Gnomoniopsis castaneae

Gnomoniopsis castaneae (synonym Gnomoniopsis smithogilvyi) is a fungus of the order Diaporthales that is the most important cause of brown chestnut rot, an emerging disease that damages the fruit of chestnuts. It also causes cankers and necrosis on leaves and on chestnut galls caused by the gall wasp, Dryocosmus kuriphilus. It has been observed to cause cankers in chestnut wood. Additionally, it can cause cankers on other chestnut species, red oak, hazelnut trees, less severe damage to some nut trees, and lives as an endophyte on other nut trees. The disease has been reported in Europe, Oceania, and has recently been found in North America; for this reason, the fungus is considered a potential threat to the reintroduction of the American chestnut.

In brown chestnut rot, Gnomoniopsis castaneae infects the kernel of the nut with browning and necrosis of endosperm and embryo. Brown chestnut rot is expressed cryptically with apparently healthy nuts found after harvest to exhibit internal rot. The fungus is believed to initially establish endophyte colonization of chestnut tissues, only becoming pathogenic with ripening of the nuts. Early on, parasitized nuts are difficult to distinguish from good nuts, rot only being detected when processed or eaten. The route of infection is uncertain but is believed to be either infection of chestnut flowers by ascospores or conidia or by inoculum entrance through shell defects.