Soil-borne Diseases

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This week NAREI in Focus will be looking at soil-borne diseases. All gardens have a range of permanent soil-borne disease organisms which are usually contained in a balanced environment where organisms, soil conditions, and hosts interact in a complex system. Garden plants only show symptoms of disease when this balance is disrupted, and pathogen organisms become dominant.

Gardeners can overcome disease by improving plant conditions, creating a hostile environment for the disease and by stimulating the growth of beneficial soil organisms.

Types of soil-borne diseases
Soil-borne diseases in the garden include pre and post-emergence damping-off, like Fusarium, Pythium and Rhizoctonia species, root rot, including Phytophthora, vascular wilts caused by fungi including Verticillium and nematodes.

Pre-emergence damping-off is where young seedlings decay in the soil before they appear above the soil surface. This occurs when conditions for seed germination are poor, such as cold, hot or very wet soil, poorly-drained soil, compacted soil or in the presence of undecayed organic matter.

Post-emergence damping-off is where stems and roots of tender seedlings are attacked at the soil line and the seedlings fall over. High salt concentrations in the soil also cause damping-off.

Root rots can affect plants beyond the seedling stage when the fungi invade internal root tissue, interfering with the supply of water and nutrients. Aboveground symptoms include loss of vigour, leaf yellowing, leaf drop, wilting starting at the growing tip, twig dieback, and sudden death.

Vascular wilts are characterised by plant wilting and discolouration of the vascular system at stems or trunks and branches.

Nematodes are microscopic, unsegmented worms. They include pest nematodes like the root-knot nematode which invades the roots and causes them to form gall-like lesions that restrict water and nutrient uptake which causes wilting.

To prevent disease spread
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.

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 grades 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 antitranspirants, 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.