Abstract

Changes in understorey management which accompany a shift to organic production potentially affect water and nutrient relationships for apple trees. Here we report field trials from both conventionally managed and biological production orchards. Changes in nitrogen and phosphate levels have been observed with changes in understorey management. These nutrient levels corresponded with changes in fruit maturity and fruit quality, but are unlikely to be the only causal factor. In three out of four years, ‘Fuji’ fruit were more highly coloured from trees with a mown understorey compared to fruit from trees with a normal herbicide strip and this largely offset a decline in yield under grassed treatments. Fruit maturity was significantly advanced on trees with a grassed understorey, and this represented a significant economy in picking operations.

With varieties which are sensitive to calcium disorders like ‘Braeburn’, our work shows changes in calcium levels with understorey management. Fruit maturity was delayed by leguminous compared to non-leguminous understories, and nitrogen levels increased with the inclusion of the legume. The incidence of storage disorders was low in fruit from orchards under biological production systems, and few differences in fruit quality among different understorey treatments were observed.

Although some understorey treatments imposed stress on the trees in addition to their effects on nitrogen levels, their overall effect was to enhance fruit quality and simplify harvest procedures. The management of nitrogen levels through changes in understorey management offers potential for increasing the proportion of Fuji apples reaching export standards, and varieties susceptible to storage disorders may also respond to changes in understorey management.

Introduction

Under any system of biological production, weed control and maintenance of a low maintenance understorey is a major consideration. Use of mulches is an important option, but these may utilise nitrogen as they break down, and may be expensive when applied to the whole orchard floor (Hogue and Neilsen 1987). In line with the work on the dynamics of pest populations (Daly and Thomas 1992), we have been looking at the effect of the understorey treatments on soil fertility and tree nutrition. Apple trees are poor competitors for water and nutrients compared to understorey species (Bould et al. 1972; Haynes 1981), and the effect of competition may be changes in fruit production, tree growth, or fruit quality. For perennial fruit crops changes in the water and nutrient balance may be all important to the quality and storage ability of the fruit. Over the past three years we have conducted trials with several horticulturally important varieties to assess the impact of biological production and particularly understorey management on tree nutrition and fruit quality. The results of a trial with conventional nitrogen sources but involving a grassed understorey treatment for Fuji, and mulching trials under a biological production system for Braeburn are presented here.

SECTION A:

Effect of grassed understory on fruit quality and production for Fuji

Methodology

Poor red skin coloration and excessive water-core levels at harvest are major fruit quality problems for apples of the ‘Fuji’ cultivar, particularly for young orchards.As a result of a field survey (Volz et al. 1991) which related fruit red colour to leaf nitrogen levels in ‘Fuji’, a trial evaluating several nitrogen treatments was established. Young trees grafted on M 793 rootstock and planted at the HortResearch Orchard at Appleby in 1988 were used in the experiment. Four rates of nitrogen application (11, 35 75, and 140 kgN/ha) were compared for their effect on leaf nitrogen levels, fruit production, harvest maturity and fruit colour. Two additional nitrogen depletion treatments at low (35 kg N/ha) and intermediate (75 kg N/ha) rates of nitrogen, but with a grassed understorey, were compared to conventional sward management which maintained a 2m wide herbicide strip in other treatments. Each treatment was imposed to two 4 tree plots in a complete randomised block design. We are focusing on the understorey treatments in the present discussion.

In 1992-1993 and 1993-1994 N rates applied to trees were similar to the previous two years of the trial, except for those on the grassed-down treatments which had approximately double the amount of N applied in 1990/91 and 1991/92. When required, fertiliser N was applied as CAN as a basal dressing (29 Sept) and as a spring dressing (13 Oct). Ammonium sulphate was applied in late spring (13 Nov) and early summer (10 Dec).

Differential hand-thinning (6 Jan) was applied to trees to give similar fruit size for all treatments (as would occur commercially). Thus grassed down treatments were thinned to 1-2 fruit per cluster, to 2 fruit per cluster for the ‘very low N’ herbicide treatment and 2 or 3 fruit per cluster for the ‘Low N’, ‘Moderate N’ and ‘High N’ herbicide strip treatments. Otherwise the trees were managed according to standard commercial practice, which is important because previous workers on nitrogen responses in fruit trees have tended to use a single harvest (Weeks et al. 1958). In contrast, we thinned trees to fruit loads appropriate to tree size, and selectively harvested fruit (3-4 harvests).

Statistical analysis: Data was analysed by ANOVA using the SAS statistical package, with 4 trees as a subsampling unit within the two replicates of each treatment. Grassing down treatments were compared to appropriate herbicide treatments using Duncan’s LSD at P<0.05.

Figure 1: Changes in fruit production and export fruit yiled in response to nitrogen and understorey treatments for young Fuji trees 1991-1994 (vertical lines represent LSDs at P<0.05)

Results and Discussion

The grassed understorey trees produced lower total fruit yields than trees under conventional herbicide management in all four years of the trial. An increase, however, in the proportion of fruit exported compensated for this decline most years (Figure 1), such that differences in export yield were relatively small . Total fruit yields were generally unaffected by rate of nitrogen. An exception to this occurred in 1993 when total yields were higher in low nitrogen treatments than high nitrogen treatments , but this may be due to differential fruit thinning (Figure 1).

The results for two contrasting years, 1992 and 1994, are summarised in Table 1. Trends for a decline in nitrogen level and fruit production with low nitrogen and grassed treatments were observed in 1991, but became more marked in 1992 (Table 1, Figure 1). Leaf nitrogen levels varied somewhat with rate of nitrogen and were markedly reduced by the grassing down treatments. As previously mentioned, fruit production and tree growth responses were complicated by differences in fruit thinning practice.

Table 1: The effect of nitrogen fertilisation and depletion treatments on fruit production and fruit quality at harvest for 1992 and 1994

Nevertheless the grass treatments clearly yielded approximately 40% less fruit than the standard understorey plots. Total fruit yields tended to increase with increasing rate of nitrogen fertilizer. The major effect observed in 1992, however , was the increase in red colour score, and higher export pack-out of grassed and very low N plots, so that there were few differences in yields of marketable fruit (Figure 1, Table 1).

Grassing down dramatically increased the proportion of fruit harvested at the first harvest date, and reduced the proportion of low coloured fruit in years 1-3 of the trial. The mean red colour score of a random fruit sample in these years was significantly higher in grassing down treatments. Since many of these effects also occurred, but without statistical significance in the very low nitrogen treatment, we believe the effect of the understorey treatment resides in it’s ability to deplete nitrogen levels. Use of a grassing down treatment with postharvest or spring fertiliser management may offer growers control over nitrogen levels and improved fruit quality in young trees. The results for year 4 were, however, somewhat in contrast to these findings (Table 1). In 1994, fruit yields and export packouts were higher, and harvest dynamics were unaffected by the nitrogen and understorey treatments. This suggests that poor fruit colour in Fuji may be a young tree problem and more work is needed to define the effects on nitrogen and understorey management in older trees.

SECTION B:

Fruit production and fruit quality for Braeburn at Winchmore

Methodology

In December 1989 three separate 1 ha blocks were converted from turfgrass (Lolium sp) to either red clover (Trifolium pratense), ryegrass (Lolium perenne) or mixed herb ley prairie grass (Bromus catharticus), timothy (Phleum pratense), red clover, chicory (Cichorium intybus) sheeps burnet (Sanguisorba minor) and sulla (Hedysarum coronarium). These three large replicate blocks with different understorey herbage treatments provided suitable sites for monitoring disease, pest and predator/parasitoid levels. Details of the methodology are provided elsewhere at this conference (Daly 1996 This volume).

To provide detailed information on herbage production, soil nutrient changes and tree growth, a sub-trial was established with each 1 ha block having 2 replicates. Each replicate was situated at either end of the block to minimise pest and disease influences. The experimental trees were five year old (grafted 1984) trees of ‘Royal Gala’ in blocks 1 and 3, with ‘Braeburn’ in block 2. After 1989, a compost treatment was included as a separate treatment applied to each of the three herbage types (2 reps per herbage type). The compost was proprietary brand (‘Bioblend^r) made from bark and animal processing wastes. The compost was applied to the treeline at a rate of 5.6 kg/tree (Table 2). A total analysis indicated that annual additions of nutrient from compost would amount to 338 kg N, 81 kg P, 81 kg K, 490 kg Ca, and 81 kg Mg per hectare.

Table 2: The effect of understorey treatments on maturity and nutrient composition (mg/100g fresh weight) for Braeburn (1993-1994)

In addition, the sub-trial incorporated a split-plot comparison of two sward mowing techniques which were either mown and returned evenly back to the plot, or mown and transferred as a mulch to the treeline area. Pea straw was added to the treeline area of the mulch treatments in July 1990, 1991 and 1992, at a rate of one small (15kg) bale/tree, to initiate the mulching process, and to help control weeds in the treeline, the mown treatment, which did not have pea straw, had a complete herbage cover throughout the pathway and treeline. We are focusing on the mown treatments in our discussion of fruit quality and tree nutrition.

Herbage sampling: Leaf samples were taken over the three sample trees in each plot from the current years extension growth (1 year old wood, youngest fully matured leaves) in the January and February period for 1990-1993. Samples were couriered overnight to a Telarc registered laboratory for chemical analysis.

Tree growth and fruit production: In the middle of the commercial harvest period, when trees were at an intermediate stage of maturity, each tree was strip-picked and total fruit weight recorded for each tree. In addition, for 1993 and 1994, a random sample of 3 fruit per tree ( count size 125) was taken at picking for assessment of harvest maturity (background colour), and for mineral analysis. Cortical cores from three fruit were bulked for digestion with nitric/perchloric acid and analysis for Ca, Mg and K by atomic absorption spectroscopy. Duplicale samples of fruit were also subjected to a Kjeldahl digestion for nitrogen and phosphate analysis. A sample (50 fruit/tree) was stored at 0°C for 12 weeks and subsequently assessed for the incidence of storage disorders.

Statistical Analysis: Differences were assessed by ANOVA and Duncan’s LSD using the Genstat statistical package. A separate contrast was conducted to compare compost to the other three understorey treatments, and residual analysis was conducted to assess the applicability of the linear model.

Figure 2: The effects of different understorey treatments on leaf analysis levels in apples for (a) phosphate (b) potassium and (c) nitrogen. Error bars are LSDs for vertical comparisons at P<0.05

Results and Discussion

Leaf Analysis: Leaf samples taken from apple trees in all four years indicated that the provision of compost had increased the nitrogen level in compost treatments. The increase was statistically significant in both 1991 and 1992, and application of compost seems to be recommended from both nutrient requirement and production standpoints (Figure 2c).

Different herbage species in the understorey affected nutrient levels, particularly for nitrogen. Total nitrogen levels were higher in the clover plots, probably because of nitrogen fixation. The grass plots which have a high requirement for nitrogen brought about low nitrogen levels in the accompanying trees. The mixed herb ley contained some legumes, but these had little effect on leaf nitrogen levels in the associated trees (Figure 2c).

Although applying compost appeared to have little effect on phosphate or potassium levels, differences in herbage species had a pronounced effect. Both potassium and phosphorus levels were increased in the ryegrass plots compared to the clover plots, with mixed ley plots giving intermediate levels (Figure 2a and 2b). Since potassium and phosphorus levels in the soil were not affected by the herbage treatment (data not presented) it seems that an interaction with nitrogen is implicated. As nitrogen levels decline, potassium and phosphorus levels increase, and we may be seeing this effect here. One explanation, is that reduced growth in trees with a restricted nitrogen supply leads to the accumulation of other nutrients, but more specific interactions in the uptake of nitrogen, potassium and phosphate have also been reported (Mengel and Kirkby 1978).

Fruit Quality 1993-1994: Overall, the fruit quality of fruit from the trial was poor in 1993, because of problems in pest control and fruit thinning. We selected relatively clean fruit for storage and assessment, but a high proportion of the fruit were unmarketable, because of factors such as insect chew, fungal spores, or small size. Calcium and nitrogen levels in the fruit changed in response to the understorey treatments (Table 2), but this had no effect on the incidence of calcium-related storage disorders. The incidence of bitter-pit and lenticel blotch was low in all treatments. In the second year of fruit sampling, fruit quality was much improved (fruit size and insect chew) and fruit were exported from the block. Calcium levels were lower in this year, but there was still a very low incidence of calcium-related storage disorders. Calcium levels were reduced by the addition of compost, and slightly by the clover treatment (Table 2).

Fruit soluble solids was also lower in the clover treatment (11.1^o Brix) compared to the other three treatments (11-8-12.0 ^oBrix), which is another indication that the clover understorey delays fruit maturity compared to the grass and mixed herb ley treatments. In both years of the trial, addition of nitrogen with compost or with the clover treatment reduced fruit firmness. It is too soon to get a clear indication of differences in fruit quality and storage in the different understorey treatments, but this work is continuing and some changes in harvest maturity and fruit mineral analysis have been consistent between years .

References

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