Tree Fruit Leader, Vol. 3(2) July 1994

Scab Resistant Apple Varieties - Problems and Potential

by Gayle Jesperson, P.Ag., Plant Pathologist BCMAFF, Kelowna

Recently, there have been several new releases of scab resistant cultivars from breeding programs in the eastern U.S. These cultivars are much improved over older resistant varieties, most of which had many undesirable characteristics. 'Goldrush', 'Enterprise' and 'Pristine' are three new varieties with commercial potential that have received favourable press this year.

Scab resistant varieties are actually "field immune"-they don't get any scab at all. The majority of scab resistant cultivars carry the dominant "Vf" gene, derived from the ornamental crab Malus floribunda 821. This gene held up for 50 years with no problems, until a resistant strain of scab was found on apples in Germany in the 1980's. This new strain (named race 6) can attack many of the resistant cultivars, but has not been confirmed yet in North America. Breeding programs will have to concentrate more on diversifying their sources of resistance for long term scab control.

Breeding programs in North America

There are three major breeding programs which develop disease-resistant apples in North America. The PRI program (Purdue University, Rutgers University and University of Illinois cooperative breeding program) released 11 named cultivars by 1992, including Prima, Priscilla, Jonafree, Redfree, Enterprise, and Goldrush. The Cornell/Geneva program in New York has also released several scab resistant cultivars, including Liberty and Freedom. Agriculture and Agri-Food Canada has a breeding program at the Kentville, Nova Scotia Research Station. They have released Nova Easygro, Nova Mac, and Nova Spy. A now defunct breeding program in Ontario released several varieties including Macfree, Britegold, Trent, Richelieu and Rouville. Other selections from the Ontario program are now being evaluated in Quebec by Agriculture and Agri-Food Canada.

Scab resistant cultivars

Currently 'Liberty' and 'Nova Easygro' are the only scab resistant varieties available at the budwood orchard. Liberty was developed at Cornell University about 15 years ago. It was the first resistant variety with commercial potential. Liberty scored well in taste tests conducted at Rutgers University in 1988 and 1989, where it was compared to Red Delicious, and three other resistant varieties: Prima, Priscilla and Freedom. Freedom and Liberty received the highest ratings for overall quality. Comments on Liberty published in the PNW Fruit Testers Association 1992 Annual Report ranged from "good firewood" to "quality surprising" and "planting more". This shows the importance of testing varieties in a number of locations. Most of the comments from the Lower Yakima Valley location were positive, reporting a sweet/tart flavour with pleasant aroma, uniform colour and attractive appearance. The tendency to produce a heavy crop of small apples appears to be the main drawback. A good job on thinning may alleviate this problem.

In a 1991 article titled "Scab Resistant Cultivars: The Good, the Bad and the Ugly", written by researchers at the University of Vermont, several scab resistant varieties were rated for overall quality. Redfree, Prima, Nova Easygro and Liberty were rated as good. Small size was again the main problem identified with Liberty. Nova Easy-gro was reported to store well, maintaining its sweetness and firmness for at least several months. Freedom fell into the "bad" category, with comments that it was not a very attractive apple, and the firmness was mediocre. Jonafree was also classed as "bad" because it is a "hard" apple, not well liked by consumers in the east, although appearance and storability were good. Priscilla was the "ugly" apple. The main problems identified were fruit drop, soft fruit, small size and poor storability. These observations were based on fruit grown at one location only.

Consumer acceptance trials at the Rutgers Fruit Research and Development center in 1991 evaluated Liberty, Jonafree, two numbered cultivars and spur Delicious. Delicious was the least preferred. Testers liked Co-op 18 the best followed by Jonafree. Jonafree was chosen best for overall appearance.

Here are some comments on scab resistant cultivars from the PNW Fruit Testers Association:

Freedom: good size with flat shape; russet prone; potential as a processing apple

Prima: very short storage life; not enough quality to compete with Gala

Redfree: No scab but no quality; subject to softening in hot weather; good quality for cool growing areas; susceptible to water core

Jonafree: quality too inferior to recommend further testing

Goldrush: very productive; good keeper; rich flavour and appealing texture; very late with hard texture but improves in storage; storage is as outstanding as Fuji's

Co-op 25: good size, colour, texture; worthy of widespread testing.

Taste panel evaluations conducted by researchers in the northeastern U.S. indicate excellent consumer acceptance of several cultivars. Goldrush and Enterprise were recently rated better than standard cultivars and other scab resistant cultivars after five months in cold storage.

Goldrush (Co-op 38) was released in 1992. It was named after its gold and bronze blush colour and rush of flavour. It is a late season variety (probably Fuji-season), equal to Golden Delicious in taste and firmness. In fact, it has strong Golden Delicious parentage in its breeding background. It is immune to scab, highly resistant to powdery mildew, and moderately resistant to fire blight. It has a high sugar content, high acid content, and is described as having very good to excellent flavour and texture. Furthermore, it keeps for at least seven months in cold storage, and longer in controlled atmosphere storage. Flavour is best after one to two months of storage time. The skin colour is green-yellow at harvest, and saffron yellow at maturity. It can develop a heavy red blush on the sun exposed side of the fruit. Fruit size is medium to large. The tree is described as having moderate vigour and a highly desirable growth and bearing habit. This is perhaps the scab resistant variety with the most potential.

Enterprise (Co-op 30) was released in 1992. It is immune to apple scab, highly resistant to fire blight and moderately resistant to powdery mildew. This is also a late season variety, maturing 2.5 weeks after Delicious in the eastern U.S. Enterprise produces medium to large-size fruit, with a medium red to slightly orange colour, smooth, glossy finish and thick skin. It has been compared favourably to Empire. Enterprise is also a very good quality winter storage apple with a long storage life of five to six months. The flavour is described as rich, spicy, slightly aromatic, and moderately acid at harvest, mellowing and improving in storage. The flesh is cream coloured, medium to fine grained and juicy. The tree is highly vigorous, producing moderately heavy crops.

'Pristine' (Co-op 32) is the most recent release from the PRI program, formally introduced in January 1994. It is immune to scab, resistant to mildew, and moderately resistant to fire blight. This is an early season "summer" apple, with medium sized, high quality fruit. It has smooth, pale green-yellow skin at harvest, which matures to a deep yellow with a moderate orange blush on the sun exposed surface. It retains quality and texture for four to six weeks in refrigerated storage. A greasy cuticle may develop after six to eight weeks of storage.

Are scab resistant varieties good for B.C.?

A good commercial quality scab resistant variety could be a positive addition to the B.C. industry.

Apple scab is the principal disease for which we apply fungicides. A block of trees that did not require scab sprays may not require any fungicide applications-a saving of time and money for the orchardist. A pink spray for powdery mildew would be necessary for some varieties with less than adequate mildew resistance. With the advent of the sterile insect release program, it could be possible to produce apples that have not been sprayed with any pesticides after the blossom period (most caterpillar pests like leafroller and budmoth can be controlled early in the season with well timed insecticide sprays).

Scab resistant varieties could also be very useful to organic producers, who now rely mainly on sulphur sprays to control scab. Sulphur may be "organic", but it also has a very high EIQ rating (see the article by H. Philip in this newsletter), and must be applied more frequently than other fungicides.

One problem with scab resistant cultivars in the east is that eliminating scab sprays results in fruit damage caused by other "summer diseases" such as flyspeck and sooty blotch. We have never had problems with these diseases in B.C. Producers of resistant cultivars in the east have also had increased problems with fruit rots. In our dry climate this is less likely to be a problem, especially if orchards are kept clean of perennial and anthracnose cankers, which are sources of Bull's eye rot. Other rots that could become more common in the absence of blossom sprays are Botrytis rot and moldy core. Not all varieties are susceptible to moldy core.

Consumer acceptance

Consumer acceptance of a new variety will ultimately decide its place in the market. It is not out of the realm of possibility that one of the new scab resistant cultivars could become the next "hot" new variety. Consumers are generally becoming a little more adventurous with food, and the novelty of a new cultivar is more likely to pay off than in the past. Growers may also be more likely to try out new varieties following the success of other relative newcomers, like Fuji and Gala. Consumers are also becoming increasingly aware of pesticide issues; apples marketed as pesticide-free or fungicide-free could probably find a market niche. Scab resistant cultivars would most likely be introduced by organic growers. Marketing would have to be accomplished initially through roadside stands, farmers markets, and specialty stores.

Trial at Kelowna Substation

A scab resistant variety trial was established at the Agriculture and Agri-Food Canada Kelowna substation in 1992 by Dr. Peter Sholberg and Dr. David Lane. This planting consists of 63 trees of 27 cultivars. The trees are planted in one row, with 1 to three trees per cultivar. This planting should provide some indication as to how these cultivars will perform in the Okanagan, and whether or not the fruit quality is acceptable. Some of the varieties include: MacFree, Prima, Nova Easygro, Jonafree, Priscilla, Sir Prize, Raritan, Trent, Redfree, Blair, Murry, Freedom, and other numbered varieties, including 12 from the Co-op series. The newer varieties, Goldrush, Enterprise and Pristine were not available when this trial was planted. Scionwood for Goldrush and Enterprise have since been obtained by the Summerland Research Station.

There is also a large project underway in the eastern United States to determine the commercial potential of selected scab resistant cultivars. More than 3500 resistant apple trees are being evaluated at 50 orchards across five states.

There appears to be a resurgence in interest in scab resistant varieties in the eastern United States. For example, during 1985 to 1989, Liberty accounted for about 0.6% of apple acreage planted in New England. An increase to 5% of new plantings was projected for 1990 to 1994.

Growers interested in the potential of scab resistant cultivars should try them out in their own orchards. Initially, it may be best to try a small number of trees of several different, but promising varieties. Scab resistant cultivars introduced by Cornell, Nova Scotia and the PRI program are now available from several commercial nurseries in the United States. With the trend of increasing quality of new scab resistant varieties, we can hopefully look forward to a wider selection of promising cultivars in the future.

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