The nation's small-business owners are increasingly worried about the economy, as conditions in most business sectors continued to decline this month, according to Discover.

Out of 1,000 owners surveyed nationwide, only 35 percent said they felt economic conditions for their businesses were improving, down from 40 percent in September. An even fewer number rated the economy as "excellent" or "good."

Overall, Discover's monthly small-business confidence index dropped to 96.8, down from 99.2 in September and the lowest level since the index was launched 15 months ago.

"Optimism among business owners is definitely clouded, and they are exercising increased caution toward business development spending," Sastry Rachakonda, the director of Discover's business card, said in a statement.