Anthracnose
Anthracnose is the primary reason why mangoes, avocadoes and some other fruit trees fail to fruit or develop blackened fruit that drops prematurely. Anthracnose is a fungal disease that is exacerbated by rain and high humidity. During dry weather the disease is virtually absent.

Application of copper based sprays immediately prior to flowering and during early fruit set and/or use of biodynamic preparations such as casuarina tea [see Do It Yourself Pest Control] can help reduce the effects of anthracnose. Some fruit tree varieties also possess inherent resistance to anthracnose.

Black Spot on Pawpaw
Adequate nutrition is as much a tool in disease control as the use of sprays.  Plants deficient in potassium, phosphorous and magnesium are more susceptible to attack by black spot and powdery mildew.  Simply increasing the potassium (sulphate of potash and/or lucerne mulch), phosphorous (rock phosphate or chicken manure) and magnesium levels (Epsom salts) helps to make plants more resistant to disease. Pawpaws are most susceptible to black spot disease during the cooler months. Spraying with sulphur or copper based compounds prior to the onset of the cool weather and watering with liquid seaweed can help reduce the severity of infestations.

Black Spot on Roses
Ensure plants receive balanced nutrition (see above). Foliage applications of seaweed sprays are recommended, not so much for their nutritional value, but for their ability to increase disease resistance in plants.  Foliage applications of liquid potash are also beneficial. Bicarbonate soda sprays are also beneficial, see [Do It Yourself Controls - Black Spot Spray]. Fishmeal and other fish-based fertilisers contain trace elements valuable in building disease resistance. Remove old canes and diseased wood as this harbours bacterial and fungal spores, insects and mites. Old wood is also less efficient at transporting water and minerals and therefore produces less vigorous growth, fewer flowers and is more disease prone.

Citrus Melanose
This disease typically affects more mature trees or plants that have a canopy of foliage that hangs close to the ground. Mulch trees with a feeding mulch such as lucerne. This will help to prevent spores splashing up onto the foliage when it rains. Lift the lower branches so that foliage is well clear of the ground. Spray trees with copper based compounds after all fruit has been harvested, thoroughly wetting the foliage, trunk and branches.  Repeat applications each year until no further evidence of disease exists. Improve general tree health with additional nutrients and water.

Damping off fungus
This disease affects germinating seeds and young seedlings. It is transferred via water and affects the water conducting tissues of the plant. Use new seed raising mix or clean compost. Wash all pots and tools in hot, soapy water then allow to dry in the sun before use. Make sure that plants do not sit in water. There is no cure for affected plants and they should be disposed of so as not to infect neighbouring stock.

Note that newly planted seedlings can appear to suffer from a similar condition, but this is typically the result of over watering or planting into a soil containing excessive amounts of fertiliser or overly rich compost and manure.

Frangipani Rust
Infected frangipani plants develop bright yellow rust pustules on the undersides of leaves. This  detracts from the appearance of plants and causes premature leaf drop. Frangipani trees that are severely affected year after year can be weakened, but the disease is rarely fatal. Rainfall and high humidity increase the incidence of the disease. Protective copper based sprays can be applied when trees first come into leaf, but are largely ineffective once rust symptoms become evident. Fallen leaves provide a source of infected material. Collecting and removing fallen leaves is one strategy to reduce infection. Any neighbouring trees that are infected will provide a wind borne source of fungal spores.

Passionfruit Scab
This disease is caused by a Cladosporium fungus. It results in the fruit of passionfruit and granadilla developing scab encrusted lesions. Heavy infestations (like the one pictured) cause fruit to shrivel and drop prematurely. Many other types of fruit and vegetables are affected by this disease. Vines grown in shaded areas appear to be more susceptible. Complete eradication is difficult in these situations, so always grow vines in full sun. Remove and dispose of infected fruit (offsite). Spray vines thoroughly with a copper based fungicide.

Passionfruit Virus
At the end of the growing season after production of a bountiful crop, passionfruit vines can look tired and in need of a well-earned rest.  In some cases, vines will show characteristic mosaic leaf yellowing that indicates presence of a virus disease.  Such diseases are often present in plants from an early age, but only begin to gain the upper hand when the vine is occupied with fruit production or stressed in some way.  Affected vines may continue to grow, flower and fruit in coming seasons, but will always lack vigour.  Unfortunately there is no cure for virus affected plants. Given their rapid growth rate, high productivity and short lifespan, passionfruit vines are best replaced every few years.

Phosphorous induced iron deficiency - ABC Gardening Talkback listener Tony of Lota sent this image of his Banksia integrifolia. Tony writes, 'A lot of the new leaves are yellow rather than dark green, and there is dieback of new growth. I have dug holes to about 700mm and filled them with compost, which has a pH of 5.5.' All banksias are sensitive to phosphorous and this banksia is suffering from the classic symptoms of phosphorous induced iron deficiency. This means that the phosphorous in the soil (found in the compost Tony added) have caused iron to become unavailable to the plant. Symptoms typically occur six to twelve months after planting or after the application of other high phosphorous products like chicken manure based fertilisers or mushroom compost. Applications of iron chelates watered around the roots will provide the iron that the banksia needs. Recovery will depend on how the plant has deteriorated prior to application. Raising the soil to pH 6.5 by adding a little Dolomite lime to the soil will also help.

Potato scab
Potato scab is a disease commonly introduced by using non-certified seed potatoes (or saving seed from previous crop) and is worse in alkaline soil and dry conditions. Prepare a new area for your potatoes, ensure the pH is not above 6, use certified disease free seed potatoes and rotate crops each season. Remember that potatoes, tomatoes, capsicum and eggplants are related crops and should not be planted after one another in the same garden bed. Certified disease free seed potatoes are available from nurseries, produce stores or online. 

Root Knot Nematodes
Nematodes are microscopic animals found in the soil, within plant tissue and within water.  Some nematodes are damaging to plants, but there are also beneficial nematodes found within organic matter that parasitise destructive nematodes.

Nematodes that attack plant roots are the most common. They cause root galls and excessive root branching.  This affects the plant’s ability to take in water and nutrients, wilting and stunted growth. Sandy soils are particularly prone to nematode infestation.

Control can be achieved by incorporating organic material into the soil, growing nematode resistant varieties, crop rotation and paying attention to hygiene procedures. For severe infestations you can treat vegetable growing areas with a mix of molasses (2 tablespoons per litre of water). This should only be used on areas that will grow nematode susceptible crops (tomatoes, potatoes and capsicum). Do not use across the entire vegetable garden as this mix has the potential to kill worms.

Sooty Mould
Scale insects and their associated sooty mould are common pests of many plants including citrus, gardenias, lillypilly hedges and other native plants.  Conventional oil based controls, see [Do It Yourself Controls - Oil Spray] can be used, but should be diluted to half strength on fine leaf natives or plants with hairs on the leaves.  Avoid using oil based products on plants with grey foliage. Commercial oil sprays include Eco-Oil.

Soap sprays are also effective, but must be used on a regular basis. Pyrethrum based products can also be used.

Stylar End Rot
This disease affects Tahitian limes and occurs where fruit is left to over ripen on the tree. The base of the fruit develops a soft rot and fruit fall to the ground. The internal flesh also shows deterioration when cut.

Fruit should be harvested while still green in colour prevent this occurring.

Zucchini Virus
This zucchini plant is showing symptoms of virus diseases. Virus diseases can be transferred from one generation to the next via infected seed and between garden plants by aphids or other sucking insects. To avoid this problem, never save seed from any plants that show poor growth or other symptoms that may indicate potential disease problems.  Remove and destroy infected plants to prevent the possible spread of the disease to neighbouring plants.