With the wet days did you have frosty nights recently? My best guess would be that this is more likely to be frost damage than anything connected with the feeding. The young leaves of the Pieris and new early flowers on the Azalea are tender and susceptible to frost damage, particularly if the Pieris was in a covered area where you bought it. You will lose these new leaves, but the plant should survive and perhaps give you a later flush of new growth.
If you recognise that this might have been the problem, then I would suggest you get the Pieris planted, give it a mulch and protect it when frost is forecast. This will give it a better chance than being in its pot.

Certainly frost damage on new growth. Cut off the withered growth down to the live greenery and the shrub will re-shoot.

I agree..it's frost damage..it will be ok ,they are quite tough,and it will regrow..

That's four for the frost damage. It happens.

Thank you very much for quick responses, everyone! Before this problem happened I went to Cotswolds to see beautiful gardens for few days and came home to find out they are suffered. It had been very cold weekend there so I guess here is Surrey must have been cold too and mulch was too thin to protect them.

I will put a pieris back to flower bed and cut back blown parts and also took dead flower head and buds from Azalea then give them enough mulch. I didn't know how critical the frost damage to new growth could be and didn't realise it till I saw these advises. I will be more careful about it from now on.

Thanks again for great helps!!!

Pieris Flaming Silver is particularly susceptible to frost damage - should really be planted somewhere very sheltered and part shady. The azalea appears to have suffered frost damage only on its flowers and buds rather than the leaves, that's just luck of the draw with the weather. Both these plants do not like to dry out though, so keep watered during dry spells.

The mulch won't prevent this kind of top growth damage. It happens because the plants are not protected from the frost in the air. I would take the advice given above and get them back in the ground. If you expect more frost put fleece over them to give them a warm canopy in which to escape the frost. If possible check on the preferred site for new plants. Bamboo knows this is susceptible to frost damage maybe you could move it to where it will be less vulnerable.

Thank you very much for more information!

I have transplanted a pieris and gave them a mulch today.

Azalea seems to be there for years and grow lots. As a beginner it looks bit difficult for me to transplanted it without damage to its root. I will wait till I have more experience and use fleece to protect it for a while. I will keep watered and buy fleece for them. There are some ok buds so I want keep them nicely.

I asked just one question and lots of people gave me helpful information and I learned lots! I do very much appreciate all your helps:)

We will look forward to seeing you blossom JujJun. Nobody knows everything and the more you learn the more you realise you don't know. An elderly friend used to say "I'll know when I'm dead because I will have stopped learning"

If your azalea has been where it is for a long time (more than 2 years) I'd leave it alone. Most years, this won't have happened to the flowers - it's just the strange weather we've had this year, being very warm indeed in March, and then winter temperatures suddenly returning just as everything's flowering. That contrast is enough to shock any plant's flowers into submission...

Yes I totally agree with you. Since I started to do gardening I find something new everyday and I realized there are too many things I don't know. I check website and gardening magazines everyday to look for answers but sometimes I just can't find right one because of lack of knowledge and experiences. I enjoy learning new things and hope I will be like everyone here who can give advice to others someday in the future!

My Azalea must have been here more than 3 years so I will leave it as it is. It was very warm, sometimes even I felt hot in March. I am glad to be able to understand why plants had problem like that. Thank you!

:)