PROGRESS TOWARDS FUSARIUM RESISTANCE IN CARNATIONS

Transcription

1 PROGRESS TOWARDS FUSARIUM RESISTANCE IN CARNATIONS L.D. Sparnaaij and J.F. Demmink Institute for Horticultural Plant Breeding, Wageningen, the Netherlands Abstract Fusarium resistance has recently been introduced as a major se 1ecti on criterion in a long-term breeding programme originally intended to incorporate resistance to Phialophora in glass-house carnations. The decision to alter the priorities in this current programme, rather than starting a separate programme for Fusarium resistance, was based on the results of several preliminary investigations. These showed that most conditions for success in Fusarium resistance breeding were met: (a) A reliable screening technique; (b) a wide variation in Fusarium resistance in the available breeding material; (c) levels of resistance, adequate to provide protection under commercial conditions in both standard 1arge-flowered and spray clones and, finally (d) resistance readily transmitted from parents to progenies. The high proportion of the available breeding material that is resistant to Fusarium is explained by a partly common genetic basis for Phialophora and Fusarium resistance. Introduction For Dutch ca mat ion growers Fusariumwilt (F u s a r i u m o x y s p o- rumf. sp. dianthi and F. redolens) is as much of a threat to their livelihood as it is for their colleagues in other carnationgrowing areas. This has led to a concentrated effort by breeders and pathologists to counter this threat. In this paper we shall give an account of our approach to this problem and of the results we have had to date. Earlier attempts to breed resistant carnation cultivars, particularly in the U.S.A., have not been very successful and most projects have been discontinued except for the work of Dr. Carrier in California. More recent work in Europe has not yet led to new resistant cultivars but has provided much information on the relative resistance of a large number of cultivars and species (Pergola & Garibaldi, 1974). In the Netherlands, resistance breeding is concentrated at the Institute for Horticultural Plant Breeding. It forms part of a broader breeding project aimed at introducing new cultivars with desirable characters such as new colours, improved keeping quality, improved winter performance and, also, resistance to Phialophora cinerescens. In fact, the programme was started 15 years ago specifically to find resistance to Phialophora wilt. In later years, after the level of Phialophora resistance had been raised, priority was given to general quality characters. Last year our breeding policy was changed for a second time by the introduction of Fusarium resistance as a major selection criterion. To justify such a change of direction in a long-term breeding programme, we had to be reasonably confident of success within a relatively short time. Had we not been so confident we would have been obliged to start a new parallel programme specifically for Fusarium res i stance. Acta Horticulturae 71, 1977 Carnations 107

2 Did we have sufficient grounds to be optimistic? For a reasonable chance of success within a relatively short time, the following conditions must be fulfilled: (1) A reliable and efficient screening technique; (2) Adequate variation in resistance, approaching the required levels in (3) breeding material tnat is not too distant from the commercially acceptable type and (4) resistance readily transmitted from parents to progenies. Clonal screening trials Preliminary investigations in 197^ and 1975 were aimed at establishing how far these conditions were satisfied in our current breeding programme. We were fortunate in having a large number of clones of diverse origin available in which we could check variability in Fusarium resistance by using the same screening technique that had long been used successfully for Phialophora. This technique consists of planting rooted cuttings in 7 cm plastic pots filled with a peat-soil mixture and inoculating them one week later by injecting 3 ml of a diluted suspension of spores and mycelium derived from cultures of different origins (around 10 x 10 6 Fusarium spores per ml) into each pot. The pots are placed on irrigation mats at 23 C. Four weeks later, the tops of the plants are cut off at the node of the fifth leaf pair and the cut surface is examined for the typical discolorations of the vascular system. Visibly infected plants are removed, the others reexamined k weeks later by cutting through the lowest node of the largest side shoot. A final check is done weeks after inoculation by cutting the plants just above ground level. In the first trial, carried out in 197^, we tested 83 large-flowered and 96 spray-type clones, each represented by a minimum of 6 plants. The clones were classified on the basis of the percentage of plants without symptoms 12 weeks after inoculation. The distribution over the various classes was as follows: Table 1 - Classification of 181 IVT clones according to percentage symptomless plants, 12 weeks after inoculation. % symptomless pi ants Number of Standards clones Sp rays k3 2b k It is remarkable that all classes of "resistance" appear to be represented. In the sprays the average percentage symptomless plants is higher than in the 1arge-f1owered standard clones. The supposedly resistant control, the cv 'May Britt' scored only 22 % symptomless p1 ants in th is trial. To check the repeatability of the screening method used, 73 clones out of the same group of clones were screened for a second time in 1975 in a new trial in 2 replicates of 6 plants per clone. The results 108

3 agreed reasonably well, now giving averages of 22 % symptomless plants for the clones that scored 0-9 % in the first test and 76 % for the most resistant clones that scored % in the first trial. It was also clear, however, that the number of plants and replicates was not adequate to detect significant differences between individual clones. Later in 1975, a first series of 21 commercial cultivars was screened in which each cultivar was represented by 10 replicates of 10 plants each. This trial showed that for a first screening of larger numbers of clones 2 or 3 replicates of 10 plants should be adequate, though a second screening with more replication is advisable before selecting a clone as breeding parent. The percentages symptomless plants in this trial ranged from 2 % for 'Romeo' and 'Sacha' to 63 t for 'Orchid Beauty' (Table 2). Table 2 - Percentage symptomless plants in 21 commercial cultivars, 12 weeks after inoculation. % symptomless % symptomless Standards plants Sp rays plants Orchid Beauty 63 Dan i1o Lonr i na 62 Al ice 48 Exqu site 40 L ilac Tommy 32 Arabe 11 a Lonvata 25 White Sim 11 Royal et te 20 Joker 11 Tony 19 Sarazis Lonbiana 8 Sam's Pride 17 Mataro Baribel 8 Fiona Starnino 15 Loii ta 8 Nicky Londuka 14 Tel star 5 Red Baron 13 Wi11iam Sim 4 Romeo Lonboti 2 Sacha Lonrubac 2 To relate the resistance found in some commercial cultivars with that in our breeding material, a trial of 104 new selections from our current programme included 8 commercial cultivars as controls (Table 3). 'Orchid Beauty' again scored high, 60 but 15 of our clones gave as good, possibly even better results. Eight clones were as poor as the susceptible cv 1 Lena'. Table 3 - Classification of 104 IVT clones and 8 commercial cultivars according to percentage symptomless plants, 12 weeks after i nocu1 at ion. Number of clones % symptomless Commercial plants Standards Sprays cultivars Lena, Heidi, Caribe Calypso, Exquisite, Alice Sc.Elegance, Orchid Beauty U

4 Simitar trials have since been carried out on a routine basis both on our own selections and on all new cultivars we could obtain. Apart from 'Orchid Beauty 1 the only source of high resistance outside our own programme was found in two selections obtained from Dr. Carrier. By the end of 1975 it had thus become clear that our breeding material contained a high proportion of Fusarium resistant clones; so high, in fact, that we can make use of it without having to fear a drop in general quality and productivity. Long-term trials under semi-practica1 conditions The next question to be answered was: What level of resistance - as measured in our screening trials - provides adequate protection under natural conditions of infection in commercial greenhouses. To obtain information on this point, a long-term trial has been laid down at the Research Station for Floriculture in Aalsmeer, comparing 24 of our clones of three different resistance levels (high, medium and low) with 6 commercial cultivars. Two methods of inoculating the soil in raised benches were applied: (a) mixing diseased soil into steam-sterilised soil in a proportion 1:30 (b) placing a layer of 3"5 cm of diseased soil at the bottom of the bench, covering this with steam-sterilised soil to 25 cm. Six months after planting, the disease was well established in plants of treatment (a) with susceptible clones dropping below 50 % symptomless plants. In treatment (b) losses were few. After 35 weeks in treatment (a), 8 clones of the highest resistance class had 88 % of the plants still symptomless; in the lowest class this was only 25 % The best commercial cultivar was again 'Orchid Beauty' with 67 % symptomless plants. See Figure 1. From this trial the provisional conclusion can be drawn that the clones with the highest level of resistance in the screening trials are likely to do well under conditions of natural infection in commercial greenhouses. As the average percentage symptomless plants in a trial may vary widely under the influence of external conditions, the use of a common control clone, representing the minimum acceptable level of resistance, is recommended. For the time being we use 'Orchid Beauty' as such a yard st i ck. Most conditions for a successful resistance breeding program thus appeared satisfied: a reliable and practically relevant screening procedure, and an encouragingly high proportion of resistant clones in our breeding material which, thanks to years of selection, was of good average quality and productivity. What remained to be investigated was whether the resistance could be passed from parents to progenies. Screening of seedlings On the basis of the results of the clonal trials a number of crosses were screened, representing three different combinations of resistant and susceptible parents: crosses between resistant parents, crosses between a resistant and a susceptible parent and crosses between susceptible parents. 110

5 % symptomless plants L weeks Figure 1 - Screening of 2k IVT clones of 3 resistance classes under semi practical conditions. Percentage symptomless plants at different numbers of weeks after planting in infected soil. -O 8 resistant clones 8 medium resistant clones A 8 susceptible clones Orchid Beauty X Wi 1 I iam Sim 111

6 Table 4 - Screening of 13 progenies of crosses between resistant and/or susceptible parents. Mid-parent values and progeny means for percentage symptomless plants 10 weeks after inoculation. Progenies of seedlings in 3 replicates. Comb i nat ion Progeny no Mid-parent va 1 ue Progeny mean Res. x Res Res. x Susc Susc. x Susc The results (Table 4) showed that there is a clear relationship between the mean resistance levels of the two parents (mid-parent values) and the performance of the progenies. The trial also demonstrated indirectly that our method of screening seedling popu1 at ions, which differs from the one used for clones,is satisfactory: Two-week old seedlings are planted in trays of 30 x 40 cm, 60 plants per tray. Two weeks later the trays are watered with a diluted spore suspension (5x 10^ spores per litre). Plants showing external symptoms are removed at regular time intervals. Though more exact data on the inheritance of Fusarium resistance must await the outcome of the screening of a number of progenies from dial lei crosses, all conditions appeared to be fulfilled for the introduction of Fusarium resistance as a primary selection criterion in our breeding programme. Relationship between Fusarium resistance and Phialophora resistance Only one, perhaps academic question was still left unanswered at the time: What is the source of the high proportion of resistant clones in our breeding material? Without a satisfactory answer we would have been obliged to look for even better resistance elsewhere. A common feature of most of our clones is that they are all derived from parents with resistance to Phialophora. The simplest explanation for the high degree of Fusarium resistance would thus be some sort of relationship between Phialophora and Fusarium resistance. A positive relationship between Fusarium resistance and Phialophora resistance could indeed be demonstrated in all Fusarium screening trials, comprising clones that had earlier been tested for Phialophora resistance (Table 5). In each case the highest proportion of Fusarium resistant clones was found among the most Phialophora resistant ones. Similarly, 112

7 the lowest proportion was found among the most Phialophora susceptible clones: Table. - Percentage clones with the highest degree of Fusarium resistance ( 60 a 70 X symptomless) in different classes of Phialophora resistance (in brackets: number of clones in each Phialophora resistance class). Phialophora resistance % symptomless plants Fusarium trial 75-1 Fusari um trial 75-3 Fusarium trial and more 6 (18) 27 (15) 60 (20) 67 ( 9) 73 (11) ) 0 ( 6) 0 ( 7) 16 (25) 24 (17) 24 (17) 0 ( 9) 0 (11) 10 (20) j 21 (14) The relationship is not a very strict one - there are many instances of Phialophora resistant clones which are very susceptible to Fusarium - and its genetics remain to be studied in more detail. There seems little doubt, however, that our breeding material is such a good source of Fusarium resistance because of a partly common genetic basis for Phialophora and Fusarium resistance. There are indications that this common basis is to be found in the Chabaud parent and some of the older French and Italian cultivars used in the earlier generations. We intend to exploit this material to the fullest but to add any interesting resistant material that comes our way. Refe rence PERGOLA, G., and GARIBALDI, A., Observations on the resistance of carnation varieties to P h i a 1 o p ho r a cinerescens and to Fusarium oxysporum f. sp. Dianthi. Eucarpia Réunion sur les plantes ornementales, Fréjus, 5-6 mars 1974: