Citrus Growing Guide Part 5: Citrus Pests, Diseases and Problems in Melbourne 24


There are several common pests and diseases that affect citrus trees in Melbourne. Generally pests only attack weaker trees. By planting your tree in a suitable location and providing the right fertilizing and watering regimes, your tree will stay healthy and strong.

Aphids

Citrus pests and diseases melbourne

The black citrus aphid is a common citrus pest

Aphids can infest new growth, mainly in spring. They are sap feeders and secrete sticky honeydew. The harmless fungus black sooty mould, often grows on this honeydew. Winged aphids are carried by wind to infest other trees and can carry the tristeza virus. Ladybirds, hoverflies and lacewings all feed on aphids. Avoid spraying small populations of aphids early in the season. Their natural predators will multiply and eventually control them. Mature trees can sustain heavy infestations without damage. Aphids can be hosed off with a jet of water, wiped off with your fingers or sprayed with white oil.

Scale

Citrus pests and diseases in Melbourne

Scale insects secret honeydew which in turn attracts ants

Adult scale insects are mostly immobile on the plant.

They feed by withdrawing sap through their sucking mouth parts. Juveniles emerge at various times of the year and spread to other plants by wind, birds, insects and on our clothing.

Some of the scales (eg Soft scales) produce honeydew on which black sooty mould may grow. These scale are often protected from predators by ants.

Small infestations of scale are not normally a problem for citrus trees. Larger numbers can be scraped off with your finger, or sprayed with a white oil solution. Controlling ants is another way to keep scale in check.

Citrus Gall Wasp

Citrus Pests and Diseases Melbourne

Citrus Galls caused by the Citrus Gall Wasp should be pruned out of the tree prior to August and burnt or disposed of.

 

The presence of large swellings or bulges on the branches of citrus trees is quite common in Melbourne.

These swellings indicate that Citrus Gall Wasp has infected the tree.

This tiny native Australian wasp lives for 3 to 4 days laying its eggs in the branches of the citrus tree. It is the tree’s ‘reaction’ to this egg-laying which produces the next year’s Citus Gall Wasp.

The traditional way to control of Citrus Gall Wasp is cutting off the swollen branches. The wasp emerges from the gall in aorund November, so you need to prune out as much of the gall prior to October to destroy the Citrus Gall Wasp life-cycle. Do not put the pruning into the compost or green-waste bin. Instead, put it in a plastic bag and put it in the garbage bin. Spraying to control the wasps are ineffective.

We’ve noticed the recent “Prune in June” campaigns have had a negative impact on many citrus around Melbourne. Take a look at our gall wasp blog post for some alternative control methods to pruning out the galls.

Citrus Leafminer

Citrus Pests and Diseases Melbourne

Avoid fertilising your citrus trees after February. Doing so will promote a flush of new growth. This leaves the tree susceptible to Citrus Leafminer infestation in Autumn.

Citrus leafminer is the larva of a tiny silvery white moth about 2mm long, when at rest. Adult moths are active at night so are rarely seen. The tiny larvae tunnel in immature leaves when they feed and leave wavy silvery trails. Leaves become crinkled, distorted and curl up at the edges when the larvae form their pupae.

Young trees are particularly susceptible and heavy infestations can severely stunt their growth. Significant infestations occur mainly on new growth in late summer and autumn.

Control of citrus leafminer is difficult. Because the leafminer is inside the leaf, sprays are generally ineffective. Affected growth, can be continuously removed to interrupt the life-cycle. Avoid fertilizer high in nitrogen, after February, to stop promoting new growth at this time.

Phytophthora

Phytophthora is the most common fungus to cause root rots in citrus. It grows on susceptible rootstocks in conditions where trees and soil remain wet for long periods. The bark near ground level becomes soft, dark-stained, water-soaked and sometimes gummy. The wood beneath the bark turns dark brown and dead bark is sometimes shed in vertical strips. Severe infection of the trunk and main roots, will kill the tree.

There is no treatment for this disease. To prevent phytophthora, only grow trees on trifoliata (or flying dragon) rootstocks in Melbourne. Select well-drained sites avoid mulching too closely to the trunk and avoid over-watering the tree.

Common Citrus Problems

There are several other “problems” that you might notice when growing citrus in Melbourne.

Summer fruit drop

Citrus normally shed large numbers of fruitlets shortly after blossoming in the spring and at early fruit set (pea size). However, it is also common for a sudden drop of small fruit (20 mm diameter) to take place in the summer, when warmer weather places stress on the tree. The problem is particularly severe in young navel orange trees and may be related to lack of water at, or soon after, fruit set. Root diseases and lack of nitrogen or trace elements may also be responsible.

Rind splitting

Rind splitting of fruit, particularly in navels, also occurs before or near maturity as a result of climatic factors. These factors specifically are drops in average temperatures and increases in relative humidity, at a time when the rate of fruit growth is decreasing. There is no control for the disorder.

Preharvest drop

Preharvest drop in the autumn, before fruit is fully mature, is a common problem with navels and grapefruit. Some of this drop is natural. Some mandarin and lemon varieties, will also shed fruit when damaged by the spined citrus bug. Brown spot infection in mandarins will result in fruit drop.

Alternate cropping (biennial bearing)

Alternate cropping (biennial bearing) is a common problem with many citrus varieties, such as Valencia orange, Wheeny grapefruit, and mandarins. After a heavy crop the tree often responds by carrying a light (or nil) crop. Once this cropping pattern is established it is difficult to return to regular annual cropping. Pruning or thinning of the heavy crop, and early harvesting will assist in reducing the problem.

Second crop fruit

Sometimes, orange trees will produce blossoms in the autumn after a stress period, and set a second crop. This fruit is often of poor quality (thick skins and low juice content). With lemon trees, however, these intermediate crops are desirable as the fruit quality remains high.

Want to know more about Growing Citrus in Melbourne?

This is Part 5 of a 5 part guide to growing citrus in Melbourne. The links below will take you to the other four parts of our Melbourne citrus growing guide.

Citrus Growing Guide Part 1: Citrus Varieties for Melbourne

Citrus Growing Guide Part 2: Where and How to Plant Your Citrus Tree

Citrus Growing Guide Part 3: Ongoing Care and Management of Citrus Trees

Citrus Growing Guide Part 4: Pruning and Shaping of Citrus Trees

Our citrus growing guides are based on my own experience, as well as the following two books. They are both invaluable resources for the home gardener based in Melbourne. I strongly recommend you have a read of both if you want to know more about growing citrus in Melbourne:

Bruce Mophett & Ian Tolley: Citrus, A Gardener’s Guide, 2009.

Louis Glowinski: The Complete Book of Fruit Growing in Australia, 1997.


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24 thoughts on “Citrus Growing Guide Part 5: Citrus Pests, Diseases and Problems in Melbourne

  • Nathan

    Hi, thanks for the useful tips.
    Just have a question.
    My lemonade lemon tree never fruits I’ve had it for a few years now and havnt received any lemons.
    Any tips would be greatly appreciated.
    Thanks again.

    • Leaf, Root & Fruit Post author

      Hi Nathan,

      That could be all manner of issues (so many that it took a 5-part blog to cover them all!). It could be anything to do with fertilizing, watering, position, soil, issues with wrong root stock, or root stock taking over or a combination of above. Without seeing the tree and knowing more about it, I wouldn’t like to guess. Lemonade trees are generally reliable producers.

      If you’ve read through our 5 part guide and think you’ve covered everything in there, then feel free to email me some more info, including a few photos and I can try and offer more advice. Otherwise a site visit might be required to diagnose the issue.

      Good Luck and Happy Gardening!

      Duncan

  • Shay

    Thank you for the useful information,
    I’m a gardener and have seen many sitrus trees with gall wasp I was cutting the brunches but I didn’t know the name to tell my costumers ,
    Thanks again
    Shay
    A man with a plan

  • Louis Vincent AGIUS

    Citrus Gall Wasp BUT do they hurt the tree I just pruned a lime tree ,took 1/4 of the tree that was loaded with limes irrespective of the swollen branched, it had a very bad infestation
    They seem to effect my lemon tree too , BUT NOT THE BLOOD ORANGE

    • Leaf, Root & Fruit Post author

      Hi Louis, thanks for sharing. Yes, the Citrus Gall seem to prefer lemon and lime trees over orange trees. I’m trialling a few different “regimes” for coping with the gall wasp. It’s a complex topic and one worthy of a blog post in its own right. I’ll put it together once I’ve confirmed my preferred regime for dealing with the gall wasp.

      Stay tuned!

      Duncan

  • Gill

    Hi, my 30 year old, grafted lemonade tree, appears to be getting something similar to blackspot on the fruit skins?, and more noticeable every year for the past 3 or 4. The fruit is still delicious.
    I live in the bayside area of Melbourne, grow it organically with straw mulch, water is sufficient unless high summer. Open foliage to get air flow etc etc.
    I have been reading things on the internet, but its all commercial stuff (USA), nothing organic etc.

    Have you any ideas?

    • Leaf, Root & Fruit Post author

      Hi Gill,
      Sounds like citrus black spot. This is caused by a fungus and once the tree has been infected, it has it for life. You can reduce the severity of the symptoms by reducing the fugal spores in the soil (through using god quality compost) and increasing air flow around the tree. You could use a copper fungicide, of which there are a few organic options avaiable, but they are still only going to treat the symptoms rather than provide a cure. Once the tree starts getting overwhelmed by the disease, it is best removed. Keep in mind that any other citrus trees that you plant in the spot in future are also likely to get infected.

      Good Luck and Happy Gardening!

      Duncan

    • Leaf, Root & Fruit Post author

      Hi Gill,
      Sounds like citrus black spot. This is caused by a fungus and once the tree has been infected, it has it for life. You can reduce the severity of the symptoms by reducing the fugal spores in the soil (through using god quality compost) and increasing air flow around the tree. You could use a copper fungicide, of which there are a few organic options available, but they are still only going to treat the symptoms rather than provide a cure. Once the tree starts getting overwhelmed by the disease, it is best removed. Keep in mind that any other citrus trees that you plant in the spot in future are also likely to get infected.

      Good Luck and Happy Gardening!

      Duncan

  • claire Spring

    My line tree 10 years old and friuiting well issuddenly being branches dying after something is eating many outside bark from branches to reveal the inner white. It must happen at night?
    What is it ? I am in Rye Victoria….claire Spr

    • Leaf, Root & Fruit Post author

      Hi Claire,

      without seeing the damage it can be hard to tell. But my guess would be rats are eating the bark. Possibly possums, but most likely rats.

      It might be worth heading out with a torch in the evening to see if you can spot the culprit!

      Good Luck and Happy Gardening!

      Duncan

  • John Brownell

    I have Oleanders growing near my citrus trees and I am constantly fighting leaf miners, are they related? Is the Oleander a cohost?

    • Leaf, Root & Fruit Post author

      Hi John,

      I’m not aware of Oleander acting as a cohost, although it may be possible. I find the best way to avoid leaf miner is to just avoid fertilizing after early Feb. Hopefully someone from our growing community has some ideas on the oleander for you!

      Please keep us updated.

      Duncan

  • Victor Boaro

    HI guys and ladies,
    I wonder if you can help me please.
    I have inherited from our daughter two citrus trees ( blood oranges ) about 50 cms high in pots that as a result of neglect have lost their leader stem . The top part has died off.
    Is a new leader stem able to be encouraged to grow to enable the plant to continue to full height or can it only remain as is.
    Regards,
    Victor Boaro
    Perth

    • Leaf, Root & Fruit Post author

      Hi Victor,

      Thanks for your inquiry. Without seeing the trees, it’s a bit difficult to say. Assuming the trees are grafted, you’d need to be sure that some of the scion was still alive to re-shoot. Otherwise, you may just be encouraging the rootstock to regrow, which won’t produce blood oranges!

      Good Luck and Happy Gardening!

      Duncan

  • Lidija Hary

    Hi

    I recently planted lotsa lemons. The plant is looking very healthy with lots of flowers and small lemons starting to form. However, in the last week or so something is eating the leaves, particularly the lower branches. Apart from seeing some ants around I am unable to I find the culprits. No sign of snails but I did see one earwig but I don’t know if that’s the problem.

    Many thanks in advance.

    • Leaf, Root & Fruit Post author

      Hi Lidija,

      Without seeing the tree it can be hard to say. However, my guess would be earwigs. It’s certainly the season for them to be doing damage. They are nocturnal, so the best way to tell is to head out at night with a torch and see if they are having their nightly feast.

      If it does turn out to be an earwig problem, post on controlling earwigs may be useful for you.

      Good Luck and Happy Gardening!

      Duncan

  • ross horsfall

    i am finding what look like bite or peck marks in my myer lemon fruit and separately from that, caterpillars ,browny/orange in colour, about 3 cm long.fruit with damaged rind seems to be o.k to use and so far hasnt deteriorated . could this be snails? the caterpillar problem has not occurred before ,tree is approx 13 yrs old ,with fruitfly traps in it. can you advise? regards , ross

    • Leaf, Root & Fruit Post author

      Hi Ross,

      without seeing the fruit it’s difficult for me to say. Does the damage look like a mechanical injury (ie something biting it) or is it more like brown or coloured “pocks” that could be caused by fungal issues?

      As for the caterpillars, you can either pick them off by hand, spray with dipel, or just wait for the predatory wasps to turn up and kill them off (I’d choose option three!). Planting plenty o flowering plants in your garden will encourage beneficial insects such as predatory wasps into the garden. This will support a healthy ecosystem and reduce the severity and impact of pest populations.

      Good Luck with the Lemon Tree!

      Duncan

    • Leaf, Root & Fruit Post author

      It’s a Passionvine hopper. Which is a sap sucker rather than a leaf chewer. So I don’t think it is what is causing damage to the leaves. It could be earwigs, or mechanical damage such as wind damage. I’d be inclined to just leave nature to sort it all out for you.

      Good Luck and Happy Gardening!

      Duncan