As podcasts attract more advertisers, podcast publishers and ad sellers are beginning to embrace tactics that are common on the internet, including the automated buying and selling of targeted ads.

Midroll Media, a podcast ad sales group that offers 200 to 250 shows daily, plans to expand programmatic sales in 2020 after a small number of tests this year.

“With the range of shows we work with and the advertisers we talk to, it makes sense to start bringing in a whole range of ad products that meet brand needs across the board—and take more of their ad dollars, frankly,” said Erik Diehn, the chief executive of Midroll parent Stitcher Inc., a podcast platform owned by broadcasting company E.W. Scripps Co.

Podcasts are shifting from a “nice to have” element of marketers’ brand campaigns to becoming integral elements, said Rachel Lowenstein, associate director of Invention+ at WPP PLC -owned agency Mindshare. That adds to the pressure to implement ad technology.

“If there is any way we can make things more efficient and targeted, it’s going to happen,” Ms. Lowenstein said.

But not everyone wants to follow the web-advertising playbook.

The vast majority of podcast ads today are sold directly, not auctioned through programmatic technology vendors. Most also are read by hosts or producers and baked into episodes during the recording, though they are sometimes edited in afterward.

Barstool Sports Inc., whose podcasts include “Pardon My Take” and “Spittin’ Chiclets,” refuses any podcast ad formats except live reads. It is on track to generate more than $22 million in podcast ad revenue this year, all from ads read by the hosts, according to a person familiar with the matter. The company has 31 different podcast series with clients that include PepsiCo Inc.’s Mountain Dew brand, Nascar Holdings Inc. and New Amsterdam Spirits Co.

“Podcasting is a great medium because there’s essentially a relationship between a host and a listener—they are in your ears, they are telling you stories and it creates an ongoing relationship,” said Barstool Sports Chief Executive Erika Nardini. “That’s also what makes advertising so effective.”

The programmatic advertising now seeping into podcasting could erode that intimacy, Ms. Nardini said. And pooling ad inventory for auction could sap prices, she added.

Others worry ad tech will encourage publishers to pack podcasts with more commercials, because it’s easier with automation than doing more live reads.

“If there are technologies that make it easy to have more ads in podcasts, it’s easy to believe that there will be more ads in podcasts,” said Mark McCrery, chief executive of podcast ad sales firm Authentic, which represents series such as “This American Life” and “Serial.”

Skyrocketing sales

Programmatic sales remain a sliver of the podcast ad business. They grew to 1.3% of U.S. sales last year from 0.7% in 2017, according to research by the Interactive Advertising Bureau and PricewaterhouseCoopers LLP.

The most common way to buy podcast ads is directly and on a quarterly basis, the mode by which 38.2% of podcast ads were bought in 2018, according to IAB research.

But podcasting is changing amid rapid growth. Apple Inc.’s podcast app offers more than 700,000 shows, the company said in June, up from 550,000 a year earlier. U.S. podcast advertising revenue is expected to reach $678.7 million in 2019, up from $479.1 million last year, according to the IAB.

Ad tech can help lure bigger advertisers that want to reach large audiences without having to strike many separate deals, said Stitcher’s Mr. Diehn.

Some publishers hope programmatic technology can help them make money off older podcast shows, especially those that cover evergreen topics, by substituting fresh ads in for new listeners.

Even without programmatic auctions, publishers can use a technique called dynamic ad insertion to serve different ads based on the listener.

“As dynamic ad insertion is more widely adopted, publishers can better monetize their back catalog and advertisers have more flexibility over their spend,” said Marty Moe, president of Vox Media Inc.’s Vox Media Studios.

Vox, which says its overall podcast revenue is in the eight figures range, already uses dynamic ad insertion and is starting to implement programmatic ad sales.

Commoditizing podcasts

The trick is figuring out how to apply ad tech to podcasting without breaking what’s working, industry executives said.

Host reads on popular podcasts can garner anywhere from $25 to $50 per thousand impressions, according to Authentic’s Mr. McCrery. That’s higher than radio ads, which can range from $8 to $15 per thousand impressions, he said. Talk-radio host reads cost on the high end of radio’s range but don’t match podcast rates, Mr. McCrery said.

Slate Group LLC, another big podcast publisher, has an eight-figure podcasting business that will account for about half of its total revenue in 2019, according to the company. The vast majority of Slate’s podcasting revenue comes from advertising—though the company does make money by producing podcasts for its paid membership program—and its advertising revenue comes almost exclusively from host reads, said Charlie Kammerer, president of Slate.

But the company also can use dynamic ad insertion for host-read ads that it records separately and places according to advertisers’ criteria. A packaged-goods brand advertising on Slate podcasts such as “Slow Burn” asked to exclude listeners in cities in Rhode Island and Ohio, for example, as part of a product test.

“Our key strategy is to create a special environment and listener experience for our audience, and part and parcel of that is the host read, which will always remain our primary strategy,” Mr. Kammerer said.

Write to Sahil Patel at Sahil.Patel@wsj.com