My H. rosa-sinensis is having a major problem. Its leaves are turning yellow overnight at an alarming rate and dropping like flies. It has been healthy otherwise, aside from the occasional aphid attack, which I take care of with a soapy spray. It is still blooming like crazy, yet I just cleaned up at least 3 dozen leaves from the ground. I don't think it is chlorosis, since the whole leaf goes yellow, including the veins. Some leaves drop while still somewhat green as well. I have fertilized it and water it well. One thing I did do recently was add sulphur to the soil to acidify it a little, as we tend to have soil with a high pH. But the leaves were yellowing somewhat even before this. Thanks in advance!