Monitoring (Apple rust mite)

Overwintering populations
The number of apple rust mite overwintering behind buds in the previous season’s extension growth should be determined in winter in orchards where a satisfactory and stable balance between the mite and the orchard predatory mite has not been established.

  • A sample of at least 20 one-year extension growths should be sampled and the number of rust mites behind three buds in each shoot, one bud towards the apex, one in the middle and one at the base of the shoot, should be counted and recorded.
  • This is most easily and comfortably done indoors using a microscope but the shoots need to be stored in a fridge until the counting is done.
  • Alternatively, it can be done in the field, bending the shoot at the point of the bud so the bark behind the bud can be examined with a hand lens.
  • The average number of rust mites overwintering per bud should be calculated.
  • If the mean number of mites exceeds 10 per bud, there is a significant risk of fruit damage.

At green cluster
Examine the undersides of the outer rosette leaves, especially round the bases using a hand lens and count the number of rust mites present.

  • Holding the leaf up to the light so more light shines through the leaf makes the mites more easily visible but care must be taken not to look directly at the sun as this could cause damage to eyes.
  • At least 25 leaves (preferably 50) should be examined per orchard.
  • An average of 5 or more mites per outer rosette leaf indicates a significant threat of damage to fruitlets.

During blossom and early fruit development (cell division)
Examine the rosette leaves as above and, importantly, also the receptacles of the flowers and the young developing fruitlets for the presence of mites.

  • The mites are often found near the point where the fruit meets the calyx and this is where examination should be concentrated.
  • If an average of one or more mites is found per flower or fruitlet, it is likely that some russeting damage will be caused.

In summer
More mature fruits are not susceptible to apple rust mite as their skin is less palatable. Mites occur on the undersides of leaves in extension shoots.

  • The undersides of leaves should be examined to ensure they remain fresh and green.
  • If any bronzing is present, leaves should be examined more closely for rust mites.
  • If large populations are present and significant bronzing is being caused, treatment may be justified.

Forecasting

Forecasting methods for apple rust mite have not been developed. However, population increase can be very rapid in hot weather and bronzing damage to the undersides of leaves can intensify rapidly.