Soil-borne diseases

Page last updated: Monday, 15 December 2014 - 2:08pm

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To prevent disease spread plant material, including cuttings, transplants, and seeds, should come from reliable sources. Research the disease history of gardens before transplanting plants from them.

Sterilise second-hand tools including pots, trellises and support material before using or reusing them in your garden. Use sodium hypochlorite at 1.2 % of available chlorine to disinfect materials.

Dispose of diseased plant material by burning or composting. Do not use the material for mulching until it is well broken down.

Preventing disease

Plant stresses

High temperatures, low soil moisture or continued flooding predispose plants to disease.

To reduce plant stress transplant during cool, moist weather when continued mild conditions are forecast.

Shade and mulch soil using medium to coarse grade bark mulches which allow rain or irrigation water to penetrate the soil. Grow transplants in well-drained soil and protect them from drying winds. Products like anti transpirants, which are sprayed onto the leaves, can help reduce moisture loss from young seedlings. Do not fertilise plants in hot, dry weather.

Seedlings should be transplanted into their permanent positions when they are sufficiently hardened but still young. Plants are generally more susceptible to transplanting injury as they age. Soil organisms will use these injuries to invade the plant.

Organic matter

Opportunistic soil pathogens thrive on rotting organic matter, which also depletes soil nitrogen, so only work well-rotted organic matter into the soil near young, growing roots. Give organic matter two to three months to break down before planting a new crop, especially in soils with a history of fungal diseases.

Salt, fertiliser and waterlogging

Salty water causes general loss of vigour in the plant and associated stunting, yellowing, wilting, leaf-loss and damping-off due to fungal infection.

Over fertilisation can also cause root injury and subsequent fungal infection.

Seeds and roots decay in anaerobic conditions, which are caused by poorly aerated soils with insufficient drainage. These conditions encourage attacks by fungi such as Pythium and Phytophthora spp. In waterlogged conditions avoid over watering, improve the drainage in clay soils with gypsum and compost or choose plants that tolerate ‘wet feet’.

Soil disinfestation

If there is a history of fungal diseases in garden beds the soil can be solarised during hot weather. Moisten the soil and work to a fine tilth to 25cm deep. Then cover the soil with a thin, transparent polyethylene sheet and bury the edges 25cm or more deep. Leave the sheeting in place for at least four weeks. Make sure the polyethylene sheet is sealed so that no air can blow under it. Solar heating is less effective at the edge of the sheeting, so treated areas should not be long and narrow. This treatment kills most pathogens, while normal soil microorganisms survive in sufficient numbers to prevent recolonisation by the disease pathogens. Solarisation also kills weed seeds and insect pests.

Crop rotation

Rotate crops of annual vegetable plants and plant crops from different plant families to break the disease cycle. If diseases occur wait two to five years before replanting the crop in the same position.

See Also