Common Citrus Diseases & Pests

Here is a list of the issues that i have come across with growing my Citrus over the years. Some you can be pro-active & spray with this or that. I like to be as natural as i can with my plants especially with anything i intend to eat. So i will only spray only if a problem seems to be getting out of control. Also since i have so many fruit trees, to spray them all would take ages to do & be quite costly. I think its very important when choosing sprays to consider the effects on other members of our ecosystem specifically BEES. Please avoid the use of nasty sprays such as Confidor which contains Imidacloprid which is dangerous to our Bee buddies. Using Horticultural Oils such as Neem or Conqueror Oil with, if necessary, added Pyrethrum should be all you need and will be fairly organic. Copper is the only other spray i would suggest for Fungal & Bacterial issues.

Verrucosis or Citrus Scab
Verrucosis or citrus scab can affect all citrus fruit but predominantly Lemons & Limes. It is a very common sight on lemons. Tiny lumps, or wart-like protrusions, appear on the skin of the fruit and leaves. It looks unsightly but the quality of the fruit inside is not affected. If you want picture perfect lemons or lots of zest for cooking, regular spraying of copper is necessary. If you're just after the juice you can let it be.

Blossom-End Rot
Blossom-end rot is due to lack of calcium- either because it's lacking in the soil or there has been drought or irregular watering so the tree couldn't take up enough nutrients to support the growth of the fruit. It is a sign of malnutrition rather than disease and is easily treated - although there's nothing to be done for fruit already afflicted. The addition of lime (a handful or two per tree) around the drip line would be an effective stand-alone remedy, but you might as well cover all bases by applying citrus food too. Citrus are gluttonous plants so proper fertilization and water management help to minimize this problem. Try to avoid allowing the soil to become too dry and then overly wet. Wide fluctuations in soil moisture inhibit calcium uptake and movement.

Sooty Mould

Sooty mold is a fungus, which causes the blackening of the leaves of citrus trees. The mould forms on the leaves as a result of honeydew secretions from insects such as whiteflies, aphids and mealybugs. Insect control is the most effective way to prevent this disease. To control the insects and prevent the secretion of their honeydew discharge, spray the tree with Horticultural Oil with Pyrethrum. When spraying the tree ensure that both the top and undersides of the leaves are adequately sprayed. A second treatment spray may be required about 10 to 14 days later depending on the severity of the insect infestation. 
To control and eliminate the mould growth that has already developed, spray the tree with Liquid Copper Fungicide. Generally one application of Copper is adequate for sooty mould control, but a second application about 14 days later may be required in major outbreaks.

Passion Vine Hoppers

The nymph passion vine hoppers are tiny wingless insects with white fluffy tails that are raised above their bodies like peacock tails. The nymphs and winged adults can jump (hop) when disturbed. From late January on the nymphs go through their final molt and change into the small brown adults that look like small winged moths that are often seen in a queue like line on the stems of plants. The adults will be be active through to autumn laying eggs in small branches of their target plants. As they withdraw their egg laying tube, they pull out small white tufts of plant material which show up as white dots on your plants. 

Control of passion vine hopper is best achieved when the insects are in the immature nymphal stages using Horticultural Oil with Pyrethrum and spray the insects directly. This works by physically blocking the insect’s breathing holes and suffocating them. This would be best done before the nymphs reach maturity and grow wings. Once adults they tend to fly off in clouds as soon as disturbed and they will be difficult to spray with the oil the nymphs can be difficult to spray also, because of their hopping habits.

Photo:John Tann

 

Whitefly

The citrus whitefly is a tiny white winged insect that is about 1mm in length. It is most commonly found feeding on the underside of the tree’s leaves. When the branches are shaken, the Citrus whitefly will rapidly take flight and can be seen fluttering around the tree. In addition to feeding on the citrus tree, the whiteflies also lay their eggs on the underside of the leaves. When the eggs hatch, the juveniles are small oval, almost transparent larva, which attach themselves to the underside of the leaves and begin sucking the sap from the leaves. As a result, the tree’s leaves begin to curl and appear to be covered with a sticky, sooty mold substance. The Sooty Mould is due to honeydew that is excreted by the whiteflies because they are not able to metabolize all of the sugars contained in the leaf sap. The honeydew becomes an attractant to ants. Over the growing season, several generations of whiteflies can emerge. To effectively control Citrus Whiteflies spray the tree with Horticultural Oil with Pyrethrum. It is hard to achieve full control of the adult flies, but several sprayings of the tree will significantly reduce the juvenile population and in doing so the overall population.

  

Aphids

Aphids, when in small numbers, do little damage to a tree, however, under favourable conditions the aphid population can grow very rapidly and cause serious damage to a citrus tree during the growing season. The aphids attack the tree by sucking the sap out of the leaves. The symptoms are very visible on the leaves in the form of multiple puckered marks, yellowing and the twisting of the leaves, which gives the appearance of deformed leaves. As the severity of the aphid infestation increases, leaf drop and twig and branch die back can be seen.
During an aphid infestation, the leaves appear to be dripping sap from the underside of the leaves. This is actually an excretion from the aphids and is called honeydew. The honeydew then becomes an attractant to ants, which feed on it. Ants are known to "farm" Aphids by transporting them up a tree to feed off their honeydew.
Aphids can be controlled using Horticultural Oil with Pyrethrum. The spray should be directed at the undersides of the leaves and other areas of visible feeding and insect concentrations. Normally only one or two spray treatments are required to achieve control. For less severe infections Aphids can be removed manually by squashing or blasting off with the hose.

Lemon Borer
The telltale sign is sawdust exuding from active holes. Other symptoms are that the tree shows poor growth, leaves are dehydrated and branches die. Holes found in branches and sawdust indicates presence of borer. You can spear the lavae with guitar strings poked directly into the holes or spray an product like No Borer Spray Injector into the holes. Remove infected wood where practical. It is a good idea to fill in the borer holes with an acyclic paint. This prevents adults entering the holes and laying more eggs. It also means that it is easy to detect new holes and further problems.
It is a must to avoid pruning between early spring to mid summer in hope that these little bugs will leave your trees alone!

Brown Scale
Scale are brown, hard scale-like insects on woody and green stems. Scale numbers build up in dry seasons, Squash manually or for large infestations spray with Horticultural Oil with Pyrethrum as needed to smother & kill the scale.

Photo:Seabrooke Leckie

Soft wax scale

White, puffy soft scale. Flick scale off manually to remove or spray with Oil if infested.

Slugs & Snails
When Slugs or Snails are present it is common to see holes chewed into leaves and the fruit may be pitted or scarred. You may also see silvery trails winding around the trunk and branches near the soil. Lifting lower branches and inspecting under leaf debris under the tree can also detect snails. A proper sanitation program around the tree is important. Clean-up and remove all leaf debris under the tree. The leaves on the ground become a good breading and hiding place for Snails. In addition place Quash snail bait on the ground around the tree trunk will kill Slugs & Snails on contact & prevent them from migrating up the tree trunk and eating the leaves. Quash is the best as its safe for Pets if they eat it accidentally.

Spider Mites
Leaves go yellow and dehydrated. Minute insects & webs under leaves. Common in hot dry weather, spray with Horticultural Oil with Pyrethrum or Liquid Sulphur spray (But not these two together!)

Leaf roller caterpillar
Leaves tightly rolled and foliage and surface of fruit eaten. Squash manually or for large infestations spray with Horticultural Oil with Pyrethrum as needed.

Mealy bug
Often the presence of black, sooty mould will be the first noticed signs. Small, white, mealy insects found in protected cavities. Squash manually or for large infestations spray with Horticultural Oil with Pyrethrum.

Yellow leaves.

Citrus are gluttons for food. When nutrients are in short supply, their leaves turn yellow and they crop poorly. Feeding with a complete granulated citrus fertiliser will fix most of the problems – or help you to avoid them in the first place – as they provide the nutrients that your citrus trees need. But if you're experiencing problems, take a closer look at the leaves.

Magnesium deficiency: Mature leaves turn yellow, with an inverted green V shape at the base of the leaf, and green on the tips. It's often seen on acidic soils. Applying Epsom salts will correct the deficiency. Dissolve a handful into a watering can of water and water into damp soil.

Nitrogen deficiency: shows by light green to yellow foliage over the entire tree in the absence of any distinctive leaf patterns. With mild deficiency, foliage will be light green progressing to yellow as conditions intensify. New growth usually emerges pale green in color, but darkens as foliage expands and hardens. With yellow vein chlorosis, the midribs and lateral veins turn yellow while the rest of the leaf remains a normal green colour. It may occur with the onset of cooler weather in the winter due to reduced nitrogen uptake by the plant from the soil. Nitrogen deficiency will limit tree growth and fruit production, while high nitrogen applications produce excessive vegetative growth at the expense of fruit production, reducing fruit quality.

Iron deficiency: occurs on young leaves. Light yellow/white leaves and green veins are the signs. It's common on alkaline soil. Treat with iron chelates applied to the soil.