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# Pear breeding and selection for Fire blight resistance

Bernard Thibault, J Pierre Paulin

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Bernard Thibault, J Pierre Paulin. Pear breeding and selection for Fire blight resistance. Working Group "Pears", Jan 1984, Avignon, France. hal-01601309

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PEAR BREEDING AND SELECTION FOR FIRE BLIGHT RESISTANCE

B. THIBAULT* - J.P. PAULIN**
Institut National de la Recherche Agronomique
Station d'Arboriculture Fruitière*
Station de Phytobactériologie**
Beaucouzé - 49000 Angers (France)

The research for a fire blight resistant variety of pear has been undertaken in the United States since some sixty years. The resistant parents were firstly choosen in oriental species : Pyrus pyrifolia, Pyrus ussuriensis or in hybrids between Pyrus communis and these species.

The poor quality of the fruits of the selected varieties prevented them from being cultivated in commercial orchards.

Before 1974 most of these varieties and some hybrids not yet named have been introduced in the collection of the INRA Research station of Angers.

In 1974 close contacts were established with North american and canadian researchers and the following conclusions were drawn out :

- The use of minor resistances in Pyrus communis could lead to less susceptible varieties with a good fruit quality if these low levels of resistance are accumulated through successives generations,

- The range of picking dates aimed at by these researchers is about the picking date of 'Williams' and the selected flavour is the one of 'Williams',

- Growers in these countries did not grow any of these new varieties.

These reasons made clear for us the need to begin in France our own breeding program in order to select new varieties with these two main characteristics :

- a low susceptibility to fire blight

- a late or medium late picking date.

The outbreak of fire blight in 1978 in the pear growing area of the South West of France, where climatic conditions are usually favourable to the disease, Leconte et al. (/1/) allowed us :

- to test hybrids in an experimental orchard near Dax,

- to establish a collection of 60 european varieties in order to know their susceptibility in our conditions.

First results of these two actions are presented hereafter.

# 1. MATERIAL AND METHODS

## A - Tests on progenies

One mother parent has been choosen in the spring of 1979. It is the variety 'Pierre Corneille' well known for its good production, good fruit quality and whose picking date is about the one of 'D. Comice' (Thibault /2/). In Canada, Quamme /3/ noted this variety 9 (highly resistant) with the USDA score established by Van der Zwet /4/.

Eleven male parents have been choosen :

- 5 were U.S. varieties known for their resistance :

* 'Maxine'

* 'Starking Delicious'

* 'Mac'

* 'Dawn'

* 'Sirrine'

I.I.F.-I.I.R. - Commission C2 - Avignon (France) - 1934/1
Acta Horticulturae 161,1984 - Working Group "Pears"
- 3 were U.S. or Canadian hybrids not yet named :

* 'Illinois 13-B-83' ('Comice' x 'Farmingdale') x 'Maxine'

* 'U.S. 62.574.13' ('Michigan 437' x 'R.C.W.'*) x ('Michigan 437' x 'R.C.W.')

* 'H.W. 602' (= Harvest Queen) ('Michigan 572' x 'Bartlett')

- 3 were old european varieties under observation by Otto 75/ in Beltsville, U.S.A. and which were given a USDA score of 8 (highly resistant) or 7 (moderately resistant) :

* 'Duchesse d'Angoulême'

* 'Figue d'Alençon'

* 'Doyenné Gris'

In April 1980 young seedlings were leaf inoculated with a one day old culture of Erwinia amylovora, strain IPO n° 108 at a concentration of about 10⁷ cell/ml in the greenhouse of the Research Institute for Plant Protection in Wageningen (The Netherlands) by H.P. Maas Geesteranus.

Seedlings still alive after this inoculation were planted in the experimental orchard near Dax where they were reinoculated in 1982 and 1983 by picking with an hypodermic syringe near the top of a growing shoot. Thus about 25 µl of a fresh bacterial suspension adjusted to 10⁹ cell/ml (strain NBP 2045) were introduced.

Every year three shoots per seedling were inoculated and the result was assessed by the proportion of the length of the blighted part of the shoot to the total length of the shoot as explained by Quamme /6/.

After these two years the results are distributed in five classes of percentage : 0-19, 20-39, 40-59, 60-79 and 80-100. The middle of each class is multiplied by the number of individual shoots in this class and the sum of the five classes is divided by the total number of shoots according to the method described by Le Lezec /7/ for apple varieties.

We obtained for each progeny an Index of Progeny Susceptibility (I.P.S.). The negative results i.e. shoots which showed no symptoms after inoculation were included in the class 1 (0-19).

Two other characteristics are noted for each progeny :

1) The severity of the attack : percentage of the total length of necrosis on the total length of shoots, taking in account the shoots which allow some disease progression only.

2) The frequency of the infection : percentage of the number of necrotic shoots on the total number of inoculated shoots.

## B - Tests on varieties

Sixteen pear varieties were planted in the experimental orchard near Dax during the winter 1978-1979. In June 1983 several growing shoots from 26 to 82 by variety are artificially inoculated as described before and each variety was assessed through the same criterions : Index of Varietal Susceptibility (I.V.S.), Severity and Frequency.

## 2. RESULTS

### A - Tests on progenies

Results of inoculation in the greenhouse are indicated on Table I. In Table II are shown index of susceptibility, frequency and severity. The percentage of hybrids which escaped infection both in the greenhouse and in the field is also indicated on Table III.

<footnote>* 'R.C.W.' = Roi Charles de Wurtenberg</footnote>
<table><caption>TABLE I
Results of inoculation in greenhouse. Female parent : 'Pierre Corneille'</caption><thead><tr><th rowspan="2">Male Parent</th><th rowspan="2">Number of seedlings</th><th colspan="3">Percentages</th></tr><tr><th>Deads</th><th>Low susceptibility</th><th>No symptoms</th></tr></thead><tbody><tr><td>Dawn</td><td>290</td><td>78</td><td>16</td><td>6</td></tr><tr><td>Mac</td><td>157</td><td>71</td><td>17</td><td>12</td></tr><tr><td>Maxine</td><td>67</td><td>75</td><td>24</td><td>1</td></tr><tr><td>Starking Delicious</td><td>94</td><td>74</td><td>15</td><td>11</td></tr><tr><td>Sirrine</td><td>201</td><td>70</td><td>15</td><td>15</td></tr><tr><td>HW 602</td><td>84</td><td>73</td><td>15</td><td>12</td></tr><tr><td>Illinois 13.B.83</td><td>276</td><td>74</td><td>21</td><td>5</td></tr><tr><td>US.62.574.13</td><td>92</td><td>62</td><td>14</td><td>24</td></tr><tr><td>Doyenne Gris</td><td>111</td><td>48</td><td>31</td><td>21</td></tr><tr><td>Duchesse d'Angoulême</td><td>45</td><td>64</td><td>18</td><td>18</td></tr><tr><td>Figue d'Alençon</td><td>244</td><td>85</td><td>11</td><td>4</td></tr></tbody></table>

<table><caption>TABLE II
Susceptibility of progenies after two years of inoculation in the field</caption><thead><tr><th>Male Parent</th><th>I.P.S.</th><th>Frequency</th><th>Severity</th></tr></thead><tbody><tr><td>Dawn</td><td>42,77</td><td>46,64</td><td>67,47</td></tr><tr><td>Mac</td><td>38,26</td><td>38,62</td><td>83,18</td></tr><tr><td>Maxine</td><td>39,49</td><td>40,15</td><td>83,45</td></tr><tr><td>Starking Delicious</td><td>34,92</td><td>36,44</td><td>78,37</td></tr><tr><td>Sirrine</td><td>35,65</td><td>44,64</td><td>67,47</td></tr><tr><td>HW 602</td><td>29,55</td><td>29,92</td><td>75,32</td></tr><tr><td>Illinois 13.B.83</td><td>36,89</td><td>37,00</td><td>82,67</td></tr><tr><td>US.62.574.13</td><td>25,58</td><td>24,03</td><td>74,86</td></tr><tr><td>Doyenne Gris</td><td>23,76</td><td>22,75</td><td>70,49</td></tr><tr><td>Duchesse d'Angoulême</td><td>30,37</td><td>38,89</td><td>62,32</td></tr><tr><td>Figue d'Alençon</td><td>52,29</td><td>63,39</td><td>76,71</td></tr></tbody></table>

<table><caption>TABLE III
Percentage symptomless after greenhouse + field</caption><thead><tr><th>Male Parent</th><th>%</th></tr></thead><tbody><tr><td>Dawn</td><td>4,05</td></tr><tr><td>Mac</td><td>7,00</td></tr><tr><td>Maxine</td><td>0,00</td></tr><tr><td>Starking Delicious</td><td>4,95</td></tr><tr><td>Sirrine</td><td>4,22</td></tr><tr><td>HW 602</td><td>11,11</td></tr><tr><td>Illinois 13.B.83</td><td>4,54</td></tr><tr><td>US.62.574.13</td><td>18,52</td></tr><tr><td>Doyenne Gris</td><td>13,91</td></tr><tr><td>Duchesse d'Angoulême</td><td>10,93</td></tr><tr><td>Figue d'Alençon</td><td>0,41</td></tr></tbody></table>
## B - Tests on varieties

Table IV indicates the results obtained with pear varieties. Criteria are the same as for hybrids. The number of shoots artificially inoculated is indicated in the last column.

<table><caption>TABLE IV
Susceptibility of sixteen pear varieties after one year of inoculation (1983)</caption><thead><tr><th>Varieties</th><th>I.V.S.</th><th>Frequency</th><th>Severity</th><th>Number of inoculated shoots</th></tr></thead><tbody><tr><td>Beurre clairgeau</td><td>14,44</td><td>75,55</td><td>16,84</td><td>45</td></tr><tr><td>Beurre Hardy</td><td>18,44</td><td>11,11</td><td>95,40</td><td>45</td></tr><tr><td>Conférence</td><td>18,44</td><td>84,44</td><td>21,37</td><td>45</td></tr><tr><td>Pierre Corneille</td><td>22,18</td><td>56,25</td><td>33,72</td><td>64</td></tr><tr><td>Dr Jules Guyot</td><td>22,88</td><td>95,45</td><td>22,88</td><td>45</td></tr><tr><td>Duchesse d'Angouleme</td><td>23,18</td><td>72,72</td><td>29,65</td><td>44</td></tr><tr><td>B.P. Morettini</td><td>23,88</td><td>88,88</td><td>28,58</td><td>36</td></tr><tr><td>Duc de Bordeaux</td><td>38,46</td><td>92,31</td><td>39,29</td><td>26</td></tr><tr><td>Santa Maria</td><td>42,30</td><td>79,48</td><td>51,22</td><td>39</td></tr><tr><td>Sirrine</td><td>43,17</td><td>74,40</td><td>54,03</td><td>82</td></tr><tr><td>Williams rouge</td><td>44,66</td><td>64,44</td><td>67,86</td><td>45</td></tr><tr><td>Doyenne du Comice</td><td>49,05</td><td>88,09</td><td>54,00</td><td>42</td></tr><tr><td>Général Leclerc</td><td>50,00</td><td>100,00</td><td>53,43</td><td>30</td></tr><tr><td>André Desportes</td><td>67,94</td><td>100,00</td><td>70,97</td><td>39</td></tr><tr><td>Alexandrine Douillard</td><td>81,81</td><td>100,00</td><td>88,13</td><td>44</td></tr><tr><td>Précoce de Irévoux</td><td>84,66</td><td>95,55</td><td>97,87</td><td>45</td></tr></tbody></table>

## 3. DISCUSSION

### A - Tests on progenies

After inoculation in the greenhouse, three progenies are noteworthy: crosses between 'P. Corneille' and 'US 62.574.13', 'Doyenne Gris' and 'Duchesse d'Angouleme'. More than a third and up to a half of the seedlings are still alive and the percentage of seedlings without symptoms is about 20 % or so.

An other cross is also noticeable: 'P. Corneille x Figue d'Alençon' where 85 % of the seedlings died after inoculation.

The two years of inoculation in the field lead to approximately the same conclusions. The progenies with 'US 62.574.13', 'Doyenne Gris' and 'Duchesse d'Angouleme' show the best level of resistance, according to our indexes.

The progeny with 'HW 602' has also a good index and 'Figue d'Alençon' has the worst.

Frequency is a more accurate characteristic when the seedlings are tested in the field since in the greenhouse only one shoot per seedling is inoculated. 'Doyenne Gris', 'US 62.574.13' and 'HW 602' are again the best male parents.

Only three families have less than or hardly more than one third of their shoots infected: they are the crosses with 'Duchesse d'Angouleme' 'Dawn' and 'Sirrine'. As far as severity is concerned, in all progenies, except with 'Figue d'Alençon', the length of necrosis is less important in the seedlings selected in the greenhouse as "No symptoms" than in the seedlings selected as "Low susceptibility" (33 % against 29,6 %). But in terms of frequency it is the contrary (13,9 % against 11,9 %).

This fact shows that in the class "No symptoms" are included seedlings with low susceptibility but also seedlings which escaped infection after inoculation for
presumably technical or physiological reasons.

All these results are valid only for the specific crosses with 'Pierre Corneille'. The value of parents could be different with an other female parent.

## B - Tests on varieties

Although the number of inoculates shoots per variety is rather high the plants which have been inoculated only once will have to be reinoculated in order to allow a more reliable classification.

The rating of 'Sirrine' is surprising : its susceptibility was considered in the USA by Lamb /8/ at midway between 'Bartlett' and 'Kieffer'. In an other experiment 'Sirrine' behave as very susceptible after flower inoculation.

'Beurre Hardy' shows a special behaviour with a very high severity on few shoots only, this resulting in a low I.V.S. This confirm the observation of Zeller /9/ in Germany where "'Beurre Hardy' was fairly resistant showing only some shoot tip infections and no spreading into the older tissue". Warm and wet climatic conditions in the Dax area may account for the higher severity (95 %) in our observations.

In contrast with 'Beurre Hardy', 'Beurre Clairgeau' and 'Conference' show a high frequency associated with a low severity. That means, in the orchard, many sites of infection with a short extension of lesions.

'Dr Jules Guyot' shows the same behaviour and this may be a useful information for the South-east pear growing area of France.

'Precoce de Trevoux' is highly susceptible as already observed by Zeller /9/.

For two varieties our observations don't agree with Van der Zwet's/10/ observations. 'Conference', which shows in Dax a low severity is noted by Van der Zwet as "much more susceptible than 'Bartlett'... the tree is destroyed by blight within a short period of time'. 'Comice' which is susceptible in Dax but score 7 (moderately resistant) by Van der Zwet. Additional years of inoculation could make these observations more concordants.

## 4. CONCLUSION

For the time being, 60 progenies including 18 000 seedlings have been made for the fire blight resistance. 13 000 have been artificially inoculated during their first year of growth allowing 3 200 less susceptible to be planted. 5 000 other seedlings have been planted before this screening.

60 european varieties and 20 north american and canadian hybrids not yet named are also established. Artificial inoculation on shoots and flowers will give more information about their susceptibility to fire blight.

## REFERENCES

1. P. LECONTE, P. LARUE, J.P. PAULIN, Climat et feu bactérien dans la région de Dax. Acta Horticulturae (1983), 141 (In press).

2. B. THIBAULT, Nouvelles variétés de poires proposées en France. Fruit Beige, n° 380 (1977), p. 251-259.

3. H.A. QUAMME, Resistance to naturally and artificially induced fire blight in the Harrow pear collection. Canad. Pl. desease Surv., vol. 57 (1977), p. 9-12.

4. T. Van der ZWET, W.A. OITIO, H.J. BROOKS, Scoring system for rating the severity of fire blight in Pear. Plant Dis. Rptr., 54 (1970), p. 835-839.

5. W.A. OITIO, T. Van der ZWET, H.J., Rating of pear cultivars resistance to fire blight. HortScience, vol. 5 (dec. 1970), 6, p. 474-476.

6. H.A. QUAMME, T. Van der ZWET, V. DIRKS, Relationship of fire blight resistance of young pear seedlings inoculated in the greenhouse to mature seedling trees naturally infected in the field. Plant Dis. Rptr., 60 (1976), 660-664.
7. M. LE LEZEC, J.P. PAULIN, Shoot susceptibility to fire blight of some apple cultivars. Acta Horticulturae (1983), 151 (In press).

8. R.C. LAMB, The Sirrine pear, Fr. Var. and Hort. digest, vol. 24, (1970), 2, p.26.

9. W. ZELLER, Field trials on the resistance of pear and apple varieties to fireblight (natural and artificial infection). Acta Horticulturae, 86 (1978), 15-23.

10. T. Van der ZWET, Possibility of combining low levels of fire blight resistance in pear. Acta Horticulturae, 69 (1977), 97-103.

**SUMMARY** : Two experiments have been carried out to test the susceptibility of hybrids and varieties to fire blight.

Young seedlings have been inoculated in the greenhouse at the frist leaf (1980). The survivors were inoculated in the field in 1982 and 1983 at Dax, South West of France. The observations were made, by progeny, on the number of damaged shoots in relation to the number of inoculated shoots (Frequency) and on the percentage represented by the total length of the necrotic part of the shoot divided by the total length of inoculated shoots (Severity). An index was obtained in classifying the shoots into five categories including in the first category the shoots with no symptoms.

For 11 progenes of known parentage (Pierre Corneille as mother parent) the progeny with US 62-574.13 and with the two European varieties "DOYENNE GRIS" and "DUCHESS E D'ANGOULEME" gave the best results. "HW 602" now called "HARVEST QUEEN" has also been a good parent.

Using the same criteria sixteen varieties have been artificially inoculated on the shoots at Dax, in Spring 1983 and are classified in a decreasing range of susceptibility.

## AMÉLIORATION ET SELECTION DU POIRIER POUR LA RESISTANCE AU FEU BACTÉRIEN

**RESUME** : Deux expérimentations ont été menées pour tester la sensibilité d'hybrides ou de variétés de poirier au feu bactérien.

Les jeunes semis ont d'abord été inoculés en serre la première année de leur pousse (1980). Les survivants ont été inoculés au verger en 1982 et 1983 à Dax (Sud Ouest de la France). Les observations ont porté, par descendance, sur le nombre de pousses atteintes par rapport au nombre de pousses inoculées (Fréquence) et sur le pourcentage représenté par la longueur totale de la partie nécrosée des pousses sur la longueur totale des pousses inoculées (Sévérité). Un index est obtenu en classant les pousses en 5 catégories en incluant dans la première catégorie les pousses n'ayant pas réagi à l'inoculation.

Sur 11 descendances contrôlées ayant toutes la variété 'Pierre Corneille' comme parent femelle ce sont celles mettant en jeu le pollen de l'hybride 'US 62.574.13' et des deux variétés européennes 'Doyenne Gris' et 'Duchesse d'Angoulême' qui ont donné les meilleurs résultats. 'HW 602', maintenant nommée 'Harvest Queen', s'est aussi révélée un bon parent.

Par ailleurs en utilisant les mêmes critères 16 variétés ont été artificielle-ment inoculées sur pousses, à Dax, au printemps 1983 et sont classées suivant un ordre décroissant de sensibilité.