Fruit Varieties Journal 46(3):145-166 1992
Breeding Apples for Scab Resistance: 1945-1990
J. A. Crosby,1 J. Janick,1 P. C. Pecknold, 11 S. S. Korban,2 P. A. O'Connon,2 S. M. Ries,2 J. Goffreda3 and A. Voordeckers3
1Purdue University, West Lafayette, IN 47907, USA.
2University of Illinois, Urbana, IL 61801, USA.
3Rutgers, The State University of New Jersey, New Brunswick, NJ 08903, USA.
The breeding of apples resistant to scab incited by Venturia inaequalis (Cke.) Wint. is a genetically-based strategy for the control of this major fungal fruit pathogen. A concerted breeding effort began with three cooperating Agricultural Experiment Stations in the late 1940s and early 1950s. Soon afterwards collaboration was extended to research workers in Canada and Europe, and later to other continents. The effort that continues today in at least 17 breeding programs throughout the world was based on a modified backcross program to combine genes for resistance to apple scab from Malus floribunda 821, and other species with commercially-accepted traits. Since 1970, 48 scab-resistant cultivars have been released worldwide of which 37 purportedly carry the Vf gene from M. floribunda 821, one of which ('Freedom') carries additional polygenic resistance from 'Antonovka,' one (Imrus) with Vf from M. atrosanguinea 804: five with other genes [one ('Rouville') with the Vm gene from M. atrosanguinea 804; one ('Nova Easygro') with the Vr gene from a Russian apple seedling from the Caucasas Mountains (R#12740-7A); one ('Murry) with Vm and/or Vf, from M. micromalus, and three ('Romus 2', Gavin' and 'Generos') with polygenic resistance only. There now exists a wide range of genotypes containing the Vf gene ranging in maturity from 75 days to 180 days or longer after flowering with large fruits, crisp flesh, good storage behavior, an a wide range in flavor and skin color. A number of selections have been identified that contain varying degrees of resistance to other diseases and pests.
A breeding program to produce apple cultivars resistant to scab incited by Venturia inaequalis (Cke.) Wint. was initiated in 1945 by J. R. Shay at Purdue University and L. F. Hough, then at the University of Illinois but later at Rutgers, The State University of New Jersey. This collaboration led to a cooperative breeding program between Purdue University, Rutgers University and the University of Illinois, known by the acronym PRI.
The major source of scab resistance used in this program was first identified by Dr. L. F. Hough. Newly appointed as assistant professor in 1942, he was responsible for evaluating a collection of Malus species and hybrids previously assembled by Professor C. S. Crandall at the University of Illinois in the beginning of this century. Unusually cool, wet weather in the s ring of 1943 resulted in a severe epidemic of apple scab which defoliated all susceptible, unsprayed apple trees. One progeny derived from the cross (Malus floribunda 821 x 'Rome Beauty') x (M. floribunda 821 x 'Rome Beauty') approximated a segregation ratio of 1 scab resistant : 1 scab susceptible. Further crosses with two resistant selections from this cross indicated that resistance was conferred by a single qualitative dominant gene, or by a block of closely linked quantitative genes, that was subsequently named Vf (Venturia resistance from floribunda). The 1:1 ratio apparently resulted from the following cross:
Vfvf x vfvf à 1 Vfvf : VfVf
It was later demonstrated that intercrossing plants heterozygous for resistance would produce the classic ratio of 3 resistant : 1 susceptible (21).
Resistance conferred by Vf had many desirable attributes: (1) it showed no evidence of being violated by new fungal races, (2) it was not linked to any undesirable effects, (3) it acted as a true dominant in the diploid, triploid or tetraploid condition, and (4) it was found in two selections, F226829-2-2 and F226830-2, that were horticulturally advanced over the original species (M. floribunda 821, which has fruit about 1 cm in diameter). These two selections formed the basis of the present scab-resistant program in apples.
Dominant genes for scab resistance were transferred by a repeated series of hybridizations of genotypes heterozygous for resistance with adapted, high quality genotypes, which are homozygous for scab susceptibly. In these modified backcrosses, half of the progeny contained dominant resistance in the heterozygous condition and were clearly identified in the seedling stage when inoculated with spores of the fungus (21). During the first three or four generations of crossing, the scab-resistant genotype was usually the pollen parent because the resistant selections were often small trees with limited flowering.
Seed from controlled pollination crosses made each spring were sent to Purdue University where they germinated after stratification and were screened for scab resistance in the seedling stage each winter in the greenhouse. This phase of the program was long under the direction of Dr. E. B. Williams of the Department of Botany and Plant Pathology. Resistant plants were moved directly to permanent positions in the field or tree and fruit evaluations; promising selections provided parents for the next breeding cycle.
The crossing process was repeated with the most horticulturally desirable of each generation of scab-resistant seedlings. After 34 generations this process, in essence, captured the dominant scab resistant gene from M. floribunda 821, replacing most of its other genes with those of the horticulturally desirable but scab-susceptible parents. To avoid inbreeding (backcrossing is equivalent to selfing in bringing about homozygosis), the high quality scab susceptible genotypes (equivalent to the recurrent parent in a classical backcross program) were varied each generation. This protocol recombined traits of the different high-quality susceptible parents. In 1990, the Purdue, Rutgers, Illinois program may be summarized as follows:
2. Stability of Resistance
The possibility of development of races of the fungus that would overcome an individual source of resistance was recognized very early in the program (38). Efforts were undertaken to find and characterize additional sources of resistance.
Subsequent studies by J. R. Shay, E. B. Williams, and D. F. Dayton identified 25 different Malus species and selections possessing high levels of genetic resistance to apple scab (19, 45). Through a series o intercrosses (tests of allelism) it was established that 11 of the 25 sources were due to the same Vf gene (45). Six loci for qualitative resistance (defined gene pools) were identified as follows:
Five different virulent races of Venturia inaequalls have been identified, four of which infect and overcome certain genes for resistance. Thus, race 5 of V. inaequalis was found at the John Innes Institute, Norwich, England (44) that overcame Vm, the Micromalus pit type resistance found in M. microma1us and M. atrosanguinea 804. Resistance in the original species sources of Vm consisted of two qualitative loci for resistance. When the two loci were separated in segregating progeny, it was found that Vm by itself was vulnerable to race 5.
Race 1 is the race commonly encountered throughout the world. Race 2, collected in South Dakota, successfully attacks: M. baccata 'Dolgo,' 'Alexis , and 'Bittercrab'; certain segregates of the Russian seedling R12740-7A (but not Vr); and 'Geneva' (M. pumila var. niedzwetzkyana o.p.). Race 3, collected in Nova Scotia, Canada, overcomes the resistance found in 'Geneva.' Race 4, found in a scab resistant seedling block in Lafayette, Indiana, successfully attacks certain segregates of the Russian seedling R12740-7A, suggesting a third locus for qualitative resistance different from that overcome by race 2 or Vr (45).
Although Vf is traditionally considered to be a simply inherited, dominant factor conditioning qualitative resistance, it is often associated with any number of quantitative resistance factors (22, 45), which make it less vulnerable to attack by new races of the fungus. Progeny segregation ratios and the frequency distributions of leaf reaction classes suggest the complexity of the genetic basis of resistance derived from Vf sources.
Reaction classes for leaf inoculations under greenhouse conditions were established early in the program: 0 = no macroscopic evidence of infection; 1 = pin point pits and no sporulation; 2 = irregular chlorotic or necrotic lesions and no sporulation; 3 few restricted sporulating lesions; M mixture of necrotic, nonsporulating and sparsely sporulating lesions; and 4 = extensive abundantly sporulating lesions (20, 37, 45).
Modified backcross progenies from V parents exhibit resistant reaction classes ranging from 2 to M to 3 after inoculation in the greenhouse (E. B. Williams, unpublished data) - When reaction classes 2, M, and 3 were considered resistant and class 4 susceptible, ratios approximating 1 resistant : 1 susceptible were observed in backcross progenies. When sporulation, however slight, is considered susceptible, a much lower ratio of resistant: susceptible is observed. In this case, rarely do the resistant fraction of the backcross progenies approach as much as a third (29). Additionally, the original M. floribunda 821 resistance was expressed as a class 1 reaction and F2 26829-2-2 and F2 26830-2 were class 2 reaction types. From these observations it is hypothesized that Vf is a simply inherited gene that confers a class 3 reaction type, and that additional genes enhance or augment the effect of V to give class 1, 2, and M reactions. It is very likely that these additional or modifier genes are complexly inherited quantitative factors. Hough et al. (22) interpreted Vf resistance as "the result of a hypersensitive reaction and that the intensity of the hypersensitivity is reinforced by quantitative genes inherited in a cumulative manner.
H. S. Aldwinkle (1989, unpublished) reported that resistant reactions exhibited by plants inoculated in the greenhouse showed a continuous gradation from hypersensitive to restricted sporulation and then on to susceptible, rather than as discrete reaction classes. This observation confirms that the qualitative resistance conditioned by Vf is genetically complex and is almost always fortified by a number of quantitative resistance factors even in seedlings identified as Class 3 reaction type.
The performance of Vf scab resistance in the field, attests to the usefulness of this gene and its genetic complexity. After over 40 years of widespread testing, in humid apple production regions around the world, Vf resistance has not been overcome by new races of V. inaequalis. All occurrences of reported breakdown of resistance have been attributed to mislabeling of apple propagules. Hough et al. (22) has documented published accounts.
We do not consider reports of low levels of scab infection on PRI resistant selections in Europe (2, 26, 41) as a breakdown of resistance. Originally reaction classes M and 3 exhibited limited sporulation in the greenhouse. 'Prima,' the first scab-resistant cultivar (8) which carries Vf, exhibits only a class 3 reaction type. Limited leaf sporulation of V. inaequalis may be seen occasionally on 'Prima' and a few other Co-op selections under optimal conditions for infection with rare minor infection on fruits-yet, they have maintained a high level of field resistance wherever they have been grown. Class 2 reaction types have always remained field immune. Except for 'Prima,' the reaction class of the Coop series of selections is unknown.
3. Co-op Selections
In the 1960s, cooperators in the PRI breeding program felt that scab resistant germplasm had progressed to the point where various high-quality selections were ready for widespread testing. In response, the "Co-op" series of releases was introduced as a mechanism to focus attention on deserving selections and to emphasize the cooperation and joint effort of the three universities behind the releases.
From 1967 to 1990, 31 Co-op selections were released for advanced testing and distributed for testing throughout the world (6, 25, 43, 47, 49, 51). During the period 1970 to 1988, seven of these selections were named, and offered to the nursery industry for propagation: 'Prima' (Co-op 2) (8); 'Priscilla' (Co-op 4) (46); 'Sir Prize' (Co-op 5) (48); 'Jonafree' (Co-op 22) (7);'Redfree' (Co-op 13) (50);'Dayton' (Co-op 21) (24), and 'Williams' Pride' (Co-op 23) (23). In addition, two other selections made at Purdue University and sent to other locations for testing were named: 'Priam' in cooperation with the Station d'Arboriculture Fruitiere, INRA, Angers, France (10); and 'McShay'in cooperation with Oregon State University (32). A scab-susceptible escape was named 'Viking' by F. H. Gilbert at the Wisconsin Agricultural Experiment Station in 1969.
A description and evaluation of each of the Co-op selections is summarized in Table 1. Co-op selections 1 - 26, released prior to 1985, have been evaluated sufficiently to establish their potential. The information for this evaluation was gathered from many different sources, including a comprehensive worldwide survey of cooperators made by J. A. Crosby in 1985-86. Most of the information on growth and fruiting comes from ongoing evaluations in Indiana, Illinois, and New Jersey. Relative disease resistance is based on observation of natural field infection in these states as well as from cooperators in other states and countries. A few recently published reports give good indication of disease incidence under epidemic conditions (26, 40, 41, 42). Information on longevity in refrigerated storage is based on data collected at Purdue University from 1984 to 1991.
Although it is unlikely that any of the currently unnamed releases in Co-op 1 through 26 will be named or widely grown, several have been and will continue to be valuable as parents for breeding. Co-op 8 and Co-op 14, recently evaluated by EMBRAPA in Rio Grande do Sul, Brazil, have proven to have low-chilling requirements, inherited from 'Mollie's Delicious.' Interest has been expressed in the release of both selections in Brazil. The Bålsgard Department of Horticultural Plant Breeding Sweden has recently evaluated Co-op 12 and found it to be well adapted to Swedish conditions. Interest has been expressed in recommending Co-op 12 as an early apple for home garden in Sweden. Co-op 15, recently evaluated in western Oregon, exhibits promise for utility in the Pacific Northwest, west of the Cascades, due to its field immunity to scab, high level of resistance to powdery mildew, and good fresh fruit quality. Goonewardene and Williams (1988) identified Co-op 17 as the most resistant of 360 PRI selections screened for different insect pests including apple maggot, Rhagoletis pomonella (Walsh), plum curculio Conotrachelus nenuphar (Herbst), and the redbanded leafroller Argyrotaenia velutinana (Walker) (15). When crossed with 'Golden Delicious,' it yielded several outstanding selections with promising fruit quality and excellent keeping ability. Co-op 17 is recommended for use in breeding as a potential source of insect resistance, firm flesh type, smooth, yellow fruit finish, and long-term retention of quality in storage. After ten years of evaluation, Co-op 25 stands out as one of the longest keeping, highest quality selections to originate from the PRI Program. The fruit are sweet with a ragrant, fruity flavor, blushed red to orange, hard, and dense with very firm, crisp and breaking texture. Harvested with 'Delicious' in Lafayette, Indiana (Sept. 20 to Oct. 1), Co-op 25 can keep until the following June in refrigerated storage, and in 1990 compared favorably in quality to 'Fuji' in April and May. Co-op 25 has not been named for release because of its marked susceptibility to fire blight. We are encouraging the testing of Co-op 25 only in areas where fire blight is not a problem.
Co-ops 27 to 31 differ from the six previous series of releases from the PRI program in that all five selections mature relatively late in the harvest season. These selections have fruited as grafted trees at West Lafayette, Indiana and/or Urbana, Illinois from 8 to 11 seasons, and are only now beginning to fruit at other sites in the U.S. All five selections show promise based on good flavor, firm, crisp flesh type, and good retention of texture and quality in storage. The utility of Co-op 28 may be limited by its susceptibility to fire blight.
4. Cultivar Releases
'Pirima,' 'Priscilla,' and 'Sir Prize' have been widely distributed since 1967 and evaluated under many environmental and cultural conditions. 'Prima' currently has some utility as a roadside market or commercial early fresh market substitute for 'Jonathan' in the mid-western U.S. and eastern Europe, essentially in areas where 'Jonathan' is grown. 'Prima 'is somewhat susceptible to winter cold injury and to sun scald on exposed bark. It is not suitable for cropping in far northern areas. More than 70,000 'Prima' trees have been sold in the U.S. from 1974-1990, and some commercial plantings of 800 to 1000 trees exist in the U.S. Midwest. 'Prima' has been used extensively as a parent in breeding programs worldwide and has been of special interest in eastern Europe. It appears that 'Prima' will have little utility in western Europe other than as parental germplasm or a pollinator. The sole licensee in western Europe reported that sales did not support the maintenance cost of varietal protection in France and Germany, and protection was discontinued. 'Prima' continues to have varietal protection in the U.S. and Denmark, and is used as a pollinizer in solid blocks of other cultivars. The U.S. Plant Patent (Reissue No. 28,435 expires in June of 1992.
'Priscilla,' has been planted on a small scale as a specialty apple for home or road-side fresh market use. More than 40,000 'Priscilla' trees have been sold in the U.S. from 1974-1990. During the early 1980s its reputation was damaged due to a mix-up in a scion orchard of a major U.S. licensee, resulting in the substitution of Co-op 3, an inferior scab resistant selection, for 'Priscilla' in six or more years of sales. 'Priscilla' has very good fresh fruit quality with firm, crisp flesh texture but the sweet, aromatic, somewhat licorice-like flavor is disliked by some consumers. The cultivar's most significant problem is its lack quality retention in storage, but if handled properly, 'Priscilla' can maintain adequate flavor and texture in refrigerated storage for 3 months. 'Priscilla' is highly resistant to fire blight. It has rivaled or surpassed 'Liberty' and 'Prima' in observations of fire blight epidemics in North Carolina (40) as well as 'Liberty, Prima,' and 'Williams' Pride' in severe epidemics in Vincennes, Indiana in 1989 and 1990. Its unique quality and high level of overall disease resistance should allow it to survive for local markets in the U.S. In Europe, 'Priscilla' currently has plant varietal protection only in Denmark. In the 1970s, the main licensee in Europe fruited 'Priscilla' and 'Sir Prize' for the first time, and displeased with them, opted not to procure varietal protection for France or Germany. Few trees were ever sold. 'Priscilla will have little utility in Europe other than as a backyard apple. The U.S. Plant Patent (No. 3,488) expires in February of 1991.
'Sir Prize': a triploid seedling from a progeny of tetraploid 'Golden Delicious', was released with an overwhelming flaw: the fruit are too tender and susceptible to bruising. Despite good quality, attractive appearance, and high productivity, this one flaw has doomed 'Sir Prize' as a commercial cultivar. Although easily bruised, the flesh is not soft, the tree is very productive, and quality is usually ranked quite high. More than 6000 trees of 'Sir Prize' were sold in the U.S. from 19761990.'Sir Prize' is not being propagated in the U.S. except on a per order basis. Plant varietal protection was never enacted in Europe and few trees were sold. 'Sir Prize' will have little utility in Europe or the U.S. other than as a home apple. The U.S. Plant Patent (No. 3,988) expires in December 1993.
'Priam,' named in cooperation with INRA, Angers, France, was reported to have utility in the Loire Valley, the Massif Central Valley, and the east of France. The fruit is quite acid and was originally intended to fill a specific niche in production in France and Belgium as a late, red, markedly-acid apple. Limited in adaptation, it was released as a stopgap cultivar. Originally selected in Indiana, it had been discarded in the second test block at Purdue because it was too highly acid for American tastes and was markedly susceptible to powdery mildew. 'Priam' continues to be tested by cultivar evaluation programs in Europe and is suggested for use in the home garden. It was never patented and is currently unavailable in the U.S.
'Jonafree' and 'Redfree' are gaining in popularity in the U.S. Productivity, good red color, competitive fruit quality, and adequate fruit storage life are factors expressed repeatedly in reports by growers. Expressions of interest by nurseries, the appearance of fruit in small scale in Indiana supermarkets and roadside markets, and the first overseas shipment of 'Redfree' apples to market in Europe in 1990 offer encouraging signals for the future. In the 5 year period 1986-1990, 22,000 trees of 'Jonafree' and 14,000 of 'Redfree' have been sold. 'Jonafree' may find a niche as a scab-resistant substitute for 'Jonathan,' ripening in the same season, with attributes such as lower acidity, redder fruit color, and similar storage life. Although 'Jonafree' has better resistance to powdery mildew, cedar-apple rust, and fire blight than 'Jonathan,' its moderate susceptibility to these diseases will require control measures. 'Redfree,' an early, sweet, summer apple, is somewhat earlier than 'Paulared' and later than 'Vista Bella'. Its mild, pleasantly-sweet flavor and four-week retention of firm-ness in storage make this a very competitive in a season filled with tart to acid, short-lived or early 'McIntosh'-like cultivars. 'Jonafree' and 'Redfree' are under evaluation in Europe. 'Jonafree' was licensed in Yugoslavia in 1979. Both cultivars are protected by plant patent in the U.S. The U.S. Plant Patent (No. 4,322) for 'Redfree' expires in October of 1995, and (No. 4,633) for 'Jonafree' expires in January of 1998.
'Dayton, 'Williams' Pride,' and 'McShay' are too new to have established a reputation for adaptation or acceptance. 'Williams' Pride' was named in honor of E. B. Williams, long-time leader of the breeding program at Purdue. It offers potential as an early summer, mildly acid, full flavored dessert apple, maturing just before 'Redfree,' that will store for up to 6 weeks. It has excellent fall quality for a summer apple but has Town excessive water core and bitter pit in some years. 'Dayton,' named in honor of D. F. Dayton, long-time coordinator of the project at the University of Illinois, may offer an alternative to or replacement for 'Prima' with more red color, more uniform fruit size and shape, and less susceptibility to rust. 'McShay' was named in honor of the late J. R. Shay, originator of disease resistant apple breeding at Purdue and former Assistant Dean of Research at Oregon State University. 'McShay,' originally selected at Purdue, was named in cooperation with Oregon State University, after many years of evaluation at Corvallis. It performs well as a scab-resistant substitute for 'McIntosh' in Oregon's Williamette Valley. 'Dayton,' 'Williams' Pride' and 'McShay' are all protected by U.S. Plant Patents.
The scab-resistant apple breeding program was conceived to increase grower efficiency by reducing fungicidal sprays. The impact of the environmental movement and ever-rising costs of energy have greatly increased the relevance of this program. Widespread adoption of disease-resistant cultivars could impact food safety, groundwater purity, protection of nontarget organisms, applicator exposure to pesticides, and also lower the cost of production.
The success of the modified backcross program to confer scab resistance in apple was due to a number of additional factors which should be underscored:
Equally important has been the bonus achieved by the coordination of apple breeding that this cooperative program engendered. In 1963 an informal organization known as the Apple Breeders Cooperative was formed which coordinates research efforts among apple breeders with special attention to disease resistance. The program is now cooperative between 16 active apple breeding programs throughout the world. These include: New York State Agricultural Experiment Station, Geneva, and the University of Minnesota, Excelsior, U.S.A.; Summerland Agricultural Research Station, Summer-land British Columbia, and Agriculture Canada Research Station, Kentville, Nova Scotia, Canada; EMPASC, Cacador and Sao Joaquim, Santa Catarina, and at EMBRAPA, Vacaria, Rio Grande de Sul, Brazil; INRA, Angers, France; Granite Belt Horticultural Research Station, Applethorpe, Queensland, Australia; DSIR, New Havelock, New Zealand; Orlovskaja Plodovo-Jagodnaja Opitnaja Stanzija, Zhilina, Orel, USSR; Swedish University of Agricultural Sciences, Bålsgard-Department of Horticultural Plant Breeding, Kristianstad, Sweden; SCPP Voinesti, Judetul Dimbovita, SCPP Bistrita, ICPP Pitesti, and SCPP Cluj, Romania; Research Institute of Pomology and Floriculture, Skierniewice, Poland; IVT, Wageningen, the Netherlands; Research and Breeding Institute of Pomology, Holovousy, Institute of Experimental Botany and O. Louda (private breeder), Prague, Czechoslovakia; and Horticulture Research International, East Malling, Kent, England.
Scab-resistant selections for advanced or intensive testing have been identified by many of these programs. Recent reports by breeders at the respective stations have identified the promising selections listed below:
EMPASC, Cacador, Santa Catarina. A number of selections from crosses with 'Gala' and 'Fuji,' deriving resistance from M. floribunda 821.
Breeding Institute of Pomology. Holovousy. Eight advanced selections given to the State Variety Trials. All derive resistance from M. floribunda 821 and quality characters from 'Britemac,' 'Golden Delicious,' 'Cox's Orange Pippin,' and Red 'Delicious.' Two other selections with polygenic resistance from 'Antonovka' (second generation offspring crossed with 'Golden Delicious'), HL 362 ('Angold') and HL 369, are promising in trials.
Institute of Experimental Botany and O. Louda, Prague. 'Harmonie' is a working name for selection 163-3, a seedling of 'Prima' x 'Sampion.' UEB 1679/4, UEB 1740/5, and UEB 1173/1 are other advanced selections currently under test in Czechoslovakia. These derive resistance from M. floribunda 821 and quality from 'Lord Lambourne' and 'Golden Delicious' as seed or pollen parents.
Horticulture Research International, East Malling, Kent. A272/5, A720/7, E11/20, and SA15/4 are all advanced selections introduced into grower trials in England. Seed or pollen parents include: 'James Grieve,' 'Cox Orange,' 'Michaelmas Red,' 'Falstaff,' and 'Starkspur Golden Delicious.' Resistance sources include Vf and Vr. A92/23, A172/2, and A567/19 have shown promise in trials in Switzerland. Seed or pollen parents include: 'James Grieve,' 'Cox Orange,' and 'Tydeman's Late Orange.' These derive resistance from M. floribunda 821.
INRA, Angers. X 3191, X 3189, X 3177, P6 R28-6, P6 R28-39, and P22 R17-66 are currently under test in the U.S. Seed or pollen parents include: 'Idared,' 'Winesap,' 'Florina Querina,' 'Gala,' 'Prima,' and 'Jonamac.' All derive resistance from M. floribunda 821.
IVT, Wageningen. IVT 78039-8, IVT 78039-18, IVT 78039-20, IVT 78039-26, and IVT 78039-27. All are seedlings of 'Elstar' x 'Prima; some seedlings of 'Priscilla' are also promising.
Research Institute of Pomology and Floriculture, Skierniewice. V-136-D2, V-144-D2, V-151-D2, V-142-D2, and VI-17-D2. Seed or pollen parents include: 'Primula' and 'Bancroft.' Resistance sources include Vf and Vr.
SCPP, Voinesti, Bistrita, and Cluj. 126 selections from crosses with 'Prima'; 2 selections from crosses with 'Priam'.
The Research Institute of Fruit Breeding, Orel. Elita 18-13-27 ('Antonovka' x PRI 240-57), selected as an elite seedling in 1988, contains Vf resistance from M. atrosanguinea 804. Elita 18-11-43 ('Skrizhapel' x 1924), Elita 18-52-13 ('Korichnoye Polosatoye' x 'Priam'), Elita 18-62-57 (814 open pollinated), and Elita 23-14-123 (814 open pollinated) were selected as elite seedlings between 1988 and 1990 and contain Vf resistance t from M. floribunda 821.
United States of America
New York State Agriculture Experiment Station. NY 6570-1-19, NY 66305-139, NY 73334-35, NY 74828-12, NY 74840-1, NY 75413-30, NY 75414-1, and NY 75441-67, offered for widespread testing in the U.S. through the New York Fruit Testing Association. Seed or pollen parents include: 'Spartan,' 'Liberty', 'Delicious,' 'Jonamac,' 'Empire,' 'Prima,' and 'MacSpur.' Most of the above selections derive resistance from M. floribunda 821.
Purdue, Rutgers, Illinois. Co-op series 27, 28, 29, 30, and 31. The process is underway to patent Co-op 30. Other promising selections to be released in an upcoming series of Co-op releases include: HFRow 34, HER4TI6, CLR13T45, TNR10T11, and CLR13T40. Seed or pollen parents include: 'Golden Delicious' and 'Prima.' All of the above selections derive resistance from M. floribunda 821.
The cultivar releases of all programs are presented in Table 2. Other ongoing efforts to utilize scab resistance exist in China.
Although none of the cultivars released are currently of widespread commercial importance, a number show promise. 'Liberty' and 'Prima' are at present the most widely planted scab-resistant cultivars in the U.S. 'Redfree' and Jonafree' are quickly gaining favor and in our estimation will become important. 'Florina Querina' (31) appears promising in Europe and initial evaluations in the U.S. are encouraging.
Most important, the Vf gene has not been overcome by the development of virulent new races of V. inaequalis and no negative pleiotropic effects of this gene have been observed. Reports of V. inaequalis infection of various Co-op selections in Moldavia have been attributed to mislabeling (I. P. Turcan, personal communication). Low levels of scab infection reported on various Co-op selections in Europe under optimal conditions for disease development are consistent with original phenotypic expressions of resistance determined in the greenhouse inoculation trials. Class 3 reaction types would allow for a low level of leaf infection in the field under optimal conditions.
There now exists a wide range of genotypes containing the Vf gene, ranging in maturity from 75 to 180 days after flowering with large size, crisp flesh types, good storage behavior, and a wide range in flavor and skin color (red, orange, yellow, and green). A number of selections contain varying resistance to other diseases including fire blight (Erwinia amylovora (Burr.) Winslow), powdery mildew (Podosphaera leucotricha (Ell & Ev.) Salm), and cedar-apple rust (Gymnosporangium juniperi-virginianae Schw.). A few selections (e.g., 'Liberty', 'Williams' Pride' and Co-op 26) exhibit high levels of resistance to all four of these major diseases. These results indicate that traditional breeding will continue to be an effective strategy for controlling some of the major diseases of apple.
The future objectives of apple breeding must be to combine scab resistance with resistance to other diseases, the most important being cedar-apple rust, powdery mildew, fire blight, and various summer diseases. Tolerance for many insect pests exists in the scab-resistant germplasm (12, 14, 15, 16) and insect resistance will become an objective in the future. Current programs are under way in California to add the Bt gene from Bacillus thurengensis to scab resistant genotypes by genetic transformation. The University of Illinois Cornell University have begun to use molecular techniques to investigate the transfer or induction of resistance on the molecular level, including the genetic transformation of apple by Agrobacterium spp. and other techniques (34). Despite all efforts on pest resistance it is clear that resistance must be combined with superior fruit quality for these improved genotypes to gain acceptance in the marketplace.
We wish to acknowledge many other individuals who have carried on the PRI program including E. B. Williams (retired), F. H. Emerson (retired), Norman Robinette (deceased), and James Ingram (retired) of Purdue University, L. F. Hough (retired) and Catherine Bailey (retired) of Rutgers University, D. F. Dayton (retired) and J. B. Mowry (deceased) of the University of Illinois, J. R. Shay (deceased) of Purdue University and Oregon State University, Maxine Thomas (retired) of Oregon State University, and Shawn Mehlenbacher of Rutgers, The State University of New Jersey and Oregon State University.