Blackleaf was originally believed to be caused by potassium deficiency, but attempts at recreating symptoms have proven unsuccessful. Blackleaf is a leaf discoloration or “blackening” often observed on the upper leaf surface, while potassium deficiency is exhibited by blackening in leaf margins. Applications of soil and/or foliar potassium to blackleaf- affected grapevines do not completely alleviate symptoms. Recent research has indicated that blackleaf is due to a combination of water stress and high UV-B radiation that is often sustained in many growing areas of eastern Washington State.
Blackleaf is characterized by a general darkening of grapevine leaves in the outermost layer of the canopy. Leaves in the outer canopy layer often display a darkened appearance or “blackening” of the leaf surface. This disorder appears to affect exposed leaf surfaces, specifically in the epidermis. The color ranges from brown to purple or black, resulting in the “black” canopy character. Symptoms can range from none to severe (Figures 1–4), and can vary spatially and temporally in the vineyard (Table 1). It is important to note that once symptoms are visible, damage has already occurred earlier in the season.
Blackleaf symptoms often become visible around veraison (late July–August), although evidence from research suggests that the actual disorder affects grape leaves soon after fruit set. In affected leaves, chlorophyll is damaged and photosynthesis is reduced. In extreme cases, this leaf damage may adversely affect sugar accumulation toward harvest. Severe cases of blackleaf can result in vine defoliation.
Vines that suffer severe blackleaf for a number of successive years are likely to decline, losing vigor and productivity. In extreme cases, vines will die.
Early water deficit, often referred to as Regulated Deficit Irrigation (RDI), has been used in some vineyards to control shoot growth. However, this early season water deficit has been linked to higher incidences of blackleaf development.