Grape (Vitis spp.)-Inflorescence Necrosis (Bunch Stem Necrosis)

Cause A physiological problem that has not been explained fully. Frequent attempts to isolate pathogens from necrotic tissue have not resulted in any fungi or bacteria. The disorder is associated with cool and/or wet weather just before bloom. Excessive vigor, shade just before bloom, and general stresses have also been associated with symptoms. The disorder resulted in low yields 4 out of 10 years in the 1980s in Oregon. Although many cultivars have shown symptoms of the disorder, it is more readily observed on own rooted Pinot Noir than on Chardonnay. Most vines in Oregon are now on rootstocks resulting in improved fruit set and thus inflorescence necrosis is rarely seen.

The problem is thought to be different from other factors that affect fruit set such as cool, wet weather that leads to poor ovule fertilization. Poor fruit set also can be from boron or zinc deficiencies and from excessive nitrogen that leads to excess vigor. In some cases, certain rootstock - cultivar combinations can induce inflorescence necrosis due to differences in nutrient uptake and utilization. The most inflorescence necrosis occurred on vines severely pruned back during dormancy and then shoot thin in spring, which resulted in only a few shoots while those with more normal shoot numbers per plant had regular bloom and set.

Symptoms The problem is characterized by necrotic flower and pedicel tissue. In severe cases, the necrosis can extend into the rachis or peduncle tissue. Necrotic florets may retain their calypteras. The disorder is observed just before bloom through fruit set.

Cultural control Avoid excessive vegetative growth through rootstock management, site selection, and judicious use of nitrogen fertilizer.

Reference Gu, S., Lombard, P.B., and Price, S.F. 1996. Effect of shading and nitrogen source on growth, tissue ammonium and nitrate status, and inflorescence necrosis in Pinot noir grapevines. American journal of enology and viticulture 47:173-180.