Walmart and its unit Sam’s Club said on Monday leafy greens suppliers will be asked to implement real-time, farm-to-store tracking using blockchain technology by next September, as the retailer tackles food-safety incidents. Walmart is among several other retailers such as Nestle SA trying to tap blockchain, a shared record of data kept by a network of computers to track food supply chain and improve safety.Walmart said on Monday that the Centers for Disease Control and Prevention (CDC) has consulted with the company to improve traceability of food products to help public officials investigate and find the source of food-borne disease outbreaks.