Question
Thanks for your quick response, there are no yellow halos on the tree but there are a large number of small ants around the black spots. The spots crack when I apply pressure and contain a brownish fluid. I am wondering if they are some sort of insect eggs and the ants are waiting for lunch. Appreciate the advise on feeding the tree many thanks

Steve
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The text above is a follow-up to ...

-----Question-----
I live in Melbourne I have a 2 year old Meyer Lemon tree planted in the garden. Well drained soil and I feed the tree each week when flowers and young fruit appear. I notices small (5mm) round black spots on the young branches, the fruit is still very young but plentiful. What is the black spot, is it going to be harmful to the fruit and how should I treat it ?  Thanks Steve
-----Answer-----
Hi Steve,

There are many things that can cause black spots on lemon leaves: frosts, fungal infestations, and micro-nutrient deficiencies.

Unless you have a very, very poor soil, I would think that feeding each week is probably unnecessary.  A better approach might be to add some organic mulch beneath the branches and add a balanced fertilizer (10-10-10) when the plant is most actively growing.  Iron supplements may be necessary from time to time if chlorosis (yellowing) is noticeable.  Also, if the pH is too high (too alkaline), the iron will be effectively unavailable to the plant.  In such cases, application of sulfur buds (granules) may help in reducing the pH.

Of the most common fungal pathogens in Florida are the Colletotrichum diseases causing anthracnose, a disease describing generalized necrosis.  When these small black spots grow bigger and coalesce, they display generalized necrosis of large parts of the leaf (or entire leaves).  

Black spots can also be due to secondary infestations of fungi that feed on other pathogens such as aphids or scale insects.

Do you see any yellow halos around the black spots?  Have you seen any insects on the tree?  

Be as descriptive as you can, that will help identify the cause of the black spots.   

John

Answer
Hi Steve,

What you have is a scale insect infestation.  The scale insects appear as small circle to oval-shaped flat spots.  The ants actually "farm" these insects for food.  Here is a nice photo:

http://www.esapubs.org/bulletin/backissues/087-3/web_pdfs_july/photo6jul06bullet

You will need to rid the plant of scales and try to remove the ants as well.  To control scale insects outdoors:

Know what you're looking for. Scale insects resemble round or oval 1/10- to 1/5-inch bumps without legs. They are hard to spot unless you look very closely.

Look for scale insects on the undersides of leaves year-round. They gather in clusters near leaf joints and on leaves and soft stems. The scale secrete a large amount of honeydew onto the foliage, which causes sooty mold, a black fungus, to grow. Black leaves and stems are also an indication of scale insect infestation.

Prune and dispose of infested branches and twigs.

Attract predatory beetles, such as soldier beetles and ladybugs, to your garden by planting coreopsis, dill, fennel and nectar plants such as salvia.

Remove scale insects from plants using a soft cloth or sponge dipped in insecticidal soap. Rinse with plenty of fresh water after washing the plant.

Purchase and release Chilocorus nigritus or Lindorus lophanthae beetles or Metaphycus helvolus, a parasitic wasp.

Spray a mixture of insecticidal soap and horticultural oil to control scale insects. The oil smothers the slow-moving insects.

Spray pyrethrin as a final resort. Pyrethrin is an organic pesticide made from a chrysanthemum. There are also chemical pesticides available.

To control ants:

Ants feed on 揾oneydew,?a sweet substance produced by insects that feed on plant sap, such as aphids and scale. Controlling these insects may help
control your ant problem.

Soapy water can be used to drench outside nests, killing some ants and forcing the others to relocate.

Drive ants out of flowerpots and outdoor nests by flooding them repeatedly.

To directly control outdoor ants, digging up the colony will destroy it. This is very labor intensive. You can also pour boiling or soapy water down the hole
of the nest.

Only if the nest is located in a dry area should boric acid be used. Sprinkle it around the openings. Always be cautious with boric acid and keep away from children and animals.

Sorry for the lengthy post, but usually when you control the scale insects, the ant population crashes.  Also, if your Lemon tree is located near a wall or other vertical surface, trim any branches that could provide a pathway for the ants.  

Good Luck and Good Gardening!