Citrus trees with root rot show a slow decline with leaf yellowing and leaf drop. It appears under the bark as white, fan shaped mycelia mats with a strong mushroom odour. Long thin black strands spread the infection from root to root and among plants as they grow through the soil. In autumn, gold clusters of mushrooms appear on the bark. If these mushrooms appear it is normally too late to save the tree and it must be removed.
To control the disease isolate infected trees with a deep trench to break the black strands (rhizomorphs). If early enough, you can expose the roots to the air (about 60cm out from the trunk). Cut off and burn any damaged roots. Burn and remove badly infected trees and roots.
Collar Rot is a fungus that thrives in damp conditions where organic matter in the soil comes into contact with the trunk (usually where the graft is). The first symptom is patches of gum oozing around the base of the trunk. The bark in the area becomes soft and as it dries out it splits. Rot will grow unless the organic matter is removed from the base of the trunk.
To prevent collar rot, ensure the base off the tree is well ventilated and kept clear off organic matter. Avoid wetting the trunk when watering and improve drainage. Avoid injuries to the lower tree (whipper snipping!) and ensure the bud union (graft) is above soil level when planting.
Apply copper fungicides in autumn as a skirt. Affected trees should have the lesions cut away with a sterilised knife (methylated spirits) and treat affected area with the copper fungicide.
A below ground symptom is the loss of feeder roots, above ground symptoms are loss of vigour and spindly growth. As the disease progresses the foliage turns yellow and twigs die back. Trees become more reliant on surface roots and are therefore more sensitive to drying out.
Nematodes feed on the roots, this will not kill the citrus tree but will impede the tree’s capacity to carry water and nutrients so yields are reduced. Nematode damage of the roots also promotes secondary disease. Affected roots appear to look knobbly, gritty and dirty when the soil is shaken form them.
Sooty mould is fungi that grow on the sweet excretions of honeydew insects (such as aphids, mealy bugs or scales). The fungi are black and dry and occur on twigs, leaves and fruit. They don’t damage the fruit directly but may block enough sunlight to interfere with photosynthesis.
To control sooty mould you need to control the honeydew producing insects as well as ants. Ants actively transport aphids and mealy bugs around the garden. Use white oil to suffocate the pests and loosen the mould.