New Fire blight and scab resistant Pyrancanthas

Alain Cadic

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Alain Cadic. New Fire blight and scab resistant Pyrancanthas. 42. Annual Conference of International Plant Propagators Society (IPPS), International Plant Propagators' Society., 1992, NA, France. hal-01601205

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THE INTERNATIONAL
PLANT PROPAGATORS'
SOCIETY

Combined Proceedings
Volume 42
1992

<figure><img src="image_2.png" /></figure>

AUSTRALIAN REGION

IPPS - DENMARK

EASTERN REGION

REGION OF GREAT BRITAIN AND IRELAND

NEW ZEALAND REGION

SOUTHERN REGION

WESTERN REGION

ISSN 0633-9143
# New Fire Blight and Scab Resistant Pyracanthas

Alain Cadic

INRA, Centre de Recherches d'Angers, Laboratoire d'Amélioration des Arbustes d'Ornement, BP57, 49071 BEAUCOUZE Cedex, FRANCE

A *Pyracantha* breeding programme was started in 1982, to obtain resistance to fire blight (*Erwinia amylovora*) and scab (*Spilocaea pyracanthae*), and frost resistance. The ornamental criterion was a heavy long-lasting fruiting period. Cross pollination of different species and cultivars in INRA'S collection produced hybrids tested against scab and fire blight. Best hybrids were then cloned and tested again in a glasshouse. Observations were made on flowering and fruiting abilities. All hybrids have endured two hard winters with temperatures falling down to -15°C without snow protection. Two cultivars were selected and released to trade.

## INTRODUCTION

*Pyracantha* is a favorite landscape and garden shrub because of the brightly coloured berries carried through the winter. They are mainly used as formal hedges or in free forms and more rarely as wall-covering plants.

*Pyracantha* is a member of the rose family (Rosaceae) and of the subfamily Maloideae which includes *Malus*, *Pyrus*, *Crataegus*, *Sorbus*, and *Cotoneaster*. This subfamily is a specific host of the fire blight bacteria (*Erwinia amylovora*) which was first discovered in the U.S.A. Extensive evaluation of the susceptibility of commercial species and cultivars of several genera has been undertaken in Angers by the Plant Pathology Research Station (Paulin, 1990).

As antibiotics are not allowed to be used to control plant disease in this country, only preventative techniques may be used to control fire blight. Breeding new cultivars resistant, or highly tolerant to fire blight, is an important part of prevention. Programmes have been developed on apple (Lespinasse and Paulin, 1989), pear (Thibault and Paulin, 1984) and *Pyracantha* (Cadic et al., 1989).

The first fire blight resistant cultivars of *Pyracantha* were released by Egolf between 1966 and 1986. Six cultivars have been described: 'Shawnee', 'Mohave', 'Navajo', 'Teton', 'Apache' and 'Pueblo' (Egolf, 1966; 1970; 1978; 1987a; 1987b), but this author is unaware of any information published on the inoculation procedure or the strain of *E. amylovora* used by Egolf to test them. In 1975, a breeding programme was initiated in The Netherlands (Bouma, 1990) but so far no cultivar has been released to the trade.

## MATERIALS AND METHODS

Breeding strategy is summarized in Table 1. Crosses between 33 clones in the INRA collection were made in 1982 and 1983 in 126 combinations, including selfing. Young seedlings bearing four or five leaves were then tested against scab in a glasshouse. Scab had been collected on susceptible taxa then cultivated on an artificial medium and induced to produce spores (conidia). These were sprinkled on
leaves and susceptibility was screened. Only young hybrids without scab symptoms were retained for a second screening.

<table><caption>**Table 1.** Pyracantha breeding strategy.</caption><tr><td>1982 - 1983</td><td>Hybridizations in Angers</td></tr><tr><td>1983 - 1984</td><td>Glasshouse selection for scab resistance in Angers</td></tr><tr><td>1983 - 1984</td><td>Selection for fire blight resistance at seedling stage in Dax Field plantation</td></tr><tr><td>1985 - 1989</td><td>Repeated field inoculations, preselection, and cloning of preselected hybrids</td></tr><tr><td>1986 - 1992</td><td>Glasshouse tests in Angers. Field plantation for further observations</td></tr></table>

At this time, fire blight was not yet present around Angers so it was impossible to perform inoculation experiments. Hybrid seedlings were taken to Dax in the southwest of France which is in a fire blight area. Seedlings were top leaf inoculated with infected scissors using a French strain (CFBP 2045) at a concentration of 10⁸ living bacteria per ml. Surviving plants were then planted into the field and inoculated again in the following years. Actively growing shoots were tip injected as soon as possible with a new French strain (CFBP 1430) at a concentration of 10⁹ living bacteria per ml. Symptoms were scored from 0 to 3, 0 meaning no symptom, 1 meaning necrosis of less than 1/3 of the inoculated shoot; 2 meant 1/3 to 2/3 and 3 meant complete necrosis. Each seedling was inoculated 5 times on more than 30 shoots. Seedlings were selected for further screening on the basis of these scores. Ten rooted cuttings of each selected seedling were containerised and placed in a controlled glasshouse at Angers. Inoculations, at a concentration of 10⁹ bacteria, were then repeated using a French strain (CFBP 1430) and an American strain (CUCM 273) of *E. amylovora*. The number of inoculated shoots showing necrosis and the size of necrotic patches were scored. Common cultivars were used as control and only the best seedlings were selected to be field planted for further observations of ornamental qualities.

## RESULTS AND DISCUSSION

Resistance to scab is quite easy to assess. Susceptible plants are quickly covered by a dark olive green mycelium while resistant plants remain free. From 7111 contaminated seedlings, 3273 were judged resistant. This resistance looks quite stable both in Dax or Angers. On resistant plants, conidia spores are unable to germinate or germination and mycelium development are quickly and actively stopped by surrounding tissues.

Resistance to fire blight is unlikely to exist and it would be better to speak of tolerance. Even tolerant cultivars may express symptoms when grown in conditions that favor the disease, or if inoculated with very high concentrations of conidia. By repeating field inoculations and using two strains in the glasshouse we tried to overcome environmental effects to assess the true potential susceptibility.

It was found that highly susceptible clones must not be used in crosses. For instance, all progenies from *P. angustifolia* have had to be discarded after the first test. One of the best parents used was the American selection 'Shawnee' and

especially in combination with the French cultivar 'Mozart' a cultivar of from *P. atalantoides*. Results from a glasshouse inoculation made in March 1989 show that selected hybrids were more effective at resisting the symptoms of fire blight than the commercial cultivars in the INRA collection (Table 2).

The range of susceptibility to fire blight follows a continuous genetic variation and environment drastically modulates plant responses to infection so that it is likely that several genes are involved in fire blight tolerance. Improvement of resistance to fire blight would then be reached by accumulating those genes by performing recurrent crosses between the more resistant hybrids. Of the 3000 hybrids originally tested against fire blight only 229 appeared to have disease resistance better than existing cultivars.

<table><caption>**Table 2.** Susceptibility to two strains of fire blight: a comparison of selected genotypes to 45 known cultivars.</caption><thead><tr><th rowspan="3"></th><th colspan="2">Strain CFBP 1430</th><th colspan="2">Strain CUCM 273</th><th colspan="2">Total both strains</th></tr><tr><th colspan="2">Genotype¹</th><th colspan="2">Genotype</th><th colspan="2">Genotype</th></tr><tr><th>LIS²</th><th>LN³</th><th>LIS</th><th>LN</th><th>LIS</th><th>LN</th></tr><tr><th></th><th>%</th><th>%</th><th>%</th><th>%</th><th>%</th><th>%</th></tr></thead><tbody><tr><td>A</td><td>6803</td><td>1904</td><td>28.0</td><td>6707</td><td>1902</td><td>28.4</td></tr><tr><td>B</td><td>9601</td><td>4047</td><td>42.2</td><td>10030</td><td>4116</td><td>41.0</td></tr></tbody></table>

¹A = Selected hybrids population, B = 45 cultivars from collection.

²IS = Length of inoculated shoots in cm.

³N = Length of necrosis in cm.

They all survived the severe frosts of two consecutive winters during which temperature fell to -15°C. Under the same climatic conditions, *P. angustifolia* has been killed, aerial parts of 'Navajo' and *P. crenatoserrata* 'Graber' were killed, and *P. atalantoides* and relatives were slightly injured (Bertrand et al., 1992).

Up to now, two cultivars have been released to the trade. Both are owned by INRA and SAPHYR, the group of nurserymen who financially supported part of the breeding program.

*Pyracantha Cadange* 'SAPHYR®' Orange comes from a cross between *P. Shawnee' and *P. atalantoides* 'Mozart' a slow growing heavy flowering and fruiting cultivar. Fruits are orange (RHS 25A to 28A), colouration is early. It is a medium-sized cultivar.

*Pyracantha CadrOU' SAPHYR®* Rouge originates from the same cross. Fruits are red (RHS 46B) turning to orange. As compared to other red-fruited firethorns, this cultivar has smaller leaves and fruits are not hanging down.

Both cultivars have been described (Bertrand et al., 1992). They were put to the trade in 1989. To fill the cultivar range, a yellow fruiting form will be introduced next year. Nurserymen are also selecting hybrids with a stronger growth and perhaps with a more erect habit.
## LITERATURE CITED

Bertrand, H., A. Cadic, and J. Belin 1992. *Pyracantha*. Origine, description et clé de détermination des principaux taxons. Collection 'Repes techniques', PHM Revue horticole Ed., 101 pages.

Bouma, A.S., 1990. Breeding *Pyracantha* for fire blight resistance. Acta Horticulturae. 273:327334.

Cadic, A., J. Belin, and J-P. Paulin. 1989. New pyracanthas resistant to scab [Spilocae pyracanthae (Otth.) Rostrup] and to fire blight [Erwinia amylovora (Burr.) Winsl. et al.] Acta Horticulturae 273:303309.

Egolf, D.R. 1966. *Pyracantha* 'Shawnee' (Rosaceae). Baileya 14(2):61-63.

Egolf, D.R. 1970. *Pyracantha* 'Mohave'. A new cultivar (Rosaceae). Baileya 17(2):79-92.

Egolf, D.R. 1978. *Pyracantha* 'Navajo' and 'Teton'. HortScience 13(4):483-484.

Egolf, D.R. 1987a. *Pyracantha* 'Apache'. HortScience 22(1):173-174.

Egolf, D.R. 1987b. *Pyracantha* 'Pueblo'. HortScience 22(3):510-511.

Lespinasse, Y. and J-P. Paulin. 1989. Apple breeding programme for fire blight resistance: strategy used and first results. Acta Horticulturae 273:285-295.

Paulin, J-P., 1990. Fire blight of Pomoideae [*Erwinia amylovora* (Burrill.) Winslow et al.]. Applied research in Europe. Agriculture/Agrimed research program. Published by CEC. EUR 12601. 279 pages.

Thibault, B. and J-P. Paulin. 1984. Pear breeding and selection for fire blight resistance. Acta Horticulturae 151:141-146.

## Cotoneaster dameri: Fire Blight (*Erwinia amylovora*) Resistant Cultivars in Germany

### Heinrich Lösing

Versuchs- und Beratungsring für Baumschulen, Bismarckstr. 49, 2080 Pinneberg, Germany

## SITUATION IN GERMANY

The bacteria disease fire blight first appeared in 1971 in Schleswig - Holstein on *Crataegus monogyna* and *Pyrus* fruit trees. It spread south, reached Hamburg in 1974, Westfalia in 1975, and finally the southern parts of Germany in 1981 (Baumm, 1989). Initial attempts at control included strict eradication of *C. monogyna* from windbreaks, especially in nurseries and around infested locations. Commercial production of common hawthorn was banned. These restrictions could not stop *Erwinia amylovora*. Nowadays people have learned to live with the disease, which is mainly found in Germany on *Chaenomeles*, *Cotoneaster*, *Crataegus*, *Cydonia*, *Pyrus*, and *Photinia*.

## CHEMICAL CONTROL

Antibiotics cannot be used for plant protection in Germany. Copper is used as a protective agent but has some disadvantages:

* Needs to be sprayed frequently

* Accumulates in the soil

* May inhibit plant growth

* Phytotoxicity problems

Since 1982 we have been working with the prognosis model based on the work of Bill'ing (1974) to optimize/minimize the use of copper during the growing season in relation to weather (Brulez and Zeller, 1981). Different plant extracts (Berberis, Mahonia, Rhus) have also shown an effect on fire blight under field conditions when used as prophylactic treatments. However, they are not yet used in the nurseries (Mosch and Zeller, 1989).

## RESISTANT CULTIVARS

Work on resistant *Cotoneaster* cultivars started in the late 1970s by Persiel and Zeller at the Institute for Horticultural Plant Breeding in Ahrensburg (close to Hamburg) with *C. dameri* var. *radicans*. This species is not very susceptible to fire blight under German conditions. Occasional problems occur during nursery production but rarely afterwards. Following artificial testing, 16 clones of *C. dameri* var. *radicans* were further tested for ornamental value in the plant selection section of the Versuchs- und Beratungsring für Baumschulen in Pinneberg. After another four years of testing, two fire blight resistant clones were selected and named:

1) 'Holsteins Resi'. Very similar to *C. dameri* var. *radicans* with leaves 3 to 4 cm long and about 2 cm wide, and growth up to 25 cm high. The cultivar is a very good