Disease monitoring (Apple scab)

Visual assessment

Scab monitoring and forecasting is an essential part of integrated disease management to rationalise fungicide use.

  • However, it is not possible in the early part of the season to base decisions on fungicide use on assessment of visible scab because of the long time (up to three weeks at low temperatures) between infection and visible scab.
  • Decisions on fungicide use are usually based on weather-related risks.
  • Searches for and assessment of scab is still important, to check on inoculum levels at the start of the season and during the season; and to gauge the success of control measures such that any modification to spray decisions can be timely.
  • Information on inoculum levels, classified as low, moderate, high is also an essential input for using ADEM.
  • Visual monitoring of scab to meet the above criteria therefore should take place throughout the season.

Visual scab monitoring (20-50 trees/orchard)

Time/growth stage (sampling unit) Scab item Threshold Action
Dormant period whole tree Wood scab Presence Remove during pruning
Whole orchard Overwintering leaf litter Easily found Macertate to encourage breakdown prior to bud burst
Green cluster - pink bud whole tree Scab on rossette leaves % infected trees Presence ≤5 = low  Intensify programme
Early blossom – petal fall

whole tree

 

Mid May

10 rosettes on 4 branches per tree

scab on rosette leaves, flowers, fruitlets

% infected trees

 

scab on leaves, flowers, fruitlets

% infected rosettes

 }5-20 = moderate

} >20 = high

presence )

 

 

≤ 2.5 = low

2.5-9 = moderate

>9    = high

 intensify programme

 

modify programme
Petal fall – harvest(every 2 weeks)extension shoots 4 per tree scab on leaves/shoots% infected shoots ≤ 2.5 = low2.5-9 = moderate
>9    = high		 modify programme
Autumn – post harvest – before leaf fall10-20 leaves/tree late scab on leaves% scabbed leaves ≤ 3 = low

>3 = high

 macerate leaves, 5% urea before leaf fall