Carving pumpkins is a Halloween tradition. Mike Valladao has turned it into a joyful form of art. Known as the “Picasso of Pumpkin Carvers,” Valladao has been chiseling whimsical faces and creatures into big pumpkins for more than four decades. This year, “Farmer Mike” celebrates 40 years of Halloween artistry at the Half Moon Bay Art & Pumpkin Festival, which is best known for its giant pumpkin weigh-off. Valladao doesn’t have any formal artistic training, but his art has taken him to different parts of the country, including an appearance on “The Tonight Show.” He has gotten requests to carve pumpkins at venues such as the San Diego Zoo, the California Academy of Sciences and the State Fair of Texas.