What to do when the new leaves of a sasanqua fall off

I bought a pot of sasanqua from the south and have been growing it for 3 years. It grew well, but starting this spring, red dots appeared on the newly sprouted leaves, and soon the leaves fell off. I tried carbendazim several times with poor results. What is causing this? How should it be prevented and treated?

Answer: Based on what you described, together with the leaves you enclosed, it can be preliminarily inferred that the new leaves were damaged by mites and then infected with web blotch disease. This disease occurs more severely from April to June and from September to October. It mainly occurs on new leaves that have fully expanded, while old leaves are less affected. The lesions mostly occur on the leaf margins or leaf tips, but they can also occur on other parts of the leaf. Initially, needle-head-sized oily spots, light green in color, appear on the diseased leaves; later the lesions gradually expand and become dark brown. The diseased tissue thickens and sometimes curls upward, and net-like protrusions appear along the veins on the underside of the leaves. Control methods: pay attention to controlling mite damage. You can spray with 25% Baleba wettable powder at 1500 times dilution, or 20% Mieshaoli emulsifiable concentrate at 2000 times dilution to kill them. In the early stage of the disease, use 75% chlorothalonil wettable powder at 600 times dilution, or 70% thiophanate-methyl wettable powder at 1000 times dilution, or Bordeaux mixture at 0.5:1:160, alternating for spray control, once every half month, for 3 to 4 consecutive times.