Abstract

During the period 1991-1994 selected apple cultivars were evaluated for susceptibility to black spot (Venturia inaequalis) and powdery mildew (Podosphaera leucotricha) in an organic orchard trial block. Initially 27 cultivars were established, with additional plantings between 1992-1993 increasing the total to 40. The primary aim of the trial was to identify disease and pest resistant apple cultivars that are suitable for use in biologically sustainable orchard production systems under New Zealand conditions.

The cultivars represent a range of material from older cultivars, current commercial cultivars, and recent releases bred with disease resistant characteristics. Most of the older cultivars have been included because of their recorded resistance to fungal diseases in the variety collection block at the Lincoln University Biological Husbandry Unit. These older cultivars could contain genes for resistance not available in present breeding programmes or in recent disease resistant selections.

Initial assessments of disease susceptibility only recorded foliar incidence of black spot and powdery mildew, but later included fruit, as the trees began to crop. A small group of cultivars have emerged displaying a high level of freedom from black spot infection. Included in this group are the surpassed old English cultivars Discovery, Merton Worcester and Laxton’s Fortune. Susceptibility to powdery mildew by the same group is variable, although generally the incidence has been low. Three cultivars bred for black spot resistance - Prima, Priscilla and Sir Prize - have displayed excellent resistance, however both Priscilla and Sir Prize have recorded significantly high levels of powdery mildew infection.

Cultivars with partial resistance are also of considerable potential for use in biological fruit production. A group of several cultivars have recorded low incidence of black spot and powdery mildew. These cultivars may be suitable for use in organic orchards operating with only minimal applications of organically permissible chemicals for disease control.

The fruit quality and storage characteristics of many of the cultivars, particularly in those bred for disease resistance, are insufficient for export. Consequently, until such time as resistant cultivars with improved flavour and storage qualities are available, most of the cultivars currently exhibiting potential for organic production are more likely to be suited to supplying local domestic markets or for processing.

Introduction

Most of the current commercial varieties grown in New Zealand are susceptible to fungal diseases and insect pests. Virtually all of these varieties have evolved since the advent of modern synthetic agrichemicals that allowed control of pests and diseases. Consequently, there was a reduced emphasis on the need for pest and disease resistance for new cultivars emerging from the breeding programmes operating at the time, and a greater emphasis placed on fruit quality and appearance characteristics. Around two-thirds of all the cultivars that have emerged from breeding programmes in the last 30 years have originated from a limited genetic base on only 5 parent cultivars - Golden Delicious, Cox’s Orange Pippin, Red Delicious, Jonathan and McIntosh (Noiton et al. 1994). Furthermore, cultivars from these parents have been grown in a wide range of geographical and climatic environments, not all of which are conducive to production of high quality fruit.

Because of this limited genetic base susceptibility to fungal diseases, principally black spot (Venturia inaequalis) and powdery mildew (Podosphaera leucotricha), constitute the greatest threat to the viability of organic apple production. Not only are current organic growers unable to use modern synthetic fungicides, they also have to do without many of the fungicides used 40-50 years ago for disease control. For various reasons, many of these older products are now unavailable because they are no longer manufactured, no longer registered, their use is not permitted in organic production, or because of unacceptable health and environmental risks associated with their use.

More recently, consumer concern over spray residue levels, together with a general change in public attitude seeking a reduction in the use of chemicals used in food production, has lead to a greater focus towards breeding for disease and pest resistance. Ultimately, disease and pest resistant cultivars may hold the key to successful organic apple production.

Methods

The aim of the Winchmore Research Orchard cultivar trial was primarily to identify disease resistant apple cultivars that are suitable for use in New Zealand biological orchard production systems. The cultivar trial block was established during the winter of 1990. The trial area was designed as a latin square to minimize the influence of surrounding cultivars, and was capable of accommodating 40 cultivars for evaluation. The initial planting comprised 27 different cultivars, with five single tree replicates. Subsequent plantings during 1991 and 1992 increased the number of cultivars to the potential maximum of 40. The rootstock used is MM106. Rows were 4.5 metres wide with trees 3 metres apart in the row.

The cultivars represented a range of material from older, surpassed varieties through to the more modern cultivars, including recent disease resistant selections. Planting material was obtained from a variety of sources. Most of the older cultivars have been included as a result of their performance and recorded resistance to fungal diseases in the Lincoln University Biological Husbandry Unit variety block. Commercial orchards and nurseries, and the occasional home garden, also provided cultivars, while the more recent selections and breeding line material was supplied by FIPIA (New Zealand Fruit Industry Plant Improvement Agency) and from the HortResearch breeding programme at Havelock North.

Black spot and powdery mildew resistance of the cultivars were determined by assessing disease incidence on the leaves and fruits during each growing season - foliar evaluation in early summer (late December) and fruit evaluation at harvest (March-April). The number of leaves on 2-3 shoots was recorded. This allowed determination of the average leaf number per shoot. Depending on tree age and size, the leaves on 3-5 shoots per tree are assessed, and the number of leaves affected by black spot and powdery mildew recorded. Using the average leaf number data, the percentage of leaves infected with black spot and powdery mildew was calculated. While the trees were young and not producing fruit, initial assessments only recorded foliage infection. As the trees developed fruit production, quality and storage parameters were measured.

Cultivars were evaluated for a minimum of two years, and depending upon their performance, may be evaluated for up to five years. Highly susceptible cultivars were removed after only two years and replaced with new material for future evaluation, while promising cultivars continue to be evaluated for the full five year duration. To date six cultivars exhibiting high levels of both black spot and powdery mildew have been assessed as unsuitable for use in a no-spray biological production system and were removed from the trial. In autumn 1993 five cultivars -Splendour, Fuji, Cantared, Red Delicious and Coromandel Red - were removed and replaced with Lawfam (Trt No.27)[this was an accidental and incomplete duplication of the Lawfam trees (Trt No.39) planted in 1992], Lundbytorp, Rall’s Janet, Dougherty and an unidentified early maturing cultivar reported to be showing black spot resistance in an unsprayed home garden. In spring 1994 two further cultivars were replaced - Hillwell, and the 1993 planted Lawfam (Trt No.27) trees. The replacement cultivars were Telstar and Liberty.

During the 1993/94 period two cultivars bred for black spot resistance - Red Free and Dayton -were identified as exhibiting signs of black spot infection. This raised concern, although the occasional development of black spot lesions on the leaves and fruits of cultivars carrying the Vf gene in areas of high disease pressure has previously been noted (Virscek-Marn et al. 1993). However, closer inspection of the affected trees at the end of the season revealed that the grafted scion variety had failed and the trees were in actuality only rootstock material. Consequently, the affected trees were removed and the data from these two cultivars excluded from the 1992/93 and 1993/94 data analysis tables.

Results and Discussion

1990/91 Season

Black spot and powdery mildew incidence are displayed in Table 1. Although not part of the fully replicated trial, the cultivars Prima, Priscilla and Sir Prize on M.793 growing in the trial guard row have displayed excellent freedom from black spot. Of these three cultivars Prima recorded no powdery mildew infection, although powdery mildew incidence on Priscilla was also very low.

A high level of resistance to both black spot and powdery mildew was recorded for six (Mother, Laxton’s Fortune, Merton Worcester, Discovery, Scarlet Pimpernel and Sunset) of the eight cultivars obtained from the Lincoln University Biological Husbandry Unit, confirming their previously reported good performance (Anon, 1986). By contrast many of the current commercial cultivars in the trial recorded high levels of black spot and powdery mildew infection i.e. Red Delicious, Splendour, Hillwell, Cantared and Fuji.

Table 1: Mean percentage of leaves per cultivar infected with black spot and powdery mildew - 1990/91 season

1991/92 Season

The 1991/92 season was one of generally low disease incidence and, with the introduction of a limited copper (no copper after December 1991) and sulphur spray programme, resulted in virtually no black spot infection and very low levels of powdery mildew (Table 2). Accordingly, very few significant differences in disease susceptibility were recorded. However, this result does highlight the ability to control black spot and powdery mildew on susceptible cultivars in seasons of low disease pressure with a minimal number of spray applications. It also demonstrates that cultivars displaying only low levels of disease incidence in the absence of fungicides can perform satisfactorily with a minimal level of chemical intervention.

Table 2: Mean percentage of leaves per cultivar infected with black spot and powdery mildew - 1991/92 season

1992/93 Season

With the removal of the limited spraying regime imposed the previous season, the 1992/93 season produced wide ranging differences in disease resistance and infection levels. Table 3 presents the results from the foliar disease assessments performed in early summer, and black spot incidence on the fruit at harvest. Because the level of powdery mildew at the time of recording was low, a ranking of infection level was awarded, rather than a fully determined incidence level. Similarly, because of the low number of trees with fruit, only a ranking of black spot incidence was performed.

The first 7 cultivars in Table 3 displayed a high level of resistance to black spot (less than 1% incidence on the foliage), despite high disease pressure this season. Sir Prize and Priscilla were the only cultivars free from black spot infection on both the foliage and fruit.

A further group showing good resistance to both black spot and powdery mildew included several of the older cultivars, such as Discovery, Merton Worcester and Laxton’s Fortune, sourced from the Lincoln University Biological Husbandry Unit organic block.

The poor performance of many of the current commercial cultivars - Red Delicious, Oregon Red, Splendour, Cantared, Coromandel Red, Hillwell and Fuji - identified these cultivars as candidates for removal from this trial to allow the introduction of other cultivars for future evaluation.

Table 3: Mean percentage of black spot foliar infection, powdery mildew foliar infection score and fruit black spot score - 1992/93 season

1993/94 season

Black spot levels were lower this season compared to previous seasons. Of the 38 cultivars remaining in the trial, 22 recorded less than 10% black spot leaf infection compared to only 8 in the previous (1992/93) season and 13 in the first year of the trial (1990/91).

Unlike black spot, powdery mildew levels were notably higher this season with 35 of the 38 cultivars recording more than 10% powdery mildew infection. Two of the more notable performances were those of Prima and Discovery. Both had low percentage leaf incidence as well as no powdery mildew infection of apical shoots. Lundbytorp, a variety of Danish origin, looks promising but is a recent addition requiring further assessment.

Table 4: Mean percentage of leaves per cultivar infected with black spot and powdery mildew - 1993/94 season

In the 1993/94 season 26 cultivars produced fruit for assessment. This was the first fruiting season for most of the cultivars, although many trees only produced small quantities of fruit. Consequently, trees were only scored visually for the incidence of black spot lesions on the fruit. As the trees develop and produce larger quantities of fruit it is intended that future assessments of black spot incidence will be of a more quantitative nature to provide more precise determinations of disease resistance.

Several distinct groupings of black spot susceptibility can be observed in Table 5. The first of these contained five cultivars recording no black spot incidence on the fruit, with three of the cultivars (Prima, Priscilla and Sir Prize) having been bred specifically for black spot resistance. Although no black spot was observed on the fruit of Laxton’s Fortune, a low level of foliar black spot infection was recorded. Conversely, Akane which exhibited no sign of foliar black spot incidence did register a low level of black spot presence on the fruit.

Table 5: Mean score for black spot incidence on cultivars producing fruit - 1993/94 harvest

The second group comprised six cultivars with very low to low levels of black spot on the fruit. Five of these cultivars were sourced from the Biological Husbandry Unit, Lincoln. The previously observed high level of resistance to fungal disease displayed by these cultivars at the Lincoln University site (Anon 1986) continues to be evident in the Winchmore cultivar trial.

The high levels of black spot incidence recorded by the remaining cultivars demonstrates the black spot susceptibility of the majority of current commercial cultivars and releases which were selected or bred primarily for fruit colour and quality characteristics. These results also illustrate the importance of disease resistance as a fundamental priority in future cultivar breeding and selection.

In an effort to increase the genetic base for apple breeding, HortResearch has been involved with the collection of seeds from wild apple cultivars growing in the Kazakhstan and Kyrgyzstan Republics (Noiton 1994). The continued good performances of Discovery and Laxtons Fortune in the Winchmore trials have also attracted attention as a possible source of genetically useful breeding material. Pollen from these two cultivars has been supplied to the Plant Breeding group at Havelock North to perform a number of new crosses.

From the data presented there is evidence of considerable variation between cultivars in their resistance to the two major fungal diseases affecting apple trees. Prima, Priscilla and Sir Prize, all bred for black spot resistance, have consistently recorded no black spot infection of foliage and fruit. Several of the older surpassed cultivars including Discovery, Laxton’s Fortune, Merton Worcester and Mother have all recorded low levels of foliar and fruit black spot infection, although their resistance to powdery mildew is quite variable. Only Prima and Discovery have displayed low incidence of both black spot and powdery mildew throughout the duration of the trial. Generally, powdery mildew levels have shown a continual increase throughout the trial period. If this trend continues, subsequent tree performance may be adversely affected. Consequently, the future performance of cultivars falling within the category of low black spot/high powdery mildew incidence, particularly those bred for disease resistance, will be monitored with interest.

Although complete resistance is preferable, cultivars with partial resistance are also of considerable potential for use in biological fruit production. These cultivars may be suitable for use in organic orchards operating a disease management regime using limited applications of permissible organic sprays. Some of the current commercial cultivars such as Braeburn, Lochbuie (a Braeburn sport) and Cox’s Orange Pippin fall within this category.

One production factor not previously discussed is that of fruit quality and consumer acceptance. Most of the cultivars in this trial displaying relatively high levels of disease freedom also tend to be older cultivars, superseded by material with improved flavour and storage characteristics. Many do not possess the fruit quality characteristics required for long term cool-storage or export marketing. Similarly, many of the newer disease resistant cultivars, although they produce fruit free of black spot, are reported to have a limited storage life (DeEll and Prange 1992) and poor eating qualities (Selby and White 1993). Consequently, most of the cultivars identified from this trial as being suitable for biological production will be limited in their suitability as export quality grade fruit. The potential of these cultivars is more likely to be in supplying local market demand for organically grown fruit or specialised niche export markets. Their suitability for various food processing applications using organically produced fruit may, however, be far less limited.

The identification of resistant cultivars with good cropping and fruit quality characteristics will provide valuable information to orchardists operating management systems with nil or low chemical pest and disease control inputs. At the 2nd International Symposium on Integrated Fruit Production, held in the Netherlands in August 1992, the selection and breeding of apples for disease resistance was given the highest priority for research and development (Wearing 1992). Ultimately, this may be the only long term solution to the elimination, or at least reduction, of fungicide applications for fungal disease control in organic orchard systems.

References

Anon, 1986. The orchard understorey: the key to sustained biological production. New Zealand & Southern Hemisphere Horticulture. 3 (11): 14.

DeEll, J.R. and Prange, R.K. 1992. Postharvest quality and storage of scab-resistant apple cultivars. HortTechnology. 2 (3): 352-358

Noiton, D., Currie, A. and Shelbourne, T. 1994. Genetic diversity for apple breeding. The Orchardist of New Zealand 67 (6): 36-39.

Noiton, D. 1994. Collecting wild apples in central Asia. The Orchardist of New Zealand 67 (7): 32-36.

Selby, H. and White, A. 1993. Breeding disease resistant apples. The Orchardist of New Zealand 66 (7): 14-16.

Virscek-Marn, M., Smole, J., Stampar, F. and Crnko, J. 1993. Results of testing the resistance of some apple cultivars to apple scab (Venturia inaequalis (Cooke) Aderh.). Zbornik Biotehniske Fakultete Univerze v Ljubljani, Kmetijstvo 61: 103-112.

Wearing, C.H. 1992. Report on the Second International Symposium on Integrated Fruit Production. HortResearch Internal Report. 41pp.