New Zealand Journal of Crop and Horticultural Science

Transcription

1 New Zealand Journal of Crop and Horticultural Science ISSN: (Print) (Online) Journal homepage: Sensitivity of apple (Malus domestica) indicator cultivars to russet ring disease, and the results of graft transmission trials of other fruit affecting disorders of apple G. A. Wood To cite this article: G. A. Wood (2001) Sensitivity of apple (Malusdomestica) indicator cultivars to russet ring disease, and the results of graft transmission trials of other fruit affecting disorders of apple, New Zealand Journal of Crop and Horticultural Science, 29:4, , DOI: / To link to this article: Published online: 22 Mar Submit your article to this journal Article views: 306 Citing articles: 3 View citing articles Full Terms & Conditions of access and use can be found at

2 New Zealand Journal of Crop and Horticultural Science, 2001, Vol. 29: /01/ $7.00 The Royal Society of New Zealand Sensitivity of apple (Malus domestica) indicator cultivars to russet ring disease, and the results of graft-transmission trials of other fruit-affecting disorders of apple G. A. WOOD The Horticulture and Food Research Institute of New Zealand Ltd Mt Albert Research Centre Private Bag Auckland, New Zealand Abstract In an evaluation in New Zealand of four Canadian apple (Malus domestica Borkh.) cultivars used as indicators for the russet ring and leaf pucker group of diseases, 'Jubilee' and 'Yellow Newtown' proved to be the most sensitive to the diseases present in New Zealand, with 'Blaxstayman' being almost as sensitive. A leaf chlorosis, and fruit russet and distortion condition of 'Fuji' apple was shown to be graft-transmissible and probably caused by a virus. The conditions causing leaf malformation and a streaked fruit russet of 'Braeburn', and a bumpy russet of 'Royal Gala' appear to be caused by genetic mutations. Of three red-skinned sports of 'Braeburn', bud-inoculated with type B russet ring, only 'Braestar' produced fruit symptoms, several seasons after bud inoculation. A test of some selected apple rootstocks from commercial nurseries indicated that they were free from type B russet ring. Keywords Malus; apple cultivars; apple indicator cultivars; russet ring disease; leaf chlorosis symptom; genetic mutation INTRODUCTION A number of diseases causing skin russeting symptoms on the fruit of apple (Malus domestica Borkh.) have been reported worldwide (Nemeth 1986). Some of the diseases have been found to be caused by viruses, but in a number of instances, although shown to be graft-transmissible, the causal H01030 Received 11 June 2001; accepted 11 October 2001 agents are still uncertain. More diseases of this nature have been found in apple trees in British Columbia, Canada, and in the Pacific north-west of the United States, than in any other fruit-growing area. In addition to symptoms in the fruit, some also have symptoms associated with foliage and blossoms (Welsh & Keane 1961; Welsh & May 1973). The relationship of a number of the diseases within this group is still uncertain but an attempt has been made to distinguish between the described types, particularly those which have been reported in British Columbia, the United States, and in New Zealand (Wood & Welsh 1989). In New Zealand, russet ring disease symptoms have been observed on orchard fruit since 1958, usually when the fruit was being graded at the packing shed (Atkinson 1971), but its occurrence in New Zealand was not reported until 1969 (Chamberlain 1969). The russet ring type disease first observed in New Zealand occurred most commonly on the cultivar 'Golden Delicious' which, at the time, was one of the most commonly grown cultivars in New Zealand (Anon. 1975). The russeting disease found affecting 'Golden Delicious' is considered to be the type B russet ring which has also been reported from Canada, United States, and Switzerland (Wood & Welsh 1989). With type B russet ring in 'Golden Delicious' there are no foliage symptoms, but fruit show russeted areas mainly in the form of narrow rings or half circles that often become broad as the fruit matures (Wood 1972a, 1979; Wood & Welsh 1989). Little was known of the incidence of this disease in New Zealand apple trees, but a visual survey of 13 blocks of 'Golden Delicious' trees in eight Hastings orchards, made by the author and staff members of the New Zealand Ministry of Agriculture and Fisheries in the late summer of 1972, showed that russet ring was widespread in the trees, although the numbers of fruit on the trees showing symptoms was low. Subsequent reports of russet ring in 'Golden Delicious' in commercial orchards showed that in most years the numbers of fruit showing symptoms was low, but in other years a considerable proportion

3 256 New Zealand Journal of Crop and Horticultural Science, 2001, Vol. 29 of the fruit was affected. The expression of symptoms is thought to be related to seasonal temperature variations, with symptoms being most pronounced when spring temperatures are below average (Wood 1972a). This has also occurred in more recent years with 'Royal Gala', in which russet ring symptoms are expressed even more conspicuously than in 'Golden Delicious' (Wood 1979), and in some seasons much of the fruit from affected trees is down-graded to process fruit or other lower paying grades (Wilton 1993). Several trials have been undertaken since the initial investigations of the russet ring group of diseases were described in New Zealand (Wood 1972a). This paper reports on: (1) the effects of three New Zealand diseases of the russet ring group on four apple indicator cultivars obtained from British Columbia, Canada; (2) a leaf chlorosis and fruit distortion condition affecting 'Fuji' apple; (3) growth stunting, leaf malformation, and russet streaking of fruit of 'Braeburn' apple; (4) bumpy russet and distortion at the calyx end of fruit of 'Royal Gala' apple; (5) the sensitivity of three red skinned sports of 'Braeburn' apple to type B russet ring; and (6) a test of some commonly used apple rootstocks for the type B form of russet ring. MATERIALS AND METHODS Investigating the effects of russet ring diseases on Canadian indicator cultivars To investigate relationships between the russet ring type diseases occurring in New Zealand and those in British Columbia and the Pacific north-west, four of the apple indicator cultivars used in Canada (Welsh & May 1967) were imported during the 1970s from the Pacific Agri-Food Research Centre of Agriculture and Agri-Food Canada, Summerland, British Columbia, for trial in New Zealand. These were 'Blaxstayman', 'Jubilee', 'Summerland Red Mclntosh', and 'Yellow Newtown'. Each of these was evaluated as an indicator for three fruit russeting diseases that had previously been found in New Zealand: type B russet ring of 'Golden Delicious'; leaf pucker and fruit russet of 'Ballarat'; and leaf flecking, bark blister, and fruit distortion of 'Granny Smith' (Wood 1972a, 1979; Wood & Welsh 1989). In this investigation, 'Malling/Merton 106' free from known virus and virus-like diseases was used as the rootstock, and the double budding technique (Posnette & Cropley 1952) used to inoculate the indicators with the sources of infection. Three replicates of each combination were planted in the field at the Mt Albert Research Centre, Auckland. In addition, two non-inoculated replicates of each indicator were planted as controls. The trees started fruiting 2 years after planting, and blossom, fruit, bark, and foliage were examined annually for 4 years. Investigation of a leaf chlorosis and fruit distortion condition of 'Fuji' apple In late spring of 1993, a row of 'Fuji' apple trees growing in a Gisborne orchard showed conspicuous distortion of the fruit, accompanied by a russet ring symptom. In addition, chlorotic areas were present on the leaves, sometimes in the form of lines or rings. About 40% of the fruit on affected tress showed symptoms. The 'Fuji' had been top-worked onto a row of 'Granny Smith' trees that, before this, had not shown any symptoms. 'Fuji' in another row, similarly top-worked to 'Granny Smith' trees originally obtained from another source, did not show any symptoms. The 'Fuji' grafted to both rows of 'Granny Smith' trees had been sourced from trees on the same orchard which had never reported any fruit quality problems. Budding scionwood was obtained in the late summer of 1994 from the 'Fuji' trees, with and without fruit and leaf symptoms, and was used as innoculum for a graft transmission trial at the Mt Albert Research Centre. Using container-grown apple seedlings under glass, two seedlings were worked with 'Fuji' buds showing symptoms, and two further seedlings were worked with buds not showing the symptoms. To determine if this condition was graft transmissible, two additional seedlings of each were worked with affected or non-affected buds, plus healthy 'Fuji', obtained from a source at Mt Albert Research Centre known to be free from virus and virus-like diseases. Two seedlings were worked with healthy 'Fuji' alone as control trees. In the following winter, the container-grown plants were transferred to the outdoors, and in spring growth was encouraged from the 'Fuji' buds. Foliage was examined for leaf symptoms from the following summer, and fruit was examined for symptoms from the time when the trees started fruiting in the 1996/97 season. Investigation of a malformation of leaves and russet streaking of fruit of 'Braeburn' apple In recent years 'Braeburn' apple has become popular with orchardists, and has been grown extensively in the apple-producing districts of New Zealand (Taylor 1995). In 1992, 4-year-old trees of 'Braeburn' growing on 'MM 106' apple rootstock in an orchard

4 Wood Sensitivity of apple cultivars to russet ring disease 257 in Hastings, exhibited abnormalities of both foliage and fruit. Leaves on these trees were small, particularly at the petiole end, and sometimes malformed with areas of chlorosis, and shoot growth was generally stunted. The fruit developed areas of russet, mainly in the form of streaks extending from the stalk to the calyx end. In 1994, the same condition was found on 'Braeburn' in several Nelson orchards, and in 1996 in an additional Hastings orchard. To determine if a graft-transmissible pathogen was involved, tests were set up with samples from both of the Hastings orchards, and from one orchard in Nelson. These were worked on container-grown apple seedlings, and the double budding technique used for testing purposes, and a similar number of trees used as in the 'Fuji' trial above. A source of 'Braeburn', free from known virus and virus-like diseases from the Mt Albert Research Centre, was used as an indicator. Investigation of bumpy russet and distortion of 'Royal Gala' apple fruit In 1993 some fruit of 'Royal Gala' apple in an orchard in Motueka exhibited russet type abnormalities. Affected fruit had conspicuous areas of raised bumpy russet at the calyx end, with severe fruit distortion. There were no symptoms on bark, foliage, or flowers. To determine if a pathogen was involved, graft transmission tests were set up with a sample of the 'Royal Gala'. These were worked on containergrown apple seedlings, and the double budding technique used for testing purposes, and a similar number of trees used as in the 'Fuji' trial above. A source of 'Royal Gala', free from known virus and virus-like diseases from the Mt Albert Research Centre, was used as an indicator. Effect of russet ring on 'Braeburn' apple Presence of latent apple virus infections in 'Braeburn' apple was first reported in 1974 (Wood 1974), and later in a number of 'Braeburn' sports which subsequently had been developed from it (Taylor 1995). These included 'Aurora' ('Joburn'), 'Braestar', 'Brillard', 'Mariri Red' ('Eve'), 'Fulford' (Robertson's strain), 'Superstar', and 'Wairarapa Red' strain. The fruit-affecting apple green crinkle disease has been known to occur in 'Braeburn' (Wood 2000), but there have been no reports of russet ring affecting it, even though it has been grafted on to trees of numerous older apple cultivars, some of which were probably russet ring infected (J. Wilton pers. comm.). Because of the current popularity of 'Braeburn' and its red-skinned sports, tests were carried out to determine if it would show symptoms when bud-inoculated with russet ring. The 'Braestar', 'Robertson', and 'Superstar' sports which had previously been heat treated for virus elimination, were available at the Mt Albert Research Centre as container-grown plants. In the spring of 1996 these were inoculated with a source of type B russet ring from 'Royal Gala' (which originated in New Zealand 'Golden Delicious') by chip-budding directly into the cultivar, two buds being placed in each tree. Two replications were made of each of the chip-budded cultivars, with a third tree of the cultivar not inoculated to serve as a negative control. Growth was encouraged from the 'Braeburn' sports in the following summer, and in the subsequent winter the container-grown plants were transferred to the outdoors. In following years, when fruiting began on the 'Braeburn' sports, they were kept under observation for fruit symptoms of russet ring. Test of apple rootstocks for type B russet ring In recent years the apple rootstocks propagated by a number of commercial nurseries in New Zealand have been tested for the presence of a number of virus and phytoplasma diseases (Wood 1996). They were not tested for russet ring as it was considered its presence in the rootstock would have resulted in symptoms being expressed when healthy material of the sensitive 'Royal Gala' was worked on them. However, to reassure nurserymen and orchardists that russet ring was not present in the rootstocks, tests for this disease in an elite collection of selected rootstocks (Mannering 1996) started in Three examples of rooted stoolbed rods of the rootstocks 'Mailing 9' (from two nursery sources), 'Mailing 26' (from one source), 'Merton 793' (from two sources), and 'Malling/Merton 106' (from two sources) were planted in containers of soil at the Mt Albert Research Centre in the winter of In spring, the container-grown plants were transferred to the glasshouse, and when growth began they were grafted with 'Golden Delicious' (free from known virus and virus-like diseases) as the indicator for the type B form of russet ring. Growth was encouraged from the 'Golden Delicious' grafts, and in the following winter the container-grown plants were returned to the outdoors. Observations for the presence of russet ring began as soon as the 'Golden Delicious' started to fruit.

5 258 New Zealand Journal of Crop and Horticultural Science, 2001, Vol. 29 It Fig. 1 A, Semi-mature fruit of 'Jubilee' apple (Malus domestica) showing russeted areas and rings of russet following inoculation with 'Golden Delicious' type B form of russet ring; B, the same on 'Yellow Newtown' fruit; C, ring and line patterns on leaves of 'Jubilee' following inoculation with 'Golden Delicious' russet ring; D, 'Jubilee' fruit showing russet and distortion following inoculation with the 'Granny Smith' disease; E, 'Jubilee' leaves showing chlorotic marks and distortion following inoculation with the 'Granny Smith' disease (healthy leaf on left); and F, russet ring on immature 'Braestar' fruit.

6 Wood Sensitivity of apple cultivars to russet ring disease 259 B Fig. 2 A, Immature 'Fuji' apple (Malus domestica) fruit with distortion and leaves with chlorosis; B, 'Fuji' leaves showing lines and areas of chlorosis (healthy leaf on left); C, foliage of 'Braeburn' showing malformed, strap-like leaves (centre and right), healthy foliage (left); D, 'Braeburn' fruit with lines of russet from stalk to calyx-end; E, 'Braeburn' fruit symptoms in the orchard; and F, 'Royal Gala' fruit with calyx-end russet (centre and right), healthy fruit (left).

7 260 New Zealand Journal of Crop and Horticultural Science, 2001, Vol. 29 RESULTS Russet ring type diseases on Canadian indicator cultivars The russet ring obtained from New Zealand 'Golden Delicious' trees caused extensive russeting on three of the indicator cultivars 'Blaxstayman', 'Jubilee', and 'Yellow Newtown' (Fig. 1 A,B). Russet on the fruit was often, but not always, in the form of rings or half circles. The fruit of 'Summerland Red Mclntosh' showed no symptoms, even though successful growth unions had been obtained with the innoculum buds. For the duration of the trial there were no bark, foliage, or blossom symptoms on any indicator cultivars (Table 1). However, in later years, a single container-grown plant of 'Jubilee' with 'Golden Delicious' russet ring, retained as a diseased stock plant, produced pale green ring and line patterns on leaves in early summer (Fig. 1C). With the leaf flecking, bark blister, and fruit distortion obtained from 'Granny Smith', extensive areas of russet, sometimes in the form of rings, affected the fruit which was sometimes also distorted, and occurred on 'Blaxstayman', 'Jubilee' (Fig. ID), and 'Yellow Newtown'. Bark blistering occurred on 'Blaxstayman' and 'Yellow Newtown'. 'Jubilee' showed conspicuous chlorotic marks on the leaves with some leaf distortion (Fig. IE), and these were present but less evident on 'Blaxstayman' and 'Yellow Newtown'. On 'Summerland Red Mclntosh', symptoms occurred only on the leaves (Table 1). There were no blossom symptoms on any of the indicator cultivars. The leaf pucker and fruit russet of 'Ballarat' caused symptoms only on 'Yellow Newtown', as areas of rough russet on some fruit. There were no bark, foliage, or blossom symptoms on any of the indicator cultivars (Table 1). There were no bark, foliage, blossom, or fruit symptoms on any of the non-inoculated control trees. Leaf chlorosis and fruit distortion of 'Fuji' On the trees propagated from the affected Gisborne 'Fuji' trees, fruit did not form till the 1996/97 season, and in that season and subsequently, no fruit symptoms were seen. On the 'Fuji' trees grown as double-bud inoculated trees, in the 1996/97 season some fruit showed small, irregular, russeted areas, but did not show the distortion that had occurred on the fruit of the original orchard trees (Fig. 2A). There were no leaf symptoms in the 1994/95 and 1995/96 season on trees propagated from the affected Gisborne trees, but in the 1996/97 season and later in the 1999/2000 and 2000/01 seasons the leaves in early spring showed irregular chlorotic marks, sometimes in the form of lines or small ringspots, with some marginal leaf distortion (Fig. 2B). Similar symptoms were observed on leaves of the budinoculated healthy 'Fuji' trees grown as double-bud inoculated trees. Symptoms were still present, but less evident, by midsummer. There were no bark or Table 1 Effects of three diseases of the russet ring group on fruit, bark, and foliage of four Canadian apple (Malus domestica) indicator cultivars. Cultivar Source of isolate Disease Golden Delicious Type B russet ring Granny Smith Ballarat Leaf fleck, bark blister, and fruit distortion Leaf pucker and fruit russet Blaxstayman Extensive russet ring, though not always in form of rings Extensive russet sometimes in form of rings with some fruit distortion; some bark blister; some leaf chlorosis No symptoms Jubilee Conspicuous russet ring Severe russet and distortion; conspicuous chlorotic marks on leaves No symptoms Inoculated cultivar Summerland Red Mclntosh No symptoms Conspicuous light yellow leaf markings No symptoms Yellow Newtown Large russet rings and irregular areas of raised russet Extensive russet sometimes in form of rings with some fruit distortion; extensive bark blister; leaves with chlorotic marks and margins uprolled Rough russeted areas on some fruit

8 Wood Sensitivity of apple cultivars to russet ring disease 261 blossom symptoms. The 'Fuji' trees propagated from the trees at Gisborne showing no symptoms similarly did not show symptoms at Mt Albert Research Centre, nor were symptoms evident where these had been bud-inoculated to healthy 'Fuji' indicator trees. Leaf malformation and fruit russet streaking of 'Braeburn' The samples of this condition obtained from two 'Braeburn' sources in Hastings, and one source in Nelson, when propagated on apple seedlings at Mt Albert Research Centre, all showed the same foliage symptoms. All showed symptoms in the first season after propagation (Fig. 2C), though there was some variation in symptom expression, with some shoot growth displaying typical symptoms, and other shoots apparently normal growth. Symptoms were most pronounced in spring, but were still evident, though less conspicuous, by midsummer. Only the trees propagated from Hastings in 1994 have produced fruit, and these showed the typical lines of russet from stalk to calyx end (Fig. 2D), similar to those observed on the orchard trees (Fig. 2E). It was noted that, late in the season as the fruit matured, severely russeted areas became depressed into the fruit skin and developed narrow black margins. The streaks of russet often coalesced so that the normal light and dark red-coloured skin became enclosed as islands. No symptoms were observed on bark or blossoms. Where budtransmission of the condition to healthy 'Braeburn' was attempted, in no instances did the budinoculated healthy 'Braeburn' trees show foliage or fruit symptoms. Bumpy russet and fruit distortion of 'Royal Gala' All fruit which have so far developed on the 'Royal Gala' propagated on apple seedlings at Mt Albert have shown the typical calyx end russet, sometimes accompanied by severe distortion (Fig. 2F). No other bark, foliage, or blossom symptoms have been observed. Where bud transmission of the condition to healthy 'Royal Gala' was attempted, in no instances did the bud-inoculated healthy 'Royal Gala' trees show the fruiting symptom. Effects of russet ring on 'Braeburn' The chip-bud inoculated red-skinned sports of 'Braeburn' started fruiting in the 1997/98 season, and were examined for fruit symptoms from that time. In the seasons up to 1999/2000, the three red sports produced a small number of fruit, none of which showed any russet ring symptoms (Table 2). However, in the 2000/01 season, several of the fruit of 'Braestar' showed russet ring symptoms, in the form of rings or russeted areas (Fig. IF). No russet ring symptoms occurred on fruit of the 'Robertson' and 'Superstar' sports in the 2000/01 season, nor did any symptoms occur on fruit of the non-inoculated control trees. Test of apple rootstocks for type B russet ring The 'Golden Delicious' which had been grafted on to the 'M9\ 'M26', 'Merton 793', and 'MM 106' rootstocks started fruiting in the 1996/97 season. Fruit was examined from then till the 1999/2000 season. No symptoms of russet ring were found on any of the fruit from these trees during this time. Table 2 Sensitivity of red-skinned sports of 'Braeburn' apple (Malus domestica) to type B russet ring. Observations in four seasons of fruiting (+, russet ring symtom present; -, russet ring symptom not present; 0, no fruit produced). Braeburn sport Braestar Robertson Superstar Season 1997/ / / /01

9 262 New Zealand Journal of Crop and Horticultural Science, 2001, Vol. 29 DISCUSSION Apple russet ring is a disease which in the past, although observed to be common in 'Golden Delicious', was not regarded as of great significance in New Zealand. However, with the introduction of the 'Royal Gala' cultivar and its subsequent popularity with New Zealand orchardists, russet ring became of increased importance because of the sensitivity in some seasons of this cultivar to the disease. It now seems likely that most, if not all, of the 'Golden Delicious' trees grown in the past were infected with russet ring, and it is possible that it was present in the cultivar when it was first introduced to New Zealand. 'Golden Delicious' originated as a chance seedling in West Virginia, United States, in 1890, and was listed in a New Zealand nursery catalogue by 1930 (Ward 1995). By 1952, it was being propagated on several rootstocks by the New Zealand Fruitgrowers Federation Nursery at Levin (Anon. 1952), becoming the fourth most widely planted cultivar by 1968 (Anon. 1970). Russet ring appeared to be so widespread in 'Golden Delicious' that at one stage it was thought that natural spread of the disease could be occurring, as there was some evidence of natural spread of type A russet ring in British Columbia, Canada (Welsh et al. 1963). However, in a trial of 'Golden Delicious' grown on the former HortResearch orchard at Appleby, Nelson, New Zealand, to determine the effects of the apple rubbery wood disease on this cultivar (Wood & Cassidy 1978), trees infected with this disease were found also to be infected with the type B form of russet ring. No spread of this russet ring occurred from these trees to adjacent healthy 'Golden Delicious' control trees in the 12 years the trial was in operation (Wood pers. obs.). These healthy control trees had been propagated from 'Golden Delicious' freed from russet ring by heat therapy. The other cultivar most adversely affected with type B russet ring in New Zealand, 'Royal Gala', originated as a red-fruited sport on trees of the standard 'Gala' growing on a Matamata orchard, and which was made available for commercial use in 1974 (Congdon 1974). 'Royal Gala', similarly freed from virus infection by heat therapy, has continued to remain free from russet ring in the orchard. In investigations of russet ring in some Northern Hemisphere countries, it has been suggested that the causal agents of many russet ring disorders are specific isolates of Apple chlorotic leaf spot virus (ACLSV) (van der Meer 1986; Desvignes & Boye 1988; Hansen & Parish 1990). In the Netherlands, van der Meer (1986) demonstrated that distinctly different strains of ACLSV could be isolated from the same tree, and that some of them induced russet ring and leaf flecking or leaf puckering on the cultivars 'Belle de Boskoop', 'Golden Delicious', and 'Lord Lambourne'. The strain of ACLSV which causes chlorotic blotches on the leaves of the apple indicator R A and line patterns on Mains platycarpa Rehd. was found to be present in all of 29 old apple cultivars tested in New Zealand (Wood 1974), and these included those with the russet ring type disease. In some seasons a number of the older cultivars have shown russet ring symptoms on the fruit ('Dougherty', 'Giant Jeniton', 'Granny Smith', 'Jonathan', 'RomeBeauty', 'Salome', 'Statesman', 'Stayman Winesap', and ' Sturmer Pippin' (Wood 1972a). Consequently it is possible that the strain or strains of ACLSV commonly present in New Zealand apple trees can induce fruit symptoms when seasonal temperatures are particularly favourable. The type B russet ring found in New Zealand 'Golden Delicious' may be a distinctive strain of ACLSV. Although standard 'Gala' was ACLSV infected (Wood 1972b, 1974), the 'Royal Gala' redfruited sport obtained from it did not display conspicuous symptoms of russet ring until topworked on to russet ring infected 'Golden Delicious'. Subsequent heat therapy eliminated russet ring and latent virus infections from 'Royal Gala' (Wood & Tustin 1989). Using a combination of heat treatment and tip grafting, in comparison with some other viruses, ACLSV was found to be fairly easily eliminated from infected apple cultivars (Wood 1973), and at a later date, so was type B russet ring, further suggesting a close relationship between the two diseases. The results of the trial evaluating the Canadian indicator cultivars (Table 1) further indicated that the russet ring found in New Zealand 'Golden Delicious' and 'Royal Gala' apple trees is the type B russet ring, as inoculated "Blaxstayman', 'Jubilee', and 'Yellow Newtown' all produced russet ring symptoms on their fruit, and in the initial trial did not produce any leaf symptoms. The leaf symptoms later found on the single plant of 'Jubilee' (Fig. 1C) appear to be the same as those photographically illustrated for 'Jubilee' ring and line pattern disease in Canada (Welsh & May 1967). This would suggest that the 'Golden Delicious' infected with type B russet ring in New Zealand may also be infected with the ring and line pattern strain of russet ring, but only display leaf symptoms in seasons when

10 Wood Sensitivity of apple cultivars to russet ring disease 263 environmental and growing conditions are particularly suitable. It is of interest that when the Canadian isolate of 'Golden Delicious' russet ring (which induced fruit but no leaf symptoms on 'Golden Delicious') was tested in British Columbia, it did not cause any symptoms on the fruit of 'Blaxstayman', 'Jubilee', or 'Yellow Newtown' (Welsh & May 1967). Differences between the environmental growing conditions of the two countries may have been responsible for this, or it could be that there are differences in the two isolates. All three of the indicator cultivars were free from latent virus infections when introduced to New Zealand (HortResearch Plant Quarantine records), whereas 'Blaxstayman' and 'Yellow Newtown' used in the British Columbia tests were infected, which could have caused symptom suppression by virus interference (Welsh & May 1967). However, this would not explain why 'Jubilee' did not show the russet ring fruit symptom in British Columbia, as it was free from latent virus infections. The 'Granny Smith' leaf fleck, bark blister, and fruit distortion disease induced conspicuous fruit symptoms on three of the indicator cultivars in New Zealand, and in addition 'Blaxstayman' and 'Yellow Newtown' showed bark blister, and 'Jubilee', 'Summerland Red Mclntosh', and 'Yellow Newtown' conspicuous leaf markings. These symptoms do not closely correlate with any described overseas, and the disease would appear to be a distinctive member of the russet ring group. The Canadian indicator cultivars were least sensitive to the 'Ballarat' leaf pucker disease, with only 'Yellow Newtown' producing fruit symptoms. Of the four indicator cultivars, 'Jubilee' and 'Yellow Newtown' proved to be the most sensitive to the russet ring diseases present in New Zealand, with 'Blaxstayman' being almost as sensitive. 'Summerland Red Mclntosh' was the least effective as an indicator. The discovery of the disease affecting the 'Fuji' apple cultivar is of interest, as it is the first new grafttransmissible disease of a virus or virus-like nature to be found in New Zealand apple trees for 25 years. The causal agent has not yet been identified but ringspots and lines of chlorosis on the leaves indicate that a virus is probably involved, and it is possible that it is a strain of ACLSV. Both affected and nonaffected 'Fuji' trees from the Gisborne orchard were found to be additionally infected with apple mosaic virus and the latent viruses stem pitting, scaly bark, and the strain of ACLSV commonly occurring in New Zealand apple trees. In character, the 'Fuji' disease could be grouped with the Canadian 'Jubilee' ring and line pattern disease, as the symptoms on leaves are fairly similar. However, the 'Fuji' disease induces fruit distortion symptoms, whereas the 'Jubilee' disease in Canada does not (Welsh & May 1967). Its present known low incidence in New Zealand means it is of low importance, but it could be of more significance if more infected trees are found in commercial orchards. Assuming that all of the russet ring disorders found in New Zealand apple trees are related, at least six distinct strains or forms appear to be present. These are: (1) the "common" form of ACLSV found in most old apple trees, inducing leaf symptoms only on specific apple indicator cultivars (Wood 1974), and only occasionally inducing russet ring symptoms on cultivar fruit; (2) the type B russet ring inducing russet ring symptoms on fruit of 'Golden Delicious' and 'Royal Gala' in the orchard, and 'Blaxstayman','Jubilee', and 'YellowNewtown' by bud inoculation, but no leaf symptoms; (3) the 'Jubilee' ring and line pattern on leaves form; (4) the 'Granny Smith' leaf fleck, bark blister, and fruit distortion form; (5) the 'Fuji' leaf chlorosis and fruit distortion form; and (6) the 'Ballarat' disease which occurs uncommonly, and primarily as a leaf pucker symptom. With the foliage malformation and fruit russeting condition of the cultivar 'Braeburn', the lack of transmission of these symptoms to healthy 'Braeburn' indicates that the condition is a genetic mutation rather than having a pathogen as the causal agent. The condition has been of concern to orchardists with affected trees. However, as usually only parts of the tree are affected, it may be possible to remove these sections, and still obtain a normal crop from the remainder of the tree. The condition causing bumpy russet and distortion of 'Royal Gala' fruit also appears to be a genetic mutation, and because of its low incidence is of minor importance. Until recently, it was thought that 'Braeburn' apple did not show visible symptoms of type B russet ring on its fruit as symptoms had not been reported on the fruit from commercially grown trees. The appearance of symptoms on the fruit of 'Braestar' in the 2000/01 season on trees which had been bud-inoculated with russet ring, shows that this can occur, but this is unusual and 'Braeburn' and its sports would not, in general, appear to be sensitive to the disease. The absence of russet ring symptoms on the fruit of 'Golden Delicious' grafted on to the selected

11 264 New Zealand Journal of Crop and Horticultural Science, 2001, Vol. 29 apple rootstocks indicates, as has been surmised, that the type B russet ring does not occur in apple rootstocks in New Zealand, and that all spread of infection in the past has been the result of propagation practices involving infected cultivars. ACKNOWLEDGMENTS Pacific Agri-Food Research Centre of Agriculture and Agri-Food, Canada, Summerland Research Station, British Columbia, for supply of apple russet ring indicator cultivars; John Wilton, AgFirst Consultants Hawkes Bay Ltd, for supply of 'Fuji' apple scionwood from affected and non-affected trees, and observation on the absence of russet ring in commercially grown 'Braeburn' apple trees; Mark Parkinson of the former New Zealand Apple and Pear Marketing Board, Hastings, for photograph of 'Braeburn' fruit with lines of russet on an orchard tree, and for supply of affected scionwood; Clive Cook, Agriculture New Zealand, Nelson, for supply of affected 'Braeburn' apple scionwood; Steven Spark, Agriculture New Zealand, Motueka, for supply of affected 'Royal Gala' apple scionwood; Fruit Tree Growers Group, New Zealand Nurserymen's Association, for supply of apple rootstocks for testing. REFERENCES Anonymous 1952: Fruit trees from the New Zealand Fruitgrowers' Federation Nursery, Fairfield Road, Levin. (Advertisement). The Orchardist of New Zealand 25 (December): 23. Anonymous 1970: Schedule of apple varieties with over 5000 trees. In: Report on the official survey of the fruit-growing industry of New Zealand Wellington, The New Zealand Fruitgrowers' Federation Limited. Pp Anonymous 1975: Apples 1973: main varieties: New Zealand trees and area by age groups and relative varietal importance. In: Report on the official survey, New Zealand fruitgrowing industry Wellington, The New Zealand Fruitgrowers' Federation Limited. Pp Atkinson, J. D. 1971: Russet ring of apple. Pp in: Diseases of tree fruits in New Zealand. New Zealand Department of Scientific and Industrial Research Information Series p. Chamberlain, E. E. 1969: Virus diseases. In: Plant pathology in New Zealand. Review of Applied Mycology 48: 6-7. Congdon, N. B. 1974: Red sport of Gala. District reports on new apple varieties: Hawkes Bay. The Orchardist of New Zealand 47 (November): 335. Desvignes, J. C.; Boye, R. 1988: Different diseases caused by the chlorotic leaf spot virus on the fruit trees. Acta Horticulturae 235 (Fruit tree virus diseases): Hansen, A. J.; Parish, C. L. 1990: Transmissible fruit disorders. Pp in: Jones, A. L.; Aldwinkle, H. S. ed. Compendium of apple and pear diseases. APS Press, The American Phytopathological Society. 100 p. Mannering, R. 1996: Nurseries in quality scheme. The Orchardist of New Zealand 69 (March): Meer, F. A. van der 1986: Observations on the etiology of some virus diseases of apple and pear. Acta Horticulture 193 (Fruit tree virus diseases): Nemefh, M. 1986: Apple russet ring. Pp in: Virus, mycoplasma and rickettsia diseases of fruit trees. The Netherlands, Martinus Nijhoff Publishers. 841 p. Posnette, A. F.; Cropley R. 1952: The rubbery wood virus and apple propagation. In: Annual Report, East Mailing Research Station, Pp Taylor, M. 1995: The blooming of 'Braeburn'. The Orchardist of New Zealand 68 (May): Ward, G. 1995: Varieties of fruit; apples. Pp in: Early fruitgrowing in Canterbury, New Zealand. Christchurch, New Zealand, The Caxton Press. 168 p. Welsh, M. F.; Keane, F. W. L. 1961: Apple leaf pucker and associated fruit disorders. Pp in: Virus diseases of apples and pears. Technical Communication No. 30. England, Commonwealth Bureau of Horticulture and Plantation Crops. 141 p. Welsh, M. F.; Keane, F. W. L.; May, J. 1963: Apple virus diseases in British Columbia: evidence of natural spread. Phytopathologia Mediterranea 2: Welsh, M. F.; May, J. 1967: Virus etiology of foliar veinflecking or ring pattern, and fruit russeting or blotch on apple. Canadian Journal of Plant Science 47: Welsh, M. F.; May, J. 1973: Blossom symptoms associated with three virus diseases of apple. Plant Disease Reporter 57: Wilton, J. 1993: Going orcharding factors you should consider. The Orchardist of New Zealand 66 (June): Wood, G. A. 1972a: Russet ring and some associated virus disorders of apple (Malus sylvestris (L. Mill.) in New Zealand. New Zealand Journal of Agricultural Research 15:

12 Wood Sensitivity of apple cultivars to russet ring disease 265 Wood, G. A. 1972b: Overcoming apple virus disease problems in New Zealand. The Orchardist of New Zealand 45 (March): Wood, G. A. 1973: Application of heat therapy for the elimination of viruses from pip and stone fruit trees in New Zealand. New Zealand Journal of Agricultural Research 16: Wood, G. A. 1974: Latent viruses of apple in New Zealand. New Zealand Journal of Agricultural Research 77: Wood, G. A. 1979: Apple russet ring and associated disorders. Pp in: Virus and virus-like diseases of pome fruits and stone fruits in New Zealand. New Zealand Department of Scientific and Industrial Research Bulletin p. Wood, G. A. 1996: Past and present status of virus and phytoplasma diseases in apple rootstocks in New Zealand. New Zealand Journal of Crop and Horticultural Science 24: Wood, G. A. 2000: Determining whether natural spread of apple green crinkle disease occurs, its absence from New Zealand clonal apple rootstocks, and the sensitivity of some new cultivars. New Zealand Journal of Crop and Horticultural Science 28: Wood, G. A.; Cassidy, D. C. 1978: Effect of rubbery wood disease on growth and yield of Golden Delicious apple trees. The Orchardist of New Zealand 51: Wood, G. A.; Tustin, D. S. 1989: Virus-free apples the benefits. The Orchardist of New Zealand 62 (October): Wood, G. A.; Welsh, M. F. 1989: Apple leaf pucker and some lesser known bark, blossom and fruit disorders. Pp in: Virus and virus-like diseases of pome fruit and simulating non-infectious disorders. SP0003. Washington State University, Washington. 330 p.