POTASSIUM
Potassium is important for fruit colour, winter hardiness, tree growth and disease resistance.
Identification
- Mid-shoot leaves lighten in colour, then turn purple and eventually necrotic spots appear along the margins of young blades
- Leaf margins dry up and roll up or down, and blades become distorted and ruffled
- Later in the season, interveinal areas of older leaves at the base of shoots turn violet brown to dark brown, especially near the clusters. This is called “black leaf”
- Severe potassium deficiency greatly reduces vine vigour, berry size, and crop yield
Often Confused With
Ozone injury – occurs after thermal inversions during the summer; only exposed basal leaves affected
European red mite – adults, nymphs and eggs visible
Late spring frost – more severe in low spots
Natural senescence of basal leaves in late summer or early fall
Scouting Notes
Symptoms become visible in mid- to late summer on mid-shoot then basal leaves. Black leaf is especially pronounced on heavily cropped vines because after veraison, maturing berries become a potassium sink.
Management Notes
Potassium deficiency symptoms occur when uptake from the soil is compromised due to waterlogging or extremely dry soil conditions or soil compaction. It can be easily leached from sandy soils. High levels of calcium, magnesium, sodium, nitrogen or phosphorus can affect uptake, especially if potassium deficient in the soil. Foliar sprays may be used but deficiency should be confirmed with tissue analyses before use.