HUNTER VALLEY, Australia — The hills are lush and green, the grapes plump and ripe. But one bite of this famed valley’s most prized product reveals a winemaker’s worst nightmare.

“It’s like licking an ashtray,” said Iain Riggs, a vintner here. “It’s really rank and bitter.”

The bush fires that raged for eight months in southeastern Australia inflicted widespread damage on the vineyards of the Hunter Valley, not directly from flames, but through the invisible taint of smoke.

Winemakers like Mr. Riggs have abandoned hopes for some 2020 vintages. Grapes that were closest to the fires are being left on the vine. Those farther away are being tested for smoke contamination, though it is an inexact science, and in some cases producers won’t know whether a wine can be sold until it has fermented in tanks.