Mildew management (Apple powdery mildew)

Using estimates of secondary mildew to make decisions on disease control

Mildew management based on assessments of secondary mildew was developed by Butt and Barlow.  In order to make mildew management work it is important to understand apple mildew characteristics that make it a successful pathogen:

  1. Mildew overwinters in the buds so it ensures a source of primary inoculum present in the orchard.
  2. Mildew spores germinate in the absence of liquid water and so can infect almost daily during the growing season.
  3. Apple shoots have a long growing season so the tree stays susceptible for several months.
  4. It is difficult to maintain adequate doses of fungicide to protect rapidly growing leaves, so fungicide cover must be good to maintain protection.

Mildew inoculum level is the key factor in determining the seasonal epidemic.  Therefore, control strategies depend on maintaining primary mildew at a low level.

  • June is a critical time for monitoring and for mildew control as this is the period of rapid extension growth and also when fruit buds are forming and sealing for next spring.
  • A high mildew incidence at this time will result in high primary blossom mildew next season.
  • Likewise, at the end of growth in late summer, when terminal buds are sealing, mildew control is important to ensure that mildew carryover in buds is as low as possible.
  • In some seasons, particularly when the summers are dry, terminal buds restart growth after harvest.  It is important to monitor this, as failure to control mildew at this time can lead to high overwintering mildew.

The objective of mildew management is to adopt a flexible approach in which fungicide dose, spray interval and spray volume are adjusted to match the level of epidemic activity.  The seasonal activity is measured by regular assessments of secondary mildew levels.

The management tools to control mildew are:

  • Choice of fungicide (eradicant or protectant)
  • Fungicide dose (25-100% of label dose)
  • Spray interval
  • Spray volume

The decisions are based on the following:

  • Mildew incidence
  • Growth stage
  • Current weather

Management of mildew in this way ensures that disease control is maintained and fungicide use rationalised.

Guidelines for decisions on fungicide use based on secondary mildew assessments

Disease rating Mildewed leaves (%) Mildewed shoots (%) Action after petal fall
Low <2-3 <10 In cool weather with rainy spells or when shoot growth is slow, take the opportunity to reduce fungicide use by reducing dose (minimum dose = 25%) or extending spray interval.
Moderate 3-9 10-30 Maintain control. Consider improving the programme by reducing the spray interval or increasing the fungicide dose (not exceeding the label maximum) especially if the weather is warm and humid and shoot growth is rapid.
Potentially high >9 >30 Improve control immediately especially if shoots are growing, irrespective of weather. Shorten spray interval, increase fungicide dose (not exceeding label maximum), possibly increase spray volume. Consider changing fungicide. Check sprayer is working correctly.