Nematodes that damage roots in the vege patch, are minute worm-like creatures, also known as eelworms. The female nematodes penetrate plant roots causing lumps to form on the roots, which affect the water-carrying ability of the roots. (These are not to be confused with the nitrogen–fixing lumps that form on the roots of legumes, see Fixing nitrogen). Root knot nematodes are more likely to occur in warm climates in soils that are low in organic matter, especially where a proper crop rotation has not been practiced.
Each female nematode can lay up to 2000 eggs, and numbers can multiply quickly. Give affected plants a foliar feed of seaweed extract tea. Seaweed contains plenty of potassium that helps to strengthen cell walls and improves plants’ resistance to pests and promotes root growth. Remove all weeds. Some weeds are hosts to these pests and can transfer viruses to plants.
Affected plants must not be allowed to become water-stressed. Badly affected plants will have to be removed. Make sure the soil is damp so that soil clings to the roots. Place plants (with attached soil) into a garbage bag. Seal the bag, leave in hot sun for a few days, then place it in the garbage. Do not compost these plants. Once the crop is harvested, proceed with methods for prevention (see below).
Allow 3 years between growing any member of the tomato/potato or melon/cucumber families in the same patch of soil. During this break, grow a green manure that is a ‘bio-fumigant’. ‘Bio-fumigants are green manures that release a gas that is toxic to nematodes. They are grown to knee height and chopped up and mixed through top soil, then covered with mulch. Green Harvest have seed for BQ Mulch (sown in cooler months) and cowpea (sown in warmer months) that control nematodes. Indian or brown mustard (Brassica juncea) and non-GM rapeseed (Brassica napus) are also effective bio-fumigants. Forget marigolds, they are more effective against northern hemisphere nematodes. During the 3 year break, brassicas or corn can be grown in the treated bed, if you have limited garden space.
When preparing beds, add a 5 cm layer of organic compost to the bed surface and cover it with organic mulch. Beneficial organisms in compost are pest nematode predators, and mulch keeps compost damp to allow microorganisms to work on restoring soil to health.
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.