RECOMMENDATIONS

Growers who have BBD-susceptible orchards do not need to turn to their nutrition programme to prevent this disorder. There is no evidence that increasing nutrient levels will prevent BBD. However, low levels of leaf nitrogen late in the harvest season seem to increase the levels of BBD in the fruit. Nitrogen levels need to be maintained on orchards where BBD has been found in the past. Avoid harvesting fruit from trees with low crop loads late in the season.

INTRODUCTION

Growers and scientists have many theories as to what influences the development of Braeburn Browning Disorder (BBD). We are still looking for an explanation as to why the disorder is present on some sites and not others.

One of the difficulties in studying BBD is the huge variation in the development of this disorder between trees and orchards. For example, in a 1994 study in Central Otago the average incidence of BBD (number of fruit with BBD) was raised by high levels in a few trees. Normally about 0 - 4 fruit per 100 have BBD, but on a few trees, up to half of the fruit on the tree had BBD. Because of this variability, statistical analysis can be hard and it is often difficult to predict trends from the wide range of data that is collected.

Ken Marsh and his team at HortResearch have been studying four Braeburn blocks in Central Otago to determine whether some of the suggested causes of BBD are valid. The team has also been investigating the possible sources of variation in BBD development.

They have shown that nutrition does not play a big part in the development of BBD. They also found that stress did not appear to affect BBD, although trees with root rot certainly have high rates of all disorders. Changes in the orchard understorey may reduce the incidence of BBD, but the results from these trials were not consistent. Likewise there are probably relationships between harvest dates and BBD as well as fruit load but once again trends differ between sites and individual trees.

NUTRITION EFFECTS ON BBD

Disorders involving internal breakdown have previously been associated with low levels of boron and phosphate. As nutrient levels vary from orchard to orchard, nutrient deficiency was suggested as a possible explanation of the variability of BBD between sites. In both the 1993/94 and 1994/95 seasons, four orchards in Central Otago received four nutrition treatments : Seniphos, Bortrac, Seniphos+Bortrac or nothing (control). Normal calcium programmes were used on the orchards.

None of the sites showed any response to the foliar sprays . Therefore, BBD is not caused by a deficiency in either phosphate or boron, and foliar sprays are unlikely to control the disorder.

In some cases, the nutrient applications, and particularly the boron sprays, affected the nutrient balance in a following season. There was no relationship to the level of BBD in 1995 however.

STRESS TREATMENTS

Braeburn trees in the trial (32 trees total) were stressed in one of the following ways :
1. partial defoliation (60% of leaves were stripped off the extension shoots)
2. girdling
3. root pruning

These ‘stress’ treatments did not have any severe effects on the trees - very little differences in either nutrient levels or maturity were detected between the stressed trees and non-stressed trees. Neither did the different treatments have any effect on BBD incidence, but this may be because any trends are obscured by large tree to tree variation and the stress treatments may not have been severe enough.

Overall, the root pruning treatment gave the highest level of BBD in the fruit.

UNDERSTOREY MANAGEMENT

A grassed understorey, used in addition to normal orchard management, has been previously shown to advance the maturity of Fuji in Nelson and Braeburn in Canterbury. As BBD was originally suspected in later harvest fruit, bringing forward the maturity by using a grassed understorey may be a means of BBD control. The three different trial treatments used were : standard herbicide treatment (control), clover (variety ‘Demand’) and clover/ perennial ryegrass (variety ‘Nui’).

After two years, the understorey treatments had no effect on the incidence of BBD, nitrogen levels in the fruit or fruit maturity.

There were, however, results which suggested that the clover/ ryegrass understorey may decrease levels of BBD. At one harvest date, BBD was reduced from 25% to 3% with the use of the clover/ryegrass treatment. Although this appears to be a big reduction, it could not be proved statistically. Further information is needed before clover/ryegrass understoreys can be recommended for BBD control.

SITE DIFFERENCES

BBD does not always occur in the same trees every year. Where BBD is severe, however, the same trees get BBD as in previous years.

MATURITY

In the trials discussed here, BBD was mainly found in late harvest fruit. However, the development of BBD could not be linked to maturity parameters. In other words, a mature fruit (as measured by starch index, soluble solids, flesh firmness and background colour) will not necessarily develop BBD.

NITROGEN LEVEL

Trees which had low levels of nitrogen in leaf samples taken after harvest also produced a high proportion of fruit with BBD. As the leaves senesce (yellow and die), they tend to have lower levels of nitrogen. Possibly trees which ‘shut-down’ early in the autumn are more susceptible to BBD. There may also be a relationship between low crop loads, low nitrogen and BBD development.

ACKNOWLEDGMENTS

HortResearch acknowledges ENZA New Zealand (International) for financial support.