By University of Tasmania researcher Penelope Measham

Cherry cracking after rainfall can be a significant setback for growers, but University of Tasmania research has been looking at the ways different management techniques can be used to increase fruit resistance to such damages. Maintaining irrigation is one way, while Measham says medium to high crop loads, spray treatments and selective pruning all have a part to play.

During the last growing season excellent progress was made in a project investigating management of rain-induced fruit cracking in sweet cherries. Results are certainly promising for the future and furthermore, there is still another season of work to be done. The management strategies assessed during the last season resulted in significant reductions of cracking and all trial sites but one experienced rainfall in the critical cracking period. Strategies focused on four key areas: crop load manipulation, spray treatments, selective pruning and irrigation.

Crop load

Trials were undertaken to assess the impact of crop load and timing of thinning on both cuticular and side cracks. Thinning (to high, medium or low crop loads) was undertaken at three different stages of fruit growth; dormant buds, full bloom and four weeks after full bloom (close to pit-hardening). In general, low crop loads experienced the highest level of cracking, the majority of which were side cracks. Thinning to low levels during full bloom induced the highest level of cracking in one trial, at 68%. Maintaining a medium or high crop load reduced cracking by as much as 50% in some trials. In addition, another positive outcome of these trials was that it showed trees are not resource limited, as medium or high crop loads did not generally show a decrease in size, firmness or sugars as would be expected.

Spray treatments

Spray trials were undertaken to assess the impact of spray treatments on cuticular cracking. Treatments included RainGard, 24/7 and SureSeal, and were compared to an untreated control. A significant reduction in cracking was seen with all spray treatments. Reductions in total cracking of up to 50% were observed in some trials. Again, the positive outcome of no reduction in fruit quality (no decrease in size, firmness or sugars) was recorded.

Selective pruning

Trials were undertaken to assess the impact of selective pruning during a rainfall event on the development of side cracks. Pruning consisted of removing the top extension growth just after commencement of a rainfall event during the two weeks prior to harvest, as could be achieved by a cutter bar on a larger scale. The hypothesis for pruning in this way was that by removing this growth, the amount of internal water held within the tree following a rainfall event (and available to move into the fruit) would be reduced.

Previous research points to internal flow of water from leaves to fruit soon after rainfall contributes to excess internal fruit water. Trials yielded highly promising results as a significant reduction in both total, and side, cracks was observed. A reduction of 50% and of 25% was seen in the two trials respectively.

The fruit from these trials has also been assessed as by removing leaves, there is also a reduction of available carbohydrates. However, fruit showed an increase in sugars under this pruning treatment but no decrease in size or other quality characteristics.

Irrigation

A trial was undertaken to assess the impact of irrigation volume on the development of side cracks. High, medium and low volumes of irrigation were applied and soil moisture difference was confirmed using soil moisture monitoring. The level of cracking in this trial was less than 1% in all treatments; the variety was harvest mature early in the season and did not experience rainfall. The cracking index was determined from mature fruit from each treatment and indicated that should a rainfall event have occurred, fruit from the low volume treatment would have developed a higher level of side cracks than from the other treatments (based on previous research showing a relationship between the cracking index and side cracking in the field).

In addition, fruit from the high volume treatment showed a significantly greater size at harvest than the other treatments with only a small loss of sugars. This was also supported by daily fruit growth patterns which showed fruit under low volumes of water experienced considerable daily shrinkage, and did not recover on very hot, dry days. Thus, maintaining irrigation to avoid water stress during all stagesof fruit development should provide some resistance to cracking through cuticular integrity, while encouraging size.

This project has been funded by Horticulture Australia (HAL) using the cherry industry level and matched funds from the Australian Government.

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