While Petrarch is primarily known for his poetry, particularly his sonnets, he also made significant contributions to Italian prose. His works, including his letters and biographies, helped to establish a clear and elegant style of Italian prose, which greatly influenced later writers.
However, it's important to note that the title of "father" is a bit of a simplification. Other writers of the 13th and 14th centuries, like Dante and Boccaccio, also played crucial roles in developing Italian prose. But Petrarch's influence on later generations of Italian writers is undeniable, making him a strong candidate for the title.