Lotto Soudal rider Ewan surged through to beat Italian Giacomo Nizzolo on the line alongside the Adriatic resort of Cattolica with Elia Viviani in third place.
Italian Alessandro De Marchi held on to the leader’s pink jersey, having taken it on Tuesday.
But the day was overshadowed by late crashes involving Pavel Sivakov (Ineos Grenadiers) and Landa (Bahrain Victorious).
Russian Sivakov, Ineos’s co-leader with Egan Bernal, fell heavily after clipping an over-hanging tree but finished the stage on a spare bike with a cut arm.
Much worse was to follow as Spaniard Landa went down in a crash also involving stage four winner Joe Dombrowski after a rider appeared to hit a race marshal signaling some road furniture some 4 kilometers (2.5 miles) from the finish.
Landa was clearly in pain and was tended to on the road, later leaving in the back of an ambulance.
Talking to broadcaster Eurosport, Bahrain Victorious sports director Franco Pellizotti said: “Unfortunately our leader crashed in those difficult final kilometers. We don’t know what happened or how it happened. Landa was in great condition.”
His team later confirmed he had suffered a broken collarbone and injuries to his ribs.
Frenchman Francois Bidard (AG2R Citroen) also left the race after fracturing his left clavicle in the same crash.
De Marchi (Israel Startup Nation) extended his lead to 42 seconds over Louis Vervaeke (Alpecin-Fenix) with Movistar’s Nelson Oliveira in third place, 48 seconds adrift.
Ewan once again proved his quality in bunch sprints as he claimed a fourth career stage win on the Giro.
“The first sprint stage didn’t go good at all, so, you know, there was a lot of pressure on me and the team to do a good job today,” he said. “The goal to start with was one (stage win), so I’ve done that. I’m pretty hungry to win as much as I can, so maybe this is good confidence for the team and myself.”
The pancake-flat 175 kilometer (108 mile) ride from Modena was drama-free until the final few kilometers as the peloton reeled in a three-man breakaway of Alexis Gougeard, Simon Pellaud, and Davide Gabburo.
“The day started in the best way, quite relaxed, quite easy, and then I think it became a really crazy circus,” race leader De Marchi said.
Thursday’s stage is a 160 kilometer (99 mile) route to Ascoli Piceno including three categorized climbs.
By Martyn Herman