Call pumpkin spice the flavor of fall.

In some ways, that can be blamed on Starbucks (NASDAQ: SBUX), with its Pumpkin Spice Latte (PSL) starting the trend. Lots of companies have followed the coffee giant by bringing pumpkin spice flavor to coffee, including McDonald's (MCD 0.62%), which has its own line of hot and cold coffee-based beverages using the flavoring.

Now, however, the Golden Arches has fired the next shot in using this variant of a traditional Thanksgiving flavor. Instead of using pumpkin spice in new drinks or bringing it back to where it started as a dessert, the burger chain has brought Pumpkin Spice Fries to McDonald's Japan.

So far, the Pumpkin Spice Fries are only being sold in Japan. Image source: McDonald's Japan.

What is a pumpkin spice fry?

While pumpkins are a symbol of Halloween in the United States, pumpkin pie -- from which the PSL takes its flavoring -- is generally associated with Thanksgiving. In Japan, however, McDonald's has released Pumpkin Spice Fries for Halloween.

A Google Translate version of the McDonald's Japan page describes the new limited-time product as a take on its classic fries with "pumpkin and chocolate sauce in Halloween style!" 

The sweet and savory treat is called the "Halloween Choco Potato," according to The Daily Meal, which says McDonald's Japan released chocolate fries earlier this year. The Halloween version can be purchased in Japan starting Sept. 28 and the company reportedly has no plans to bring them to its home market as of now.

Why is McDonald's doing this?

Often, fast-food chains use foreign markets to test a product before releasing it in the U.S. In this case, McDonald's may be gauging reaction to the bizarre mashup in a market that has already responded well to chocolate fries in the form of what's called "McFry Potato Chocolate Sauce," a mix of white and milk chocolate sold in a packet that can be added to fries.

The concept of Pumpkin Spice Fries sounds outrageous, but it's really just a variant of the salty/sweet/savory mixtures like bacon as an ice cream topping that have become increasingly popular in recent years. In some ways, this new product also borrows from another Thanksgiving treat -- brown-sugar-glazed sweet potatoes topped with marshmallows.

That bizarre dish, which mixes savory and sweet, has appeared on holiday tables for decades. McDonald's may be tweaking the recipe to substitute its signature fries for sweet potatoes and chocolate for marshmallows, but it may be an idea whose time has come.