What is turning into a record heat wave has prompted the Manteca Chamber of Commerce to cancel the weekly farmers market at Library Park that would have started today at 4:30 p.m.Chamber Executive Director Joann Beattie said a number of farmers had indicated they would not attend as the excessive heat would impact the quality of fruits and vegetables being sold. Beattie said the chamber is also concerned about the general health of those attending. The high is expected to reach 108 degrees by 4 o’clock today.