The new consumer phenomenon of smartphone app usage is expected to keep growing at a rapid clip, with downloads tripling over the next five years. But with that growth comes challenges for the mobile industry as competition heats up.

Downloads from all app stores will reach 6.67 billion applications by 2014, up from two billion this year, Vikrant Gandhi, analyst at market research firm Frost & Sullivan and author of "An Insight into the U.S. Smartphone Application Storefront Market," told InternetNews.com.

Right now, Apple's (NASDAQ: AAPL) App Store, which blazed the trail for the cottage industry of smartphone apps, claims more than one billion downloads with 65,000 programs available.

It was followed by Google's (NASDAQ: GOOG) Android Market, Research in Motion's (NASDAQ: RIMM) BlackBerry App World, Palm's Pre Catalog (NASDAQ: PALM) and Microsoft's (NASDAQ: MSFT) store, which is due later this year for Windows Mobile.

Verizon Wireless (NYSE: VZ), the nation's largest wireless carrier, is opening the VCast Store later this year, however it will differ from existing carrier stores by opening its network to developers through APIs.

Gandhi said the following factors are fueling the download app growth: free content subsidized by mobile advertising and mobile commerce; high-quality user experiences driven by next-generation devices and advanced networks; strong support from key industry players; the vast inventory of apps appealing to a broad and diversified segment of users; and operator acceptance.

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