Reducing the Incidence of Fungal Diseases
Grapes are subject to attack from a wide range of pests and diseases. Black rot, Botrytis, downy and powdery mildew are all caused by wet weather and if allowed to move from the leaf to the grape can cause considerable wine grape yield loss and quality deterioration.
The most effective control is through protective fungicide applications, opening up the grape crop canopy can reduce disease pressures. A balanced crop nutrition programme will also reduce the incidence of fungal diseases.
Crop Nutrition and Fungal Diseases
Potassium and Nitrogen
One key benefit from getting the potassium:nitrogen ratio right, is the reduction of fungal diseases in wine grape, such as Botrytis. When leaf nitrogen levels are high, plant growth increases and the concentration of plant exudates (e.g. sugar and amino acids) in the fruit rises. When combined with low Potassium-levels (N:K ratio over 2.5) this stimulates the spread of the disease on leaves and fruit. By balancing the nitrogen and potassium levels,(N:K ratio around 1.2) the effects of Botrytis have been seen to minimize.
The K:N ratio effect on disease is dependent upon the absolute level of both nutrients. So, for example there will be no benefit from an increased potassium concentration on the level of Botrytis where high, or excessive, N-levels also occur at harvest.
Calcium
Calcium is beneficial in aiding plant resistance to diseases such as Botrytis on the vine. It does so by strengthening the cell wall membrane, thereby reducing disease impact, and also by directly inhibiting the activity of a specific pathogen enzyme (polygalacturonase), which dissolves the middle lamella of cell walls. Calcium sprays applied pre-veraison to the fruit are most effective.
