Hello, Haxuan. Absent any other symptoms, it sounds like your plant has azalea leaf spot, a fungal infection in humid settings with wet weather.

Some cultural controls that can help you are actually just good sanitation techniques... things like: Improving the air flow around the plants; Watering the soil instead of the leaves; Disposing of plant debris under and around the plant; Replacing the existing mulch with new mulch; Increasing the amount of sunlight. Drier weather will eventually/naturally stop the progress of this infection. However, the spores are present on leaf surfaces by now so the problem will reoccur next year.

The ASA recommends using leaf spot fungicides like Maneb or Ferbam beginning at the end of the bloom period; then continue at 2-week intervals through growing season or as long as young leaves are present.

Notes: 1) Bayleton fungicide is also recommended but read the label for special instructions 2) follow the instructions on the labels and use the recommended dosages to prevent developing fungal strains that resist these fungicides 3) wear protective goggles/gloves/clothing whenever you use chemicals.

Good luck haxuan,
Luis

haxuan, I thought I would add a note to the previous response. The above named fungicides were designed to work against fungi common over here. These fungii may not be the same ones that are causing the problem in your azaleas. So, it is difficult for me to know if they will work well for you. Try those but be aware that you may have to get local fungicies designed to work against the fungi common in your country.

If the fungicides that the ASA recommends are available to you and they work for you, that will be great. If they are not available or they do not work well, feel free to try any local brand that says it works against azalea leaf spot.

Have a great day,
Luis

Hi!
Approximately a year ago I planted a hedge of Azaelias. In our subtropical climate, I selected a part-shade location. They have doubled in size since planting. But in recent months (Autumn & Winter) I have noted they are looking a little sick.
I have taken a photo of the leaves and would like to attached a photo of the leaves (not sure how - this is my first forum). Currently here in Australia most Azaleias are in flower. I have planted them in sandy loam soil, heaps of pine mulch to keep away weeds and have fertilised with both organic and phosphate fertilizer. There apperars to be a testure to the bottom of the leaf (possibly small brown sap spots). The top of the leaf is much yellower that it should be.
What treatment should I spray on these plants?

Thank you very much Luis.
From the list of causes you gave, I found "humid settings with wet weather" could be the possible one that is affecting my azaleas since we're now in the middle of rainy season in Vietnam. If that's correct, perhaps all I should do is wait for the weather to get drier.
There's another thing to azaleas in our region. I've noticed (through photos only) that your azaleas bloom once a year (?) and they give a lot of blooms all at once so that you see a curtain of blooms. Here, my azaleas bloom all the year round and each plant gives out a few blooms at a time. What causes this? the climate? the culture? the pruning technique? the fertilizers? the soil?. (Please note that my azaleas are at our second home, up the mountain where the temperature is from 60-80 all the year round, with six months of rain and six months of sunshine.)
I'd appreciate your comments.
Have a nice weekend gardening.

Xuan

Xuan, 'Here, my azaleas bloom all the year round and each plant gives out a few blooms at a time. What causes this? the climate? the culture? the pruning technique? the fertilizers? the soil?. '

We have different type/cultivars of azaleas that are mostly once blooming because they are more suitable to our drastic differences between summer/winter seasons.
Bloom time and longevity of it depends on the parentage and climatic conditions/zone where particular plant grows.
For example, down south from where I'm people have no problem to grow Satsuki hybrids and they have them blooming at once for about 2 weeks. In my zone I have to plant them in a places that are very protected not only from the winter cold and winds, but especially from the winter sun. Thus they are in places that are fairly shaded by other evergreens all year around. Such setting causes them to bloom very late by evergreen azaleas standards (mid-late June), but blooms opens in succession and I have a very long lasting (almost a month) display of individual flowers, not a mass effect.
Good luck.

hellen, is azalea lace bug common in your area? Your symptoms sound like sucking insect damage and lace bug would appear as you describe - check to see what controls are available in your location.