Two glassy-winged sharpshooter nymphs were found last week on grapevines for sale at Costco’s Novato location, putting Marin County on high alert about a threat to the wine and agriculture industries. The sharpshooter is a vector for Pierce’s disease, a bacterial infection that blocks water movement within the plant, steadily killing it. The disease is incurable, and, if left unchecked, it can wipe out an entire vineyard in a couple of years. “It’s the highest…