By Igor Ilic

ZAGREB (Reuters) - Croatia's president named Andrej Plenkovic, head of the conservative HDZ party, as prime minister-designate on Monday, with the 46-year-old former diplomat to lead a coalition government that needs urgently to tackle a struggling economy.

HDZ won 61 out of 151 seats in a snap election on Sept. 11 and struck a coalition deal with a small center-right reformist party Most (Bridge) last week. The coalition also has support from ethnic minorities which won eight seats.

Plenkovic now has 30 days to win parliament's approval for his cabinet and must quickly work out a way to spur modest growth of about 2 percent in order to manage high public debt at about 85 percent of GDP and unemployment of 13 percent.

"I want to form a cabinet as soon as possible. The work of the future government will primarily be focused on the economy. We're now starting serious work," he said after being named, as was expected.

Plenkovic secured 91 signatures from deputies, with additional support not only from ethnic minorities but also from other smaller parties.

"Plenkovic has proved he has support of a majority of deputies," President Kolinda Grabar-Kitarovic said.

Highlighting the tough task facing the new government, Zagreb Economic Institute analyst, Maruska Vizek, said: "Making the public firms professional and efficient, reform of the bloated public administration, streamlining of public spending and improvement of the investment climate as well as work of judiciary will be the key tests for the government's success."

(Reporting by Igor Ilic; Editing by Louise Ireland)