Growth retardant lifts fruit output

Press, 25 July 1986, Page 13

Growth retardant lifts fruit output

A plant growth retardant, known as PP333, could help increase New Zealand stone fruit production, a Central Otago scientist believes.

Dr Tad Jacyna said result from his initial experiments indicated that PP333 could be very effective in controlling excessive branch growth in high density plantings of stone fruit trees.

It might have a key role to play in ensuring the success of high density plantings. “The more trees per hectare, the higher the yield per hectare, provided the grower can stop any excessive growth after the tress have filled their allotted spaces in the orchard,” he said. PP333 is not yet available to New Zealand growers, but if experiments such as those being carried out in Central Otago continue to show positive results, the growth retardant could be on the market within three years.

In the case of sweet cherries, PP333 together with another type of plant growth regulator, could the way for high

density plantings of this stone fruit species, said Dr Jacyna, who is M.A.F.’s horticultural scientist in Alexandra.

Until now, the natural strong growth habit of many sweet cherry cultivars and lack of cherry dwarfing rootstocks made this species unsuited for high density plantings. Dr Jacyna said his results also indicated that PP333 could have a direct beneficial effect on stone fruit yield and quality. “By reducing the tree’s vegetative growth (that is, the growth of branches and leaves), the retardant allows more of the tree’s resources to be directed into producing fruit. As a result, fruit size and yield increases.”

However, Dr Jacyna stressed that it was too early to draw any firm conclusions from the trials.

"We have only one season’s results and although these are promising we need further evidence,” he said. PP333 is a chemical compound which suppresses the plant’s production of growth promoting hormones. n

Its potential offers hope to many New Zealand stone fruit orchardists who are increasing the density of their plantings but face problems with excessive branch growth and overlapping tree canopies. “This excessive growth is very undesirable,” Dr Jacyna said, “as it prevents good sunlight penetration into the canopy of the tree. This, in turn, depresses flower initiation, fruit set, yield and quality.” Dr Jacyna said many growers were planting stone fruit at densities of over 1000 trees per hectare, compared with traditional rates of between 400 and 600 trees per ha. “It is more profitable,” he explained, “to have many small trees rather than a small number of big trees. “The yield per hectare is higher because more sunlight penetrates the canopies of small trees; as well, small trees are easier to spray and harvest, reducing the cost of management” Dr Jacyna said growers had hoped that excessive branch growth in high

density plantings could be controlled by early and continued heavy cropping. However, it had become clear that not all types of stone fruit were sufficiently precocious or heavy cropping in their early years. Orchardists would then, apply hand pruning to try to control the unwanted vegetative growth, but this was not always successful because pruning sometimes had the opposite effect of stimulating such growth. PP333 could greatly reduce the need for this sort of pruning, Dr Jacyna said.

In August, 1984, Dr Jacyna began four experiments, investigating the use of PP333 in high density plantings of stone fruit trees. Two trials were carried out on bearing apricot and nectarine trees, while the other two were on non-bearing two-year-old sweet cherry trees.

PP333 gave good control of growth in the apricot, nectarine and cherry trials during the same year as it was applied and to continue to

be effective for at least another season. Results indicated that the method of application was much more important than the rates of chemical used, Dr Jacyna said.

“We obtained much better growth control by applying this chemical through the soil than as a foliar spray. “Also, we think we can apply much lower rates of PP333 without reducing its effectiveness.” In the apricot and nectarine trials, trees given a soil application of PP333 produced larger fruit than untreated trees. As well, PP333-treated nectarine trees produced more fruit per hectare.