Our expert. David Williams responded: 
Randomly select at least 5 trees in each block or variety that you wish to assess. If the trees are taller than the height you can reach without standing on your toes, take a picking ladder with you. If your trees are trellised and you cannot easily sample all sides then use one panel of trellis as equivalent to a full tree. Starting from the top of the tree randomly look at 25 fruit from each of the 4 sides of a tree (or each of 4 trees in a panel of trellised trees) as you work down the tree.
Do not pick the fruit but carefully inspect them without excessive lifting or twisting that would dislodge them. Record presence or absence of damage for each fruit and record the type of damage. By the time you have finished a tree, or a panel, you will have inspected 100 fruit.
By inspecting 5 trees, or 5 panels, randomly selected throughout the block you will have the number of damaged fruit out of 500 fruit, and you will have the number damaged by each pest or disease. If you multiply those numbers by 2 and then divide by 10 (or else multiply by 0.2) the result is the % of damaged fruit to 1 decimal place