Bougainville yellow leaves with dark green spots

[BougainvilleIT]

Good morning everyone, a few days ago my 5 bougainvillea spectoglabra show several leaves turning yellow with round dark green spots. Within a few days the affected leaves fall off. These are adult plants about 3 meters high in 60cm pots, in Milan, Italy. I'm asking for help to understand what this is. Thank you.

 

[Sluggy]

Usual suspects to blame. Incorrect watering, needs fed, heat stress etc. I am sure you will receive more practical solutions. They are desert type plants so overwatering and poor drainage is a problem let the top few inches dry out before watering and allow to drip dry and they do not like the cold

 

[BougainvilleIT]

Thanks for the answer. Normally I keep them with fairly dry soil and they've always reacted well, even with strong heat. At a specialized store, they recommended treating with difenoconazole, which I did, but I don't notice any improvement.