What to do about plant diseases

Plant diseases are a natural part of the growing process. But it's certainly not a pleasant part of it. While it's difficult to isolate your garden from attack, there are a few things you can do to deal with plant diseases.

Non-infectious diseases are caused by environmental factors, such as nutrient deficiencies and lack of water. Infectious diseases are usually caused by fungi, bacteria and viruses and can be spread in a number of ways.

• Humans: via hands, shoes, pruning tools and machinery.
• Insects: via bodies and mouthparts.
• Water: from groundwater, rainwater and water from overhead irrigation.
• Wind: carries fungal spores and other pathogens long distances.

Quick tip: To avoid spreading disease, wipe your pruning tools with a cloth dipped in tea-tree oil after pruning each plant.

• Tend to plant disease outbreaks quickly. Check your garden regularly and remove infected leaves, flowers, fruit or entire plants.
• Never take cuttings from plants that are diseased. As an added precaution, disinfect pruning shears and other pruning tools after each cut, or before starting to cut another plant, to avoid the unnecessary spread of disease.
• Keep pruning tools sharp, as jagged, rough cuts made by blunt tools may become infected.
• To effectively control over-wintering spores, spray fruit trees with a lime- or a copper-based fungicide in winter.
• Don't compost anything diseased or infected by a virus — this includes weeds, plant leaves and prunings. Your compost heap may not be hot enough to kill them. Instead, place them in a bag and put it straight in the garbage bin.
• To improve air circulation, prune out the centre of plants subject to leaf diseases.
• To reduce the chance of soil-borne disease, viruses and pests, avoid growing members of the same vegetable family in the same part of your garden in successive seasons.
• Keep insects that spread viral diseases, such as aphids, under control. Squash them, hose them off with a jet of water, spray them with soapy water or try another home-made remedy, such as garlic spray.
• Check tree trunks in autumn, when fungal growths flourish. Bracket fungi on the tree trunk indicates that the tree is probably rotten inside and may need to be removed. Consult a professional.
• Take care of the root systems of trees, because damaged roots are susceptible to root rot. Don't park the car over them, don't cut them and don't use weed killers near them.
• Check for root problems. Look for wilting when the plant doesn't need water, the tips of twigs or branches dying and splitting or oozing from low down on the trunk. The plant may also be unsteady in the ground if its root system is dying.
• To reduce the likelihood of root rot, allow the soil to dry out slightly between watering.

Yellow leaves may be a result of normal aging, and not a problem at all — but they can also indicate deficiencies and more serious problems. Here's how to tell what's what.

• A few older leaves yellowing is a normal sign of aging. There is no treatment needed.
• New leaves pale yellow is a sign of iron deficiency. Try iron chelates to increase iron supply.
• All leaves pale yellow means nitrogen deficiency. Try a more complete fertilizer.
• Yellow "v" on leaves means magnesium deficiency. Offer magnesium sulphate.
• Yellow mottling/marbling is a virus infection. Remove badly affected plants, otherwise do nothing.
• Yellowing and excessive leaf drop is due to weather that is too dry or too wet. Water accordingly to keep plat at optimal hydration levels.