can be sown in all areas as a green manure crop where you intend to sow tomatoes next spring. Broad beans inhibit the growth of fusarium wilt – a fungal soil disease that can affect a wide range of plants, including tomatoes. If grown as a green manure, the plants are slashed when knee high. Broad bean seed sold for green manures may be called fava, or faba, bean.
Peas, broad beans (and sweet peas) like a humus-rich soil with a pH of around 6.5. They will need an application of complete organic fertiliser (see post on Fixing nitrogen). Legumes also need the presence of molybdenum and cobalt in soil for good growth, and an application of seaweed extract tea to the bed before sowing, will ensure these trace elements are available.
Try to avoid periods of heavy rain when sowing legumes because they can rot before germinating in cold conditions. Having said that, we had a 98% germination rate for our peas that endured a week of heavy rain after sowing in a raised bed. The seed had been saved from last year’s crop and had not been treated with anything. I am at a loss to understand why major seed manufacturers feel the need to coat their legume seeds with toxic fungicides.