It's called sooty mold and is common on citrus trees. It's unsightly but quite harmless. The mold grows on the sweet excretions, honey dew, of sap sucking insects - aphids, scale and so on. Get rid of the sap suckers and the mold will fade away due to lack of food. Try hosing the sap suckers off. If that doesn't work you could consider garlic spray or glue spray. An insecticidal soap spray like natrasoap or horticultural oil I would use as a last resort.

hi ray!

thanks for the information.

I'm searching for garlic spray now!

Wishful, both garlic and glue spray are easy to make at home. If you're interested, email me and I'll send you some recipes.

g'day wishful,

scale is very common on citrus but also thrips and mealy bug can cause this side effect, the sooty mould won't affect fruit quality just looks unsightly to us gardeners, the scale insect could cause damage to young growth as they suck the sap.

anyhow the insects are most commonly there due to ants and their symbiotic realionship, so for me i always use a white oil for the bugs there is a homemade recipe on my page, and then you need to move the ants on.

look to growing companion plants around the base of the citrus tree to deter the ants, garlic or garlic chives is a good one.

len

Here is a link that might be useful: lens garden page

Does hosing aphids off with water ever work? I swear that the little blighters just crawl back up again. You'd have to squash them, I think ...

Yes, hosing is one of the many options, but it's not a permanent solution. Nothing really is. Belting them with something like pyrethrum will kill those in situ, but won't stop more turning up later.

I like squashing them!

When you squash them you know those particular aphids won't be coming back. Of course, 500 others might fly in from neighbouring gardens but, so far as I know, the squashed ones stay dead. Unless there's an army of zombie aphids on the rise ...

aphida can be washed off with a strong hose and doing this daily for a few days can give some apparent control but again as i know it they don't fly in ants again bring them in and harvest them for their secretions, so when you wash one lot off the ants have others stored on the roots of the plants and just simply bring them back out.

used to be a good barometere to coming heavy rain/storms especially for rose owners as aphids seem easier to see on them when you see the ants taking the aphids below then loof for the rain.

aphids are just way to easy to control and for me aren't as damaging as other pests so why waste chemicals? control the ants. there are also lady bugs that feed on aphids.

len

Here is a link that might be useful: lens garden page

Not sure if this could work but, buy an insect net and place it over your lime tree then get 10 lady bugs (some horticulturist stores so them), they will eat all the aphids, remove the net and they will stick around as long as there are other little insects to dinner on, then they will fly away.