PRIOR TO 1920 most breeding work with strawberries was conducted by private breeders. However, a comparison of the chapter on early breeders with that on breeders of today indicates that a change had taken place in the interim, for today's breeders are almost all employed at experiment stations where the work is supported by either the federal or the state governments. Tax-supported experimental work originated most of the varieties grown today; and the work continues, producing new varieties and adding to the knowledge about strawberries in general. Central to this work, is that conducted by the U.S. Department of Agriculture at Beltsville, Maryland, where extensive systematic breeding has been proceeding without interruption for forty-five years. Much of the work done there has affected, either directly or indirectly, the work at the state experiment stations, for the U.S. Department of Agriculture works cooperatively with many stations on the strawberry, and in the past has done much to disseminate breeding materials, ideas, and techniques to all those actively engaged in breeding. (See also under North Carolina and Wyoming.)
LOCATION AND PERSONNEL. A project for the improvement of the strawberry was undertaken by the U.S. Department of Agriculture early in 1920, with George M. Darrow in charge, and the work located at Glenn Dale, Maryland, fifteen miles out of Washington, where Dr. Walter Van Fleet was breeding roses and chestnuts and had made some strawberry crosses. Before his federal service, Dr. Van Fleet had bred strawberries in New Jersey, and had named the Early Jersey and Late Jersey varieties. After Dr. Van Fleet's death in January 1922, the strawberry work was greatly expanded. The first crosses were planted at the Glenn Dale Horticultural Field Station in 1921 and one-half acre fruited in 1922. For two years (1921 and 1922) crosses were made in the field, but the loss of crossed flowers by frosts was so great that the later crosses were made in an unheated sash glasshouse (Fig. 14-1). The seedlings also were raised in it. In 1922 four acres and the following year about eight acres of seedlings were grown. G.F. Waldo joined the team in 1926.
In 1928 strawberry breeding was initiated at Willard, North Carolina, in cooperation with Charles Dearing of the North Carolina Department of Agriculture, and later, in 1936, in cooperation with the North Carolina State Experiment Station, of which Prof. E.B. Morrow was in charge until 1956. In 1928 also breeding was begun at Corvallis, Oregon, in cooperation with the Oregon Experiment Station, with Prof. Carl Schuster in charge. In 1932 the central breeding work was moved from Glenn Dale to Beltsville, both in Maryland, where it has been continued ever since. From 1930 to 1932, George M. Darrow was in charge at Corvallis. Since then G.F. Waldo has been in charge there. From 1941 to 1945 E.M. Meader was associated in the work at Beltsville. From 1946 to 1957 D.H. Scott was associated with the work, and has been in charge from 1957 to the present. Dr. R.J. Knight, now at Chapman Field, Miami, Florida, assisted from 1957 to 1961, and Dr. J.N. Moore was associated in the work from 1961 to 1963. In 1939 cooperative breeding was begun with the University of Maryland, with the resulting seedlings being grown near Salisbury, Maryland. Since about 1946 most of the seedlings, first tested in the Beltsville greenhouse benches for red stele resistance, have been fruited at the University of Maryland Research Farm at Salisbury. In 1959 cooperative work was started at the University of Southern Illinois at Carbondale, with Dr. R.C. Blake in charge.
The staff at present is:
Willard, North Carolina
OBJECTIVES. In the early years of the work the three specific objectives were origination of a commercial variety of superior dessert quality for eastern states, origination of improved canning varieties, and origination of superior freezing varieties. A survey of all the commercial strawberry areas of the United States in 1914 to 1917 showed Marshall as the standard dessert variety. Thus, on the Boston Market in 1916, ordinary market berries were selling for 8c a quart and those of Marshall at 35c a quart. Marshall is not fully hardy and is very subject to leaf and virus diseases in eastern states. As a result of breeding for higher flavor, four dessert varieties -- Dorsett, Fairfax, Narcissa and Southland -- were introduced in 1932 and 1933. Southland proved frost-susceptible, all Dorsett plants lacked hardiness and were found to have virus, and both have been discarded. But clean stocks of Fairfax were found and propagated; and it continues to be grown as a standard dessert variety of northeastern United States. Narcissa was grown to a slight extent in Oregon, but was too soft. Redheart was introduced in 1932 as a very firm, high flavored, canning variety, but canning of strawberries has decreased substantially in the United States and with it the need for a canning variety. It is still grown in Italy. Furthermore Redheart is very subject to virus diseases. Blakemore was introduced as a shipping and freezing berry in 1930, and soon became the leading variety in the United States, a position which it held until 1962, when Northwest became the leading variety.
This first early work indicated that breeding for a specific objective was practicable and through the years varieties have been bred and introduced to fulfill specific objectives. In general, since 1940, the breeding work has had as one ultimate overall objective, the replacement of all varieties with new varieties resistant to the red stele root disease. Another objective for the South has been ideal shipping varieties -- firm, large, high-flavored, attractive, and productive. Although the first ones, Daybreak, Eleanor Roosevelt, and Fairmore, failed due, at least in part, to infection with virus diseases, Massey and Albritton have succeeded in North Carolina. The objectives listed for the federal work in 1937 were: "(1) greater resistance to leaf, crown, and root diseases; to virus diseases, and possibly to nemas (eelworms); (2) greater resistance to high and low temperatures and to drought; (3) better adaption to long and short days; (4) better dessert quality under adverse weather conditions; (5) increased firmness and toughness of skin; and (6) better adaption to specific uses." (Darrow, 1937). Present objectives are similar: varieties resistant to red stele, firmer than Blakemore, larger than Pocahontas, more productive than Midway, as highly flavored as Suwannee, and as good for freezing as Midland. At Cheyenne, Wyo., (see also p. 253) beginning in 1932 the objective has been a home garden and local market variety that would stand -40 F. without mulch protection just as the wild ones do. Cheyenne 1, 2, and 3, Sioux, Arapahoe, Radiance and Ogallala were introduced and served the purpose. The latter three are still grown. Still larger, firmer, and more productive varieties are being bred.
METHODS OF BREEDING IN MARYLAND. As stated above, since field crossing was found in the early work to be hazardous, an unheated greenhouse (Fig. 14-1) was used. Vigorous plants were potted (usually in 5-inch pots) in late summer and fall and placed in the cold greenhouse. They received enough cold to break their rest period and crossing began in March, about a month earlier than was possible in the field. Low temperatures and low light intensity of midwinter caused the first flowers of many varieties to open without stamens, and emasculation of such flowers was unnecessary. By screening the ventilators insects were largely excluded; regular spraying for insect control was used; and the work of crossing was consequently made far easier, since protection of flowers against insect crossing was also rendered unnecessary. In the greenhouse, stamens rarely open before the petals unfold, and it is thus possible to emasculate and pollinate at the same time, just as the flowers first open. Most of the crossing and harvesting of seed is done before field work begins outside. Although the unheated greenhouse was moved from Glenn Dale to Beltsville, when the work was transferred there, and was used until 1963, heated greenhouses for crossing have been used at all stations for many years. The use of four to six hours of supplemental light, during the winter when light intensity is low, helps the flowers to develop more anthers and pollen, and is used particularly in Oregon.
Each year the crosses are planned, and a breeding chart is prepared which outlines these proposed crosses. As an example of breeding method and objectives, a part of the breeding chart used as a guide for 1945 at Beltsville, Maryland, is given below.
|Mother parent*||Pollen parents||Purpose|
|1 Midland (62) x||Oreg. 1491, 1509, 1765, 1775 (continued)|
x Fairpeake & F. virginiana Sheldon, N.D.
|2 Md-3205 (52) x||Oreg. 1491, 1509, 1775, US-3366, & x self||Production|
|5 Aberdeen (35) x||" " " " & Fairpeake||Resistance|
|9 Massey (67) x||Eleanor Roosevelt, Midland||Size|
|14 US-2827 (66) x||Midland||Aroma|
|15 Suwannee (72) x||" (also 2189)||Flavor|
|20 F. virg.,Sheldon,|
N. Dak. x
*Note: The number preceding the mother parent variety indicates the group cross number; other crosses (Nos. 3, 4, 6, 7, 8, 10-13, 16-19, 21-28) omitted. The number following the mother plant variety and enclosed in parentheses indicates the Vitamin C content of the mother plant.
A total of 28 sets of mother parents were planned. Potted plants were brought into the greenhouse and the above plan followed as fully as possible.
When emasculating, the thumb nail is generally used to remove the stamens, corolla and calyx at one operation. If done with ordinary care no resulting injury to the pistils has been observed. When working with small flowers, such as those of F. vesca, sharp-pointed scalpels or tweezers sharpened to a cutting edge are used to remove the flower parts. Even though emasculation may not be necessary, the calyx and petals are always torn off when the flowers are pollinated in order to identify the crossed flowers (see Fig. 14-4a and b).
Especially when grown under glass, the first flowers to open on clusters of many varieties are pistillate (the stamens, if present, do not contain good pollen). For this reason it is often difficult to obtain pollen at the beginning of the season. The first flowers of Marshall and Fairfax contain abundant pollen, while the first flowers of such varieties as Howard 17, Missionary, Blakemore, Bellmar, and the everbearing varieties usually contain none. The primary and secondary flowers contain far more pistils than the later flowers on a cluster, and hence can set far more seed when pollinated. Many of the later flowers on a cluster do not set at all. Thus, every effort is made to pollinate the earliest flowers on each Cluster, using the late flowers of the perfect-flowered varieties as source of pollen.
To obtain pollen, the practice has been to pick flowers having pollen-bearing stamens just as the petals are unfolding. The stems are left about a quarter-inch long and if the pollen is to be used within a few hours the flowers are allowed to wilt. As they wilt the anthers crack open and more pollen is available than if the anthers dehisce on the plants. If pollen is needed later, it can be held from one to four days by placing the flowers and large buds in shallow vessels of water and allowing them to open. When examination shows the anthers fully opened, the flowers are used directly on those to be crossed, being held so that the stamens touch the pistils. The flower is twirled by its stem in such a manner as to cover all the stigmas with pollen. A flower with abundant pollen may be used to pollinate four to six flowers or even more. Some breeders first collect pollen, then use camel's hair brushes to apply it (Fig. 14-5). When pollen is scarce, brushes may be used to advantage. There has seemed to be a greater chance for error, through accidental mixture of pollen, when using brushes, than when using the flowers themselves.
The comparative ease with which an abundance of pollen may be obtained, owing to the readiness with which blossoming may be advanced or retarded by placing the potted plants in warmer or cooler parts of the greenhouse, and by bringing in additional plants from the field, have made studies of pollen storage unnecessary. It has been observed, however, that pollen spoils soon under moist conditions, but when kept dry it apparently has been fully effective even when held for several days. Crandall (1912) has reported slightly better results with pollen four days old than with fresh pollen.
At first seed was saved by mashing ripe berries on paper toweling, or newspapers, but since Morrow's use of an electric blender all seed is cleaned by putting berries in a blender from one half to two thirds full of water (Plate 14-1), turning on the blender for about twenty seconds, and saving only the seed that sinks. The seed is surface dried, then stored in vials, or small envelopes, and placed in a refrigerator until used. In one test by Meader the germination was 92 percent after ten years storage.
The seed is sown in sterilized soil or on pulverized sphagnum, about 1,000 seeds to a 12 x 18 inch flat. Since Piringer's tests at Beltsville showed the need of light on moist seed for their germination, care is taken that strawberry seed is sown on top of sphagnum or very shallowly, so that light can penetrate to the moist seed.
The speed of germination varies greatly, depending chiefly on the mother parent. Seed of F. virginiana germinates more rapidly than any other we have observed, seedlings sometimes appearing four days after sowing (Darrow, 1927). Seed of some garden varieties and of some strains of F. chiloensis may not germinate for two, or even four weeks.
When two or three leaves have grown, the seedlings are pricked into greenhouse benches, or into flats (Plate 14-2a).
Waldo devised a test for red stele resistance about 1940. In late fall seedlings are planted in greenhouse benches whose soil has mixed into it the roots of red stele affected plants, or is taken from a field in which the red stele disease had killed out plants, or whose soil is inoculated with cultures of specific strains of the fungus. After a growing period of two to three months the temperature is lowered to a range of 35 to 50 degrees for four to eight weeks and the soil kept wet. The roots of vigorous plants grown in such soil become affected, if susceptible, and only resistant seedlings are saved (Fig. 14-6) for a fruiting test. Because occasional seedlings escape infection, all selections are retested.
FIELD TESTS OF SEEDLINGS. Various methods of growing and fruiting seedlings have been used. The two most used in the U.S. Department of Agriculture are (1) growing single plants of each seedling and (2) growing and fruiting small blocks of runner plants of each. Both methods have advantages -- the single plant approach because of the large numbers (about 8,000) which can be grown per acre at an 18-inch spacing between plants, as opposed to the small numbers grown at the 4- x 4-foot spacing for small blocks (about 2,750). Single-plant selections however, must be propagated; hence their second fruiting is delayed until the third year. At the 4- x 4-foot spacing, second-test rows of selected seedlings can be set at once and the second fruiting test made the very next year. Tests in North Carolina indicate that this method is quite efficient in selecting superior seedlings (Plate 14-2b).
In making first selections of seedlings, wooden stakes are usually placed beside each selected plant, to be left or removed at subsequent surveys of the seedlings (Plate 14-4a). Each field of seedlings is surveyed as often and as thoroughly as possible, with an ideal period of twice weekly. When a seedling is to be saved for further testing, it is given a serial number by which it is known thereafter. Notes are made of critical characters of each selection and usually of the cross as a whole, but not of each seedling. Often, detailed notes are made of each seedling, or of samples, of particular crosses that were made for special reasons (Plates 14-3a and 14-3b).
Second tests of selected seedlings are usually conducted in short rows, five plants set in a 10-foot row proving to be sufficient. Third and later tests are in replicated 10-foot rows set for comparison with other selections and varieties.
NUMBER OF SEEDLINGS. In the early days of breeding work small numbers of seedlings of many crosses were used. Later, larger numbers of fewer crosses have been grown. The original cross of Howard 17 x Missionary, from which Blakemore and Bellmar were selected, had a population of only 64 seedlings, while the cross of Tennessee Shipper x Midland, from which Pocahontas and Dixieland were selected, had about 3,000 seedlings and 107 selections were first made in 1948. But later, large numbers of seedlings of the Howard 17 x Missionary were raised from which the Suwannee was selected and named. Very much greater numbers of seedlings are now necessary, since the infected-soil bench test for red stele resistance has been adopted. Since the bench tests started, probably not over 10 percent of the seedlings have been set in the field for fruiting, with the remaining 90 percent discarded as too susceptible to red stele.
NUMBER OF SELECTIONS. Before 1933 for the six years (of twelve) when records were complete 37,084 seedlings were fruited and 1,013 selections made, of which 400 were still under test. A total of 1,126 selections were still being tested out of the 1,880 selections made up to 1933.
Consecutive numbers are usually given selections; thus, selections in 1922 were numbered 1 to 100, of which No. 44 was later named Southland. Now selections at Beltsville are labeled as U.S. 25; those selected at Salisbury, Maryland, Md.-U.S. 25; at Corvallis, Oregon, Oreg.-U.S. 25; at Willard, North Carolina, N.C.-U.S. 25; etc.
Not all selections saved are chosen for possible naming and introduction as home garden or commercial varieties. Some are the best seedlings of the crosses that were planned in attempts to incorporate new qualities into the commercial strawberry, and are to be used in further crosses (Plates 14-4a, 4b, and 4c and 14-5a, 5b, and 5c). Up to 1934 as many as 20 selections of species were used in breeding to explore their possible value in a breeding program. Selections of these, and other species crosses made in later years, have been used for further breeding.
RESULTS. Up to 1965 about 1,500,000 seedlings have been grown from crossed seed, many thousands of selections made, and 40 varieties introduced. Since selections have accumulated more and more desirable genes over a period of forty-five years it is to be expected that many selections now being tested contain a great many of the desirable genes, and many varieties already may be named from the selections now on hand. Of the 38 varieties introduced, 23 are still in the trade and 15 have been dropped.
Two new varieties, Sunrise and Earlibelle, are just being named and released for propagation and introduction. Sunrise, for introduction in the fall of 1965, originated from a cross of US 4152 x Stelemaster made in 1952 and selected in 1954 at Salisbury, Maryland. It ripens early, about with Dixieland and Blakemore, is medium in size, light bright glossy red, firm, light pink flesh, but too pale for frozen pack, and is resistant to three races of red stele and to Verticillium wilt. It is susceptible to spot and is suggested for the area from Maryland west to Missouri. Earlibelle, for introduction in the spring of 1965, is from the cross Albritton x Md-US2101 and was selected in 1956. It is earlier and more productive than Albritton in eastern North Carolina. It is medium large, glossy bright red turning deep red, firm with bright red rather tart flesh, and adapted to frozen pack. It is resistant to leaf spot and leaf scorch and is suggested for the area from North Carolina southward.
About 100,000 seedlings a year are being grown, including those at all stations, but of course far fewer are fruited as a result of the red stele bench tests.
Some of the discarded varieties were dropped because of specific reasons. Narcissa, introduced for Oregon and Washington, although hardier than Marshall was not sufficiently superior to it in flavor and productivity; therefore Marshall continued to be the variety widely grown there. Redheart, Northstar, Daybreak, Maytime, and Starbright were all lost because they were not tolerant of virus diseases. Southland proved too frost-susceptible. The reasons for the failure of Eleanor Roosevelt, Fairmore, and Brightmore are not so clear, though Fairmore may have been too susceptible to virus and Brightmore too small. Fairland and Temple are still grown but, as all stocks are infected with virus, they may be dropped soon in favor of Midway, Surecrop, and other new sorts resistant to red stele. The four varieties, Cheyenne 1, Cheyenne 2, Cheyenne 3 and Sioux, although small and soft-fruited, were hardy and high flavored. In their place Arapahoe, Radiance, and Ogallala are hardy, good flavored, larger, and everbearing and have much wider appeal. Massey has been replaced in North Carolina by the larger, firmer, still more handsome and more reliable Albritton. Bellmar, which is still raised a little, although large and less acid than Blakemore, is not so firm and not quite so early.
Beginning about 1946 at Beltsville, Salisbury and Willard and about 1950 at Corvallis, seedlings and selections have been kept relatively free of virus by isolating and spraying them to control insects.
Undoubtedly many selections of the past would have become important varieties if they had not become infected with virus which weakened them before they were introduced. In addition, much good genetic stock also was lost because of this. Now that techniques are used to keep the seedlings and selections free of virus, advances should be more rapid.
KEEPING SEEDLINGS AND SELECTIONS FREE OF DISEASE. In the 1940's when techniques first made it possible to tell whether plants were approximately virus-free and when control measures were available, seedling and selection fields were isolated and regular spraying programs adopted. Later, when damage by the meadow non-gall-forming nematodes was found to be more serious than that of gall-forming ones, nematode control was added to the program. Plants of selections to be introduced are now indexed by grafting to indicator plants to make sure they are free of virus. Virus-free runner plants are rooted in sterilized soil, and propagating stocks for nurseries raised in screenhouses to ensure, as far as possible, clean stocks.
Nurseries introducing new varieties obtain plants from screenhouse stocks and propagate them in fields isolated from non-virus-free stocks. They observe a regular dusting, or spraying, schedule, to control vectors of virus diseases, and treat their fields for nematode control. Recent research has shown that a so-called "latent virus," which produces no distinctive symptoms, greatly reduces plant vigor and yield of some varieties and steps are being taken to eliminate this virus where it occurs. New varieties free of this virus are already being introduced. A chief problem at present is the attainment of complete freedom from the non-gall-forming nematodes in virus-free stocks sold by nurseries to growers.
INTRODUCTION OF NEW VARIETIES. In 1928 when it was proposed to introduce the Blakemore, a committee of the American Nurserymens' Association, instituted to cooperate with the U.S. Department of Agriculture, was asked to suggest three established nurseries of the eastern United States, where Blakemore seemed adapted, who were large propagators of strawberries and who might be willing to help introduce the variety. The Committee suggested three nurseries on the Delmarva Peninsula and all agreed to cooperate. Blakemore was introduced in 1930, using stocks propagated at the North Carolina Coastal Plain Station at Willard, North Carolina. The arrangement was satisfactory and was continued for other varieties with the approval of the nurserymen and the Committee of the Nurserymen's Association. These nurserymen have introduced many of the later varieties, always at a nominal price. Other nurseries have attempted to propagate virus-free stocks of the new varieties, but have found the necessary procedure too difficult for their conditions. However, some are doing this through cooperation with their states.
The U.S. Department of Agriculture has issued Circulars and Releasal Notices concerning each introduction giving the name and a brief description of the variety, the purpose for which it is adapted, the area where it has been tested, and often the varieties it should supersede. Methods of introduction have been worked out for varieties resulting from the work at Cheyenne, and those resulting from the cooperative work at Willard, North Carolina, and at Corvallis, Oregon.
SUMMARY. During forty-five years, continuous breeding has been carried on by the U.S. Department of Agriculture with specific objectives: (1) firmer, higher-flavored, commercial shipping varieties; (2) varieties for processing, both freezing and preserving; and later, (3) red stele resistant and hardier sorts. Since the first variety was introduced in 1929, from 25 percent to 40 percent of the United States strawberry acreage has been planted to varieties of U.S. Department of Agriculture origin. Much firmer, much better processing varieties and many good varieties resistant to some strains of red stele are available. Desirable genes have been concentrated in current varieties so that the level of available varieties is much higher. As an example, the firmer Blakemore replaced Missionary in eastern North Carolina, then the larger and higher-flavored Massey replaced Blakemore, and finally the still larger, more attractive and firmer Albritton has replaced Massey. Efforts are being made to breed hardier, still larger, and more productive varieties, which at the same time, still retain the high flavor, firm skin and flesh, and superb glossy color of the Albritton.