If you have noticed your lemon tree getting a coating of black soot like stuff, your tree may have an infestation of citrus whitefly.
The citrus whitefly came to New Zealand from Australia in around 1995 and because it has few predators in NZ it has become common, particularly in the North Island. It is a small sap sucking insect which like other aphids and whitefly produces a sweet honeydew waste product from its rear. Sooty mould grows on the surfaces of leaves covered in the sticky honeydew.
The combination of whitefly sucking sap from the plant, spreading other disease and the loss of photosynthesis due to the covering of sooty mould can be very damaging to the health of the tree and the yield of fruit.
So if you see your citrus tree looking sooty it may not be enough to treat it with a fungicide alone.
Control of these whitefly is achieved with the use of spraying with mineral oil such as Kiwicare Organic Super Spraying Oil. The oil should be applied directly onto the insects as it works by suffocating them. It can be used in conjunction with copper spray such as Organic Copper Oxy to control the sooty mould.
A woman goes for a job picking citrus. The farmer is surprised at the woman's CV.
"You seem a bit over qualified" He says "I see you have a PhD! Have you ever picked lemons before?"
"Yes" She replies. "I've had three husbands and I bought a BMW last year."