Sick Lavender in South Africa

I live in South Africa, Johannesburg. It is currently spring and I have just bought myself different varieties of lavender for the garden.
One of the lavender plants, which I believe is English lavender is struggling to grow. It is quite young and small but there are a lot of yellowing leaves and in some parts the leaves have turned black. I think I saw some kind of mite on the plant, some look like whitish spots and others appeared to be green mites. I am a beginner gardener and therefore know very little about the types of diseases or pests that can affect lavender. Please advise. I have searched everywhere for information and the only thing I have come up with is that it would be good to attract ladybirds to the garden by planting fennel.

The symptoms you describe point more to a sick root system than to pests. Pests will attack a weakened plant and may certainly speed up the demise of your plants. Lavender does not like to dry out completely -the roots die when that happens, but it abhors soggy soil too. Also too much fertilizing is bad for it. Since you did not describe your treatment of the plants, it is hard to know what needs to change.
Do check our web-site at www.richters.com click on "News&Info" on the left-hand side, then on "Q&A", then type in "lavender" and lots of helpful questions and answers will come up.