Crop Monitoring

There is a strong correlation between the crop load during the first six or seven weeks after full bloom and fruit size at harvest. Measurements of the effect of thinning over this period show that for each weeks delay in thinning off surplus fruits, fruit size at harvest is reduced by 5 to 10 grams depending on the variety's genetic fruit size potential and responsiveness to thinning. Hand thinning at this stage is not practical so this is why we use the chemical thinners.

Studies by Hort+Research in the various fruit growing districts show that for Royal Gala varieties the amount of heat, expressed as growing degree days (GDD) above 10 OC between full bloom and seven weeks later has a marked influence on both harvest fruit size and also the length of time between full bloom and harvest.

A paper titled "Early Prediction of Apple Fruit Size Using Environmental Indicators" by C J Standley et al, published in Acta Horticulturae 557 pp 441-446, gives the results of their studies.

This paper shows harvest date for Royal Gala will range from about 130 days from full bloom in a warm district which accumulates 300 GDD by 50 days after full bloom to about 155 days in a cool district accumulating only 110 GDD.

Fruit weight at harvest for light cropping trees can be around 260 grams where 300 GDD are accumulated in the first 50 days. On the other hand, if only about 120 GDD are accumulated in the first 50 days after full bloom, the average fruit size will struggle to make 140 grams which is 130 count average fruit size.

There is also a reasonably good correlation between fruit size at 50 days after full bloom and size at harvest. This relationship between fruitlet size and size at harvest gets better as one gets closer to harvest, even so comparison of fruit size growth curves at 50 or 65 days after full bloom between various orchard blocks, or with historical data gathered in previous seasons, will give very useful information for fine tuning crop husbandry.

Historical information for Hawke's Bay shows that in big size years, average fruit size at 65 days after full bloom was 45.5 mm while in small fruit size years, fruit size at 65 days from full bloom was 42 mm, a difference of only 3.5 mm which by harvest ranged between 4 and 9 mm difference.

Last season, using growth curve data and full bloom dates, we were able to forecast reasonably accurately harvest dates and also predict fruit size range.

Last year for Royal Gala was an early season. Full bloom was about 10 days earlier than normal and post bloom temperatures were above average. The early season growth advantage carried through to harvest, giving seven to 10 days earlier harvest. It was possible to predict the earlier harvest at least six weeks before harvest which is necessary these days for planning the spray programme so that withholding periods can be met.

This year we are going to be looking at a much later harvest season. It's possible that unfrosted blocks which received dormancy breakers will come in at a similar time to the normal harvest last year. Where frost damage is involved much of the early set fruit has been lost so it's only the later flower that is likely to set. In these blocks harvest period will be delayed relative to undamaged blocks. They may also struggle for fruit size due to their crop being carried on later, weaker flower.

Over the last two or three years we have been developing a good database on fruit sizing. This is based on selecting typical branches early in the growing season and measuring all the fruit on these marked branches several times during the growing season. The first fruit measurement is made about 45 days after full bloom just prior to hand thinning, followed by measurements at 65 days to give a comparable reference date fruit to other years, then later measurements to detect any signs of the fruit sizing stalling should crop loads be a little on the heavy side.

Another measurement is made about a month out from harvest to determine if the crop will make the target fruit size range.

These later measurements are very useful for predicting fruit size range and identifying where fine tuning of crop husbandry practices will pay dividends.

For this type of monitoring to be really valuable it's necessary for accurate dates of full bloom to be recorded so that it's possible to find the right starting date for the growth curve data.

Incidentally there is little point in trying to measure fruit size earlier in the season than 45 days after full bloom. Early in the season differences in fruitlet size may be more related to fruitlet age than its' potential final size.

The 45 day after full bloom measurement has potential to form a basis for "size" thinning the crop, particularly if used in conjunction with crop loading data.