Chrysanthemums are the central symbol of life, vital energies, sexuality in the story. Just like Elisa herself, they need nurturing and attendance. She lives a boring and unfulfilled life in a dull, loveless marriage, and she has never had a chance of finding her true self, except in the garden. That is why she commits to gardening so eagerly: because she couldn't bear to see those flowers withered. At the end of the story, she sees her chrysanthemums on the road, thrown away, "a dark speck" as Steinbeck says. They are wasted, just like Elisa's life, love, and happiness.
It's a term for lung disease caused by silica dust.
