LONDON — It turns out, the hay bales caused it.

Sitting in a cafe a stone’s throw from the Royal Opera House in London, the opera singer Pretty Yende of South Africa whipped out her smartphone to show an Instagram photo of herself in a surgical mask during rehearsals for her London debut in the opera “L’Elisir d’Amore” in May. She had come to London two weeks earlier for rehearsals for her role as Adina, and for the first two days she could not stop sneezing.

“I thought, ‘It is London, it is worse than Milan, and it is this time of year and you expect it,’ ” she said referring to pollen allergies and adding that when she is not traveling she resides in Milan. “But the moment I entered the rehearsal room the worse it became,” and in her apartment that night she was convinced she was getting a cold.

But the next day she realized that the small rehearsal room was filled with raffia — palms that are found in tropical parts of Africa — made to look like hay bales for props for the opera, and it (or the dust on it) got her allergies churning, and she had to take a week off. She asked for a mask to use during rehearsals, and found that it actually helped her become more conscious of her sound. Once the raffia was moved onto the large stage, her allergies calmed down.

“It was interesting technically because I found and learned things I was not aware I was doing,” she said. “The sound that comes out, I do not need to open my mouth so much.”