Disease monitoring and forecasting (Apple powedery mildew)

Mildew is always present in the orchard and routine control measures are usually needed from around green cluster until vegetative growth ceases.

Monitoring mildew is essential to rationalise fungicide use and/or to check that control measures are effective, even in a routine fungicide programme, as it is very easy for the mildew incidence to increase unnoticed.  Once primary mildew levels are high, achieving successful control is much more difficult.

Assessment of primary mildew

  • At pink bud stage the level of primary blossom mildew can be estimated by counting the number of infected and healthy blossoms on lengths of branches on at least 10 trees taken at random in the orchard.
  • The number of trees assessed is dependent on orchard size and time available.
  • Usually on each tree 10 blossoms are examined on each of four branches.
  • Record the percentage of mildewed blossom trusses and categorise:
Categories of primary mildew blossoms or shoots
Low ≤ 0.5%
Moderate  0.5 - 2.0%
High >2.0%

At petal fall the incidence of primary vegetative mildew is assessed on emerging terminal buds.

  • First approximately count the number of new extension shoots on each of a few representative trees in the orchard and calculate the mean number of extension shoots per tree.
  • Then on at least 10 trees selected at random (the number of trees depends on the orchard area and the time available) in each orchard, record the number of extension shoots with primary mildew per tree.
  • Using the estimate of total number of extension shoots per tree, calculate the percentage of primary mildewed shoots and categorise as above.
  • Primary mildew levels below 2% mildewed shoots or blossoms are considered satisfactory, but preferably levels should be less than 1.0%

Assessments of primary mildew provide information on the inoculum level for the coming season and also provide information on the success of the previous season’s programme.

  • Primary blossom mildew will give an indication of mildew incidence in the early part of the previous season and primary vegetative mildew a measure of mildew control at the end of the previous season.
  • This may give indications of the need to modify the programme for the coming season.

Assessment of secondary mildew

Assessment of secondary mildew gives a measure of mildew activity in the current season and is an essential part of checking on the efficacy of control measures or as a basis for decision making on chemical control.

  • For more reliable assessments of secondary mildew, repeat assessments should be made on the same trees in each orchard.
  • However, in practice this is often too time consuming and as disease assessments are usually combined with pest assessments, there may be a need to examine different parts of the orchard at each visit.
  • The number of trees sampled again is dependent on orchard size and time available (see below).
  • In mixed orchards, sample the most susceptible varieties or a mixture of varieties.

Number of trees to sample for assessing secondary mildew in an orchard block

Area of block not more than: Number of trees to sample
Acres Hectares
1.5 0.50 10
2.5 1.00 15
4.0 1.5 20
6.0 2.5 25
7.5 3.00 30
10.0 4.00 35
  • Assessments should be done regularly during the growing season at 7-14 day intervals or as often as a spray decision needs to be made.
  • On each assessment date, select at random four actively growing extension shoots per tree, avoiding ones with primary mildew or pest problems.
  • Select the youngest unrolled leaf as the reference leaf.  Examine the five leaves immediately below the reference leaf for colonies of mildew, which are often not easy to see.
  • On each assessment date record the number of either mildewed leaves or mildewed shoots and express as a percentage of leaves or shoots examined.  These two measures of secondary mildew can be converted (see below).

Equivalence of percentages of leaves and shoots with secondary mildew colonies

Mildewed shoots Mildewed leaves Mildewed shoots Mildewed leaves
5 1-2 55 18
10 2-3 60 20
15 4 65 23-24
20 5-6 70 26
25 7 75 29-30
30 8-9 80 33-34
35 10 85 38
40 12 90 44
45 14 95 53
50 16 100 80

Levels of secondary mildew are categorised as follows:

Categories of secondary mildew shoots
Low ≤ 10%
Moderate 10 – 30%
High >30%