Why?
Breeding pest and disease resistant pipfruit varieties is regarded as the only longterm solution to reduction and elimination of fungicide use. Reduction and elimination is necessary to meet the decreasing residue tolerance levels on imported fruit set by overseas markets. Consumers in these same markets are demanding pesticide-free fruit.

Another limitation of relying on the use of pesticides is the decrease in the number of these chemicals available to the growers. Due to increasingly stringent registration conditions many existing chemicals are withdrawn, and only few new compounds become registered. A clear advantage to the growers in using resistant cultivars is the reduction in production costs by saving time and money otherwise spent on spraying. This is a genuine contribution to the enhancement of a the green image of New Zealand overseas.

Which pests and diseases?
The most important diseases in apple are black spot (Venturia inaequalis) and powdery mildew (Podosphaera leucotricha). Both fungi can affect tree production by weakening the trees, but their economical effect can be measured more directly by the number of fruit affected by the fungi. With the aid of fungicides growers are able to minimise the losses caused by these diseases, but at a considerable cost.

Black spot, Venturia inaequalis.	Powdery mildew, Podosphaera leucotricha.

In the orchard fire blight (Erwinia amylovora) and collar rot (Phytophthora cactorum) cause tree-death, when affected trees are left untreated. Preventative treatment of fire blight consists of spraying with streptomycin during blossom time when the conditions are favourable to the disease. Once trees are infected, the only means of control is by eradicating the affected branches. Little can be done to prevent collar rot infection except improving soil drainage. Once infection occurs, trees can only be saved by either cutting out the affected tissue, or by injecting the tree with a fungicide at an early stage of infection. Woolly apple aphid (Eriosoma lanigerum) causes galling on all the parts of the apple tree, resulting in slow growth and poor production. The parasitic wasp Aphelinus mali, which is normally present in New Zealand orchards, can keep the pest in check, but with variable success.

Collar rot, Phytophthora cactorum.

Woolly apple aphid, Eriosoma lanigerum.

Sources of resistance
The main sources of resistance used in the HortResearch breeding programme are major gene resistances derived from crabapple. The V_f gene for black spot resistance is derived from the ornamental crabapple species Malus floribunda. Most black spot resistant cultivars developed around the world carry this gene, but there are some selections that carry the V_r (from M. pumila) or V_m (from M. micromalus) gene.

Major gene resistances to powdery mildew are derived from M. robusta (Pl₁) and M. zumi (Pl₂), which we both use in our breeding programme. The apple cultivar 'Northern Spy' has a longstanding reputation for its major gene resistance to woolly apple aphid (Er). Since early this century this resistance has been used to develop woolly aphid resistant rootstocks such as MM.106 and M.793. Much later it was shown that the cultivar was also very resistant to collar rot and a useful breeding parent for this resistance. We have used 'Northern Spy' in several breeding lines to incorporate both resistances into new scion cultivars.

The advantage of using existing cultivars as breeding parents for resistances is, that new cultivars can be selected directly from their progeny. With crabapples an extensive back-crossing programme had to be set up to combine the resistances with good eating quality. This work was initiated in the USA, while most breeding programmes in Europe followed suit. By importing resistant selections and pollen or seed thereof, we mainly have sourced three breeding programmes to introduce resistance into the New Zealand apple breeding programme: Purdue-Rutgers-Illinois (USA), Angers (France) and East Malling (Great Britain). A large new source for resistances forms the Apple Breeding Population, developed at HortResearch by Dominique Noiton.

Screening for resistance
The selection of resistant individuals in progenies takes place in the early stages of the life of the plants. Screening generally is carried out by inoculating the seedlings with the pathogen, followed by incubating them under optimum conditions for the pathogen.

Inoculation of an apple seedling with fire blight.

The time of inoculation varies from pathogen to pathogen. Resistance to both black spot and collar rot can be tested at a very early stage shortly after germination, while the screening for fire blight and woolly apple aphid takes place when the seedlings are about 2-3 months old. Powdery mildew is an exception, since no inoculation is carried out because the pathogen cannot be grown on an artificial medium. The seedlings are subjected to natural infection in the field and evaluated for their resistance after the second year. Earlier is not possible due to the delayed expression of the resistance.

Cultivar breeding
The resistance breeding programme is integrated with the cultivar breeding programme, lead by Allan White. In the first programme the groundwork is carried out by developing advanced breeding lines with preferably multiple resistances and good eating quality through back-crossing. These will be used in the cultivar breeding programme to develop new pest and disease resistant cultivars which have an eating quality equal to, or better than our current main export varieties 'Braeburn' and 'Royal Gala'.

These and other high quality cultivars have been used in many crosses as breeding parents for their excellent quality characters. Some of them are known to carry some levels of resistance to pest and diseases themselves. For example, Pacific Rose and 'Red Delicious' do not need sprays against powdery mildew, while the latter also is highly resistant to fire blight. 'Granny Smith' is lowly susceptible to both fire blight and collar rot, and several local market cultivars are known to carry fair levels of resistance to black spot and powdery mildew, eg. 'Discovery', 'Merton Worcester', 'Laxton's Fortune' and 'James Grieve'. We make use of these cultivars to enhance the major gene resistances derived from crabapples.

Update on the resistance breeding programmes for pip and summerfruit
By Vincent Bus and Mike Malone - HortResearch, Havelock North

Pipfruit
Over the last two years, HortResearch has made over 100 selections which carry resistance to black spot. High priority is being given to the development of cultivars which also are resistant to powdery mildew. Several selections look promising and will be evaluated in grower trials in co-operation with ENZA New Zealand (International).

The HortResearch breeders and geneticists are aiming to have more than one gene conferring resistance to black spot in the plant. Multiple resistant cultivars, which have more than one resistance gene, are less likely to lose their resistance to the disease over time.

Glasshouse resistance to black spot

Susceptibility : uninhibited growth and abundant sporulation by fungus	Intermediate resistance : crinkled leaves, chlorosis and limited sporulation	Hypersensitivity : ‘pin-point’ lesions caused by dying leaf cells after infection by black spot.

HortResearch has imported advanced selections from overseas to use in the pear breeding programme. The main objective of the pear resistance breeding programme is to develop cultivars which are resistant to fire blight and black spot. In the commercial pear breeding programme, it is common practice to select against susceptibility to diseases. So far several promising selections have been made from mainly interspecific crosses between local and imported varieties.

Summerfruit
The peach and nectarine breeding programme will develop varieties tolerant to bacterial blast (Pseudomonas syringae), bacterial spot (Xanthomonas campestris pv. pruni), brown rot (Monilinia fructicola), and leaf curl (Taphrina deformans).

Varieties have been imported from overseas which are resistant to the bacterial diseases and brown rot. These lines are currently in quarantine. Once released, they will be screened for natural infection and then used in the breeding programme.

A number of local lines have been screened for resistance to leaf curl. One of the better lines will be used in crosses this coming spring.

Acknowledgments
HortResearch acknowledges the support of ENZA New Zealand (International) and the Foundation for Research, Science and Technology for their support.

Source:
Submitted by Helen Percy from HortResearch Technical Updating Seminar Proceedings, 1996 and HortResearch Fact Sheet "Developing pest and disease resistant pipfruit cultivars".