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Predicting orchard behaviour is critical to fine tuning block performance.
Over this last season by analyzing climatic data as the season unfolded, we were able to predict harvest data and likely fruit size ranges from as early as 50 days after full bloom. The estimates made at that time based on flowering dates and growing degree days base 100C for the first 50 days after full bloom showed that harvest date would be about 14 days later than last year, and fruit size would be at least one count size smaller. These predictions have come to pass.
Earlier in the spring we were using winter chilling data based on the South African developed "daily positive chill unit" method to predict optimum timing for dormancy breaking sprays. One of the main difficulties we have in using chill unit data is knowing where to start counting chill units. To date, we have been using an arbitrary date such as 1st May or from the date of the first air frost. If we are going to try and predict completion of deep dormancy, it is necessary to establish accurately when dormancy commenced.
By keeping good records on the time of autumn leaf colouring and leaf fall on a block by block by variety basis, the commencement of dormancy for each variety and block can be established. When 50% of the leaves are showing full autumn tints or have fallen is the stage when winter chilling data accumulation should begin.
The amount of winter chill requirement varies by species, and even variety within a species. The strength and uniformity of bud break is dependant on the amount of winter chilling, but can also be modified by the level of stored carbohydrate reserves and nutritional status of the plant. Trees which have good nutrient reserves, particularly nitrogen, and have accumulated good carbohydrate reserves because they held onto healthy photosynthetically active leaves for a long period following harvest will not require as much winter chilling for good bud break as trees which defoliated earlier in the autumn or suffered depletion of reserves because of late harvest of a heavy crop or suffered foliage damage by insect or disease infection.
For this reason, it is absolutely critical to maintain healthy leaf condition right through until natural leaf drop occurs.
Partial defoliation, such as removal of excessively shading branches within the tree, will not have detrimental effects on carbohydrate accumulation because it allows better light penetration within the canopy, so the remaining leaves are able to compensate for the leaves which have been lost.
Where premature leaf drop has occurred, it is probable that next year's cropping will be adversely affected in some way.
Low nutrient levels, particularly nitrogen and magnesium are often associated with premature leaf drop.
Pest and diseases likely to cause foliage problems include black spot, powdery mildew, red mites and leaf curling midge in apples. Pears and nashi do not suffer from powdery mildew, and nashi does not suffer from black spot either, but both can be stripped of foliage by pear slug.
Incidentally the same insect that attacks the pears will also attack cherries and plums doing considerable damage here and if severe in cherries may delay bud break by up to three or four weeks.
Plums and susceptible peach and nectarine varieties are also prematurely defoliated by stonefruit rust.
Keeping accurate records of the various phenological development stages by variety and orchard block basis will enable better prediction of future cropping behaviour to be forecast because it will enable more precise use of climatic data.
Critical dates include:
- Time of 50% autumn leaf colour, or 50% leaf fall.
- Green tip terminals and older spurs.
- Full bloom or petal fall king bloom of terminals and spurs.
Refinements to these dates could include the spread of bud break and flowering, and pinpointing the "T" stage of fruitlet development. The "T" stage of fruitlet development is when the fruit stem forms a right angle with the base of the fruit, as the fruit changes shape from its juvenile pear shape to the typical mature fruit shape with a sunken stem end cavity. In the case of apples, this stage indicates the completion of the cell division stage.
Some research scientists specializing in apple fruit development and maturity consider the period between the "T" stage and harvest to be relatively constant for a particular variety.
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