Bunch Stem Necrosis, also known as BSN, is a physiological disorder that causes rachis and connected berries to abort. Cabernet Sauvignon and Cabernet Franc are more susceptible than other varieties to late-season BSN. The exact cause of BSN has not been identified, but many studies have demonstrated that pathogens are not involved in it.
(Photo credit: Mizuho Nita)
Black Rot is caused by a fungal pathogen, Guignardia bidwellii (syn. Phyllosticta ampelicida), that causes infection on leaves, berries, as well as the shoots and rachis of grapevines. The leaf symptom of black rot is characterized by brown lesions with dark edges, containing dark fungal fruiting bodies. On shoots and rachis, it causes dark necrotic lesions that contain the fungal fruiting bodies. On fruit, the fungal fruiting bodies develop in a concentric “bull’s eye” pattern, and the infected berries eventually mummify. In some cases, black rot is mistaken for Phomopsis and vice versa, where infection on the rachis causes berries to drop.
(Photo credit: Mizuho NIta)
Mechanical breakage – damage to the rachis may cause portions of the cluster to shrivel and mummify.
(Photo credit: Mizuho Nita)
Insect damages
Grape Tumid Gallmaker – The grape tumid gallmaker (Vitisiella (formerly Janetiella brevicauda (Felt)) can cause swelling of clusters early in the season.
(Photo credit: Michigan State University Extension)
Grape Cane Girdler – the grape cane girdler (Ampelogylpter ater) is an insect that punctures the green shoots, causing the stems to become prone to breakage. Tissue above the girdle will eventually die.
(Photo credit: University of Kentucky)