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California’s Prop 37 could make or break some natural food brands’ reputations. This state proposition would require a food company to tell shoppers whether a food product contains GMOs.

Annie’s (NYSE: BNNY) and

Natural Grocers (NYSE: NGVC) support GMO labeling, and many of these companies’ customers gladly pay higher prices to avoid food that contains GMOs.

Kellogg’s (NYSE: K) and

General Mills (NYSE: GIS) oppose this proposition, which could put these large companies’ natural food brands at risk.

Annie’s goes even farther than just showing support for Prop 37. A visit to Annie’s

Facebook page shows that Annie’s supported and funded a campaign for a national GMO labeling law, which an article at Annie’s website explains. A federal GMO labeling law could provide significant advantages for Annie’s and Natural Grocers, while Kellogg’s and General Mills could lose customers, so many food companies will watch Prop 37 closely. If Prop 37 fails in California, a federal law isn’t very likely, but if it passes, federal regulators could use the California GMO law as a model.

Natural Grocers’ reputation matters even more because it just went public. Natural Grocers needs to show that it won’t sacrifice food quality to provide a better return to shareholders, and some comments on the company’s Facebook page already mention this concern. Natural Grocers’ support for Proposition 37 could reassure wavering customers. The company has made several posts about its support for Proposition 37 on its blog, including this recent post. Proposition 37 won’t affect Natural Grocers directly because it hasn’t expanded into California, but its support for the proposition could bolster its reputation in Mountain West states.

Proposition 37 puts Whole Foods (NASDAQ: WFM) in an awkward position. Whole Foods has its own GMO free store brand, 365 Everyday Value, and shoppers specifically seek out Whole Foods stores because they don’t trust the food quality at cheaper supermarkets. Although Prop 37 could improve sales for some Whole Foods product lines, the company risks irritating its suppliers by supporting the proposition, since California would force Whole Foods’ suppliers to label their food. Whole Foods hasn’t come out in favor of Proposition 37, but the grocer encouraged its suppliers to move away from GMOs by selling organic food or joining the Non-GMO Project. This decision disappointed shoppers who wanted Whole Foods to take a stronger stand, which could help the much smaller Natural Grocers attract some Whole Foods customers.