The natural resistances of apple and pear trees to insects and diseases have had a major impact on the New Zealand pipfruit industry. For example, in the late 19th century, the apple industry was doomed due to the devastating effect woolly apple aphid (WAA) had on the trees. However, in 1880 it was shown in New Zealand that Northern Spy, a cultivar introduced from North America, was resistant to this pest. It became very popular as a rootstock and by the turn of the century, New Zealand nurserymen had developed a thriving export trade by shipping nursery trees on blight proof Northern Spy rootstocks to Australia, South Africa, Argentina and Chile. This variety still has a major influence on apple growing in New Zealand through the use of WAA resistant rootstocks derived from Northern Spy.

Resistance of apple and pear to pests and diseases is dependant on the interaction between the host and the pathogen as influenced by environmental conditions. The health of a tree, which is influenced by the orchard location, soil conditions, tree nutrition and cultural practices, determines its susceptibility, while the amount and virulence of the pathogen or pest influence their pathogenicity.

This article focuses on the resistance side of the HortResearch pipfruit rootstock breeding programme, involving WAA, crown rot and fire blight. In a further article the need to breed new pipfruit rootstocks is discussed.

Woolly apple aphid Eriosoma lanigerum
WAA is easily recognized by the woolly appearance of the waxy exudation covering the purple-red aphids. Above ground the insects thrive best under warm, humid and calm conditions, avoiding direct sunlight. While feeding on branches, twigs, shoots and roots, the aphids inject toxins, causing the formations of galls (Fig. 1). Often the bark tears, which gives the insects easier access to feed, and also allows secondary infections to occur. As a result the trees grow slowly and produce small fruit.

Fig. 1 Photo by: Peter Spring
Woolly apple aphids have caused the formation of galls while feeding on a susceptible rootstock seedling.

The commonly used WAA resistant rootstocks in New Zealand, M793 and MM106, are both direct descendents from Northern Spy. M793 was selected from a M2 x Northern Spy cross, made at the John Innes Institute at Merton in Great Britain at the beginning of this century. M2 was one of the Malling rootstocks (M-series), which performed well in Europe, where WAA was less of a problem due to the colder winters destroying the aphids. In 1922, this was followed by a larger cooperative programme between John Innes Institute and East Malling Research Station to breed more WAA resistant rootstocks with good horticultural characteristics. Northern Spy was crossed with Malling rootstocks and some other resistant cultivars (Ben Davis, Winter Majetin and M793), which resulted in the Malling-Merton (MM) series. The strong resistance from Northern Spy was found to be simply inherited, which makes it an excellent parent for use in a breeding programme. Many other sources of WAA resistance are known, in both dessert apples (Malus domestica), eg Carrington Red, Irish Peach, Ivory's Double Vigour, Jonathan, Duchess of Oldenburg, and the crabapples, eg Malus fusca, M. halliana, M. hupehensis, M. baccata genuina and M. robusta R5 (Robusta 5). Some of them have been used in rootstock breeding programmes.

Seedling populations can be screened for WAA resistance at an early stage in the greenhouse or the nursery by placing aphids or a short piece of infected shoot at the base of the rootstock seedlings. These methods are preferred to natural infestation in the field, which is time consuming and unreliable due to seedlings that escape infestation.

Crown rot Phytophthora cactorum
Phytophthora cactorum is a fungal disease that can infect both the roots and the above ground parts of the apple tree. In the first case it is referred to as crown rot, in the latter case as collar rot or aerial phytophthora. Crown rot is also caused by other Phytophthora species, especially P. megasperma, while in Japan Phytophthora cambivora is a major cause of the disease.

Phytophthora is a soil fungus that enters the tree through wounds in the roots or tree parts close to the ground. Trees are most susceptible to infection in springtime and when planted in poor draining soils. The bark and phloem turn dark brown and die, and in advanced stages the disease spreads into the xylem. The disease often is not noticed until the stem is nearly girdled and the tree shows yellowing of the leaves in late spring or purpling in late summer, while the fruits remain small and become highly coloured. Complete girdling leads to tree death. Therefore it is important that collar rot infections are recognized at an early stage by the presence of slightly sunken bark tissue just above the bud union. On cutting, the stem will show a distinct edge between the healthy and affected tissue in inactive lesions (Fig. 2), but in active lesions the discoloration changes from brown passing through orange merging imperceptibly into the healthy bark.

Fig. 2. Photo by: Peter Spring
On cutting of the stem, collar rot is easily recognized by the brown colour of the affected tissue.

Again Northern Spy was found to be highly resistant to the disease, which explains why many rootsocks with this cultivar as a parent, are resistant to both WAA and collar rot. Other resistant apple cultivars are (Red) Torstein, Woolbrook Russet, Royal Jubilee, Wagener and King's Acre Bountiful. Valuable sources of crown rot resistance for apple rootstock breeders are some of the dwarfing rootstocks (M9, M27 and Ottawa 3), and some crabapples, eg Novole (M. x sublobata), M. angustifolia and M. x magdeburgensis, which transmit high levels of resistance. In Japan M. prunifolia and M. sieboldii were found highly resistant, while from a M. prunifolia x M9 cross ten dwarfing rootstocks, Morioka #1 to #10, were selected, most of them highly resistant to crown rot.

Screening for crown rot resistance is usually done by flooding 10-day-old seedlings with zoospores of pathogenic isolates of P. cactorum. Only the most resistant seedlings are expected to survive the application of the fungus at high inoculum concentration and favourable conditions for the fungus, although with this method escapes are possible. Since other species of Phytophtora can cause crown rot, these are often included in the test. The flooding method is unreliable when applied to mature trees. An indication of their resistance is provided by using the excised shoot method, where pieces of shoot are tested in vitro by placing shoot cuttings in a culture of the fungus.

Fire blight Erwinia amylovora
Although resistance to fire blight is more important in scion cultivars, it is a desirable character particularly in pear rootstocks. Since the commonly used quince rootstocks are very susceptible, there is a need for resistant selections to prevent the disease spreading in the nurseries.

The bacterial disease fire blight, indigenous to the state of New York (USA), derived its name from the symptoms it causes on trees, which is blackening of the leaves as if they were scorched by fire (Fig. 3).

Fig. 3. Photo by: Peter Spring
A susceptible pear seedling showing typical symptoms of fire blight.

Often the blossoms are the first point of entrance for the disease, from where it progresses into the spurs, branches and the stem of the tree, forming cankers and finally leading to tree death. Affected shoots are often easily recognized by their characteristic bent shape, the "shepherd's crook". Infected areas become water-soaked and in warm, humid weather often show droplets of bacterial ooze. From here the bacteria are dispersed by rain, wind, birds and most importantly by insects to other trees. Aphids, flies and leafhoppers have been associated with the disease transmission, but especially bees are very efficient disseminating agents for fire blight. The bacteria can enter the tree through any wound caused by insects, weather (e.g. wind and hail) or pruning.

In apple particularly (Red) Delicious and, to a lesser extent, many of the scab-resistant cultivars (Liberty, Priam, Prima, Priscilla, Redfree and Sir Prize) are known to withstand fire blight infection. Rootstocks M7, Novole and Robusta 5 are highly resistant, and most of the MM rootstocks lightly to moderately susceptible (Table 1). In pear Pyrus ussuriensis, P. calleryana, the cultivars Oregon 18 and Old Home (OH), and many seedlings of an OH x Farmingdale cross appear highly resistant. In contrast, most of the common dessert pear cultivars (Bartlett, Beurré Bosc, Beurré d'Anjou, Doyenné du Comice, Packham's Triumph and Winter Nelis) and rootstocks (Quince A and C) are highly susceptible to fire blight. The same applies for the popular cultivars of Asian pear, with Hosui and Nijisseiki being susceptible, and Kosui showing intermediate resistance.

Table 1. Pest and disease resistant apple rootstocks.

Rootstock	WAA	Collar rot	Fire blight	Apple scab	Mildew
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Alnarp 2	-	-	--	+	-
Aotea 1	+	+
Bemali	-		+
Budagovsky 9	--	+	+	+	-
Budagovsky 490	-	+	+	+	-
Geneva 65	-	+	+		+
M2	-	+	+	+	+
M4	-	+	+	+	+
M7	-	+	+	+	+
M9	--	+	--	+	+
M13	-	+	+	+	+
M25	+	+	-
M26	--	+	--	+	+
M27	-	+	-	+	+
M793	++	+
Mark (MAC9)	--	+	--
MM106	++	-	+	+	-
MM111	++	+	+	+	-
Novole	+	+	+	+	+
Ottawa 3	--	+	-	+	+
P1	-	+	-		+
P2	-	+	-		+
P18	-	+	+		+
P22	--	+	-		+
Robusta 5	++	+	++	+	+
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++ very resistant
+ resistant
+ intermediate
- susceptible
-- very susceptible

Rootstocks usually increase scion susceptibility to fire blight, eg by promoting early flowering in the life of the tree and increasing total flower numbers per tree, which creates more infection sites for the disease.

To screen seedling populations for fire blight resistance, the young plants are inoculated by cutting the youngest fully developed leaf with scissors dipped in a bacteria suspension. With mature trees bacteria suspension is injected in the shoot tips with a hypodermic needle. In both cases the seriously affected plants with lesions of over 30% of the shoot length are discarded. To prevent indirect infection of the rootstocks, they are screened against susceptibility to WAA and against tendency to suckering.

Other resistances
By planting virus-free trees, viruses have become a minor problem in orchards. To prevent reinfection of the tree from the rootstock, it is desirable that rootstocks are free of the common viruses, chlorotic leaf spot virus (CLSV), apple stem-grooving (ASGV) and apple stem-pitting (ASPV). Many of the older rootstocks such as M7, M9, M25 and Budagovsky 9 (Bud9) are known to be systemically infected by these viruses without showing any symptoms. A simple test of screening rootstocks for viruses is to bud them with a susceptible cultivar. In the same way new rootstocks can be screened for resistance to viruses. However, tolerant rootstocks still have to be virus-free if they are grafted with susceptible scion varieties.

In the 1970s the Aotea programme was started in New Zealand to select apple rootstocks with resistance to root canker, which is a decline and replant disease of apple, causing severe problems in the Nelson, Marlborough and Otago regions. The main basidiomycete fungus implicated with this disease is Peniophora sacrata, to which none of the commonly used rootstocks showed acceptable levels of resistance. Over 4600 plants were tested for resistance, of which finally nine with good horticultural characters, all of the species M. sieboldii, were selected. The most promising one was named Aotea 1 and released as a commercial rootstock.

Overseas research has been carried out on resistances to locally occurring diseases and pests in rootstocks, including voles. Novole, for example, is highly resistant to the pine vole. Sugar Crab, M. honahensis and M. x micromalus are resistant to the meadow vole.

Although they are of lesser importance for rootstocks, resistance to apple scab and powdery mildew, characters that many rootstocks possess to some extent (Table 1), are an advantage for growing apple trees in the nursery. More important is the effect of rootstocks on the susceptibility of the scion, as described for fire blight. For example, Cox's Orange Pippin were found to be more susceptible to powdery mildew and apple scab on M1 than on M13 and M15, which warrants further research.

Breeding resistant pipfruit rootstocks in New Zealand
The apple rootstock germplasm collection in New Zealand contains old (M- and MM-series), as well as recently selected rootstocks. New apple rootstocks, for example, are the Cornell-Geneva (CG) series from the USA and the AR-series from East Malling, of which some selections have been imported and others are nominated to be imported. Most of these selections are reported to carry at least two of the three resistances the New Zealand rootstock breeding programme is focusing on: to WAA, crown rot and fire blight. These selections are being tested for their resistances under New Zealand conditions.

Currently in the apple rootstock breeding programme a range of material is studied, including the New Zealand A-series selections from the Aotea rootstock programme. This series has been selected for their resistances to WAA, Phytophthora and root canker, but little is known on the genetics of these resistances. Seedlings of a cross between M9 and Aotea 1 have been observed, and selections have been made for further trialling. Also new populations have been developed and the range of resistance tests will be extended to include screening for fire blight.

The New Zealand pear rootstock germplasm consists mainly of a few fire blight susceptible quince clones and many seedlings of vigorous Pyrus species, of which some carry fire blight resistance. In the past growers have used, for example, fireblight resistant P. ussuriensis seedlings. However, the pears grown on these rootstocks appeared to be susceptible to black end disease, a condition that leaves the calyx of the fruit hard and black. Future pear rootstocks therefore will be selected for their fire blight resistance and avoidance of black end.

The rootstock breeding programme is linked with the HortResearch biotechnology programmes involving the identification and transformation of genes that express desirable traits in horticultural crops. Particularly pest and disease resistance genes are targeted in speeding up the breeding process.

Acknowledgements
This research is funded by the Foundation for Research, Science & Technology and the New Zealand Apple & Pear Marketing Board.

Source:
Bus, V. 1994. Pest and disease resistance in pipfruit rootstocks. The Orchardist 67(9): 57-60.