M. laxa infects the aerial parts of the host plants to give a variety of symptoms, including blighting of blossoms, buds, leaves and twigs; cankers on woody tissues; and rotting of fruits. It is considered to be a pathogen of blossoms and twigs more than of fruits. It is the main fungal disease of stored peaches and nectarines in Europe.
Blossom and Twig Blight
This is the primary feature of brown rot caused by M. laxa in spring. The first symptom of blossom blight is necrosis of the anthers, which proceeds to the floral tube, ovary and peduncle. Infections may extend into and then girdle the twig. As infected flowers wilt and turn brown, they often become affixed to the twig in a gummy mass. In wet weather, infected flowers become covered with greyish to tan conidiomata. On apricot, plum and prune, all blossom parts are susceptible. Apricot is the most susceptible to blossom blight, followed in order by prune, sweet cherry, peach, sour cherry and plum. Symptoms appear first on sepals, anthers and stigmata as the blossoms open. Blight symptoms are expressed when the fungus girdles the peduncle.