For several years now the zucchini in my amateur garden have always failed in the same way. The fruits start to grow but stay in an arrested development state (their size never goes beyond 12cm, the first photo below represents one of my largest ones) and their skin are punctured with small, superficial holes. The zucchinis also suffer from powdery mildew but that's harmless, right ? The garden is located in South France, in a place with plenty of sunshine and heat. Can anyone diagnose this or suggest anything ?
Update : since some answerers asked, to fertilize I used some horse manure that I buried before putting the plants (note that this is an organic farming question as indicated by the tag below).