G.M. Darrow, The Strawberry: History, Breeding, Physiology
STRAWBERRIES ARE ATTACKED by many pests, both insect and disease, which vary widely in their destructiveness and their distribution. Many of these pests are world-wide, while some are more serious in particular locations. The examination which follows reviews those pests known to the United States. Characters, such as resistance which are shown by certain varieties and noted by associates, are also included.
Very little is known of resistance to insects. Consequently resistance to specific insects is not discussed. Varieties that have thick leathery leaves seem to be more resistant to leaf mites. No real resistance to aphids is known, although they seem to multiply less freely on some varieties than on others. The bud weevil, which feeds on pollen, does not affect pistillate varieties, but all perfect-flowered varieties seem to be affected.
Knowledge of diseases which affect strawberries is somewhat more comprehensive. The lists on pages 390, 391 indicate what varieties are known to be resistant to various diseases. This information is important, for many diseases are being and others can be controlled by raising resistant or tolerant varieties. Systematic breeding is under way in many places to obtain more varieties resistant to various troubles. In the case of some diseases the need is urgent and breeding for resistance is more extensive than with other diseases. The demands to be met vary. For instance, all varieties introduced on the Pacific Coast, because of the difficulty of controlling aphids there, must be relatively tolerant to virus diseases, that is, slow to lose vigor when infected.
In the U.S. Department of Agriculture program at Beltsville, Maryland, since 1940 all breeding work has been directed toward varieties resistant to the red stele root-rot; and this is now a part of the program at Corvallis, Oregon, and other places. Once the knowledge of diseases, especially viruses, is more complete, the present means of dealing with them may seem clumsy stop-gap attempts, but these means are what seem presently most appropriate in respect to the amount and kinds of information now available.
Nematodes are threadlike worms too small to be seen easily by the unaided eye. Many species live in the soil and several attack the roots of the strawberry; many others live on the above-ground parts of plants, and several cause serious damage to the buds and leaves of the strawberry. Sanitary measures are the usual method of control.
ROOT NEMATODES. The northern root-knot nematode (Fig. 21-1) and the meadow or root-lesion nematode enter strawberry roots to feed. They stay alive when plants are dug and shipped, and consequently are readily spread in nursery stocks. Other nematodes, like the sting and dagger nematodes, feed on the surface of roots and are usually dislodged when the plants are dug. Most root nematodes are more destructive in sandy than in clay soils. Soil fumigation to kill the sting nematode has been found necessary to raise good crops in Florida. Black root in many soils is due to meadow nematode, since the black-root fungi enter roots injured by this nematode (Chen and Rich, 1962). Fumigation of the soil is generally practiced in California to control the complex of soil troubles, including black root, and is beginning to be a standard practice in the eastern United States also. Little is yet known of resistant varieties. Fresno, Torrey, Siletz, and F. ovalis (Canada 5157 selection) may have some resistance.
BUD, LEAF, AND STEM NEMATODES. The bud nematodes, causing spring and summer dwarf, live in the most succulent tissues of tiny buds and very small leaves. The flower buds are often killed by the feeding of the"spring dwarf"species, resulting in flowerless plants. Buds and small leaves are killed or dwarfed by the "summer dwarf" (Fig. 21-2). Both diseases are commonly spread by infected stocks. The bulb and stem nematode is chiefly a pest in the Pacific Northwest. No resistance to these nematodes is known in varieties. Leaf nematodes are important in parts of Europe, but not in the United States, so far as is known.
Red Stele (Brown Core) Root-Rot Resistance
This disease of the root system was first noted in Lanarkshire, Scotland, about 1920, in Illinois in 1930, and in California and Maryland at about the same time. Its country of origin is still unknown. It is serious as far south as eastern North Carolina and Arkansas.
In moist, cool weather of late winter and early spring, this fungus disease grows rapidly up the center part of the roots, the stele, which turns reddish while the cortex of the root appears normal (Figs. 21-3 and 21-4). If the season is dry, infected plants may die before blossoming; if the season is wet they may blossom, but die before the fruit ripens. Affected plants at first show the dull, bluish-green color of wilting plants and may die in a few days. The fungus spreads through soil by swimming spores so that wet or low spots in a field are usually first affected. The disease is spread by drainage water, by soil on machinery and shoes, and by infected plants. In well-drained sandy soil it is rarely serious; yet, when the soil freezes deeply and then the top soil is thawed but wet, it may be severe. In the summer the disease is inactive except in areas with very cool summer temperatures. Summer spores of the fungus may live in the soil for many years.
The fungus has from several to many races which differ in their ability to infect different varieties; all named varieties are susceptible to some race. Because of Reid's (1933) and Colby's (1935) and Darrow's (1937) success in early tests of breeding resistant varieties, extensive breeding work was promptly started. Hundreds of thousands of seedlings have been grown, and most desirable characters of strawberries are now in resistant varieties. Progeny of a cross of two resistant parents gave twice as many resistant seedlings as a cross with but one resistant parent.
When it was realized that this fungus produced races that differed in their infectivity, resistance tests of many species, of hundreds of varieties, and of hundreds of thousands of seedlings were made. Resistance to some strains was found in both chiloensis and virginiana and in a few varieties, chiefly Frith, a little-known variety of Scotland, and in Aberdeen of unknown parentage of New Jersey. Marshall, Oberschlesien and Perle de Prague show resistance to some strains. Highest resistance in species has been found in the Yaquina chiloensis selected by Waldo on the coast of Oregon. So far this selection has been resistant to all strains in the West but not to all in the East. Its seedlings in crosses with varieties have shown the highest percentage of resistance yet found. Resistant varieties grown on the Pacific coast have obtained their resistance largely from selections of chiloensis, those of eastern states from Aberdeen, and those of Europe from both Frith and Aberdeen. On page 390 are listed varieties known to be resistant to some strains. General resistance, that is, resistance to red stele disease and not just to certain races of the fungus, may occur.
Leaf Disease Resistance
In general, damage to leaves is probably greatest by leaf spot, Mycosphaerella fragariae (Figs. 21-5 and 21-6A), next by leaf scorch, Diplocarpon earliana (Fig. 21-6B), and least by leaf blight, Dendrophoma obscurans (Fig. 21-7), although the latter disease is sometimes serious on Dunlap and Robinson, especially in Illinois, Indiana, and Michigan. One of the greatest factors for reduced loss has been the resistance of Howard 17 to all three leaf diseases, since Howard 17 composes such a large part of the ancestry of today's strawberry varieties. Blakemore and probably Fairfax inherit resistance from Howard 17 and have passed it on to their descendants. Plakidas (1948) of Louisiana and Bolton (1958) of Canada showed that there were races of the leaf spot fungus and that varieties differed in susceptibility to these races. Blakemore was severely affected by one race in Arkansas in 1957 and now is considered non-resistant there. No immunity to leaf spot has been found, although high resistance can be found. Propagation of virus-free stocks, if properly isolated, can reduce to a minimum the amount of infective material of leaf spots in fields. Leaf spot enters leaves through stomata; leaf scorch penetrates between epidermal cells. Varieties showing resistance are listed on page 391.
Kennedy and King (1962) have reported that a bacterial angular leaf spot is severe in humid weather in Minnesota. Robinson, Sparkle, and Trumpeter were most severely affected, while F. vesca, Grenedier, Catskill, Massey, Arrowhead, Earliness, Sioux, Vermilion, and Wisconsin 214 were relatively less affected. Other leaf spots are "purple leaf spot," similar in appearance to leaf scorch, with which it is usually confused, and widespread in the South, "leaf blotch," and Cercospora spots, whose economic importance is undetermined.
Powdery Mildew-Resistant Varieties
Mildew may affect strawberries in nearly all regions. Its characteristic appearance is an upward curling of the leaves (Fig. 21-8) as the fungus destroys the surface layers. As the disease progresses, the underside of the leaf reddens and affected parts die. Over half the leaves on a plant may be killed, and flowers and green berries may not develop. The skin on ripening berries may be affected and not grow with the berry, so that the seeds are entirely on the surface and the skin may crack (Fig. 21-9). If the leaves are not affected more than in Midland, little loss of yield occurs, but the loss may be very serious with Atmore. Catskill, Dunlap, Klondike, Marshall, and Sparkle have been notable resistant varieties. For others see page 391.
Verticillium wilt affects strawberries in northern United States, in southern Florida, and along the Pacific Coast of California. It rarely causes damage from Maryland southward, including northern Florida. This fungus is present in nearly all soils of the United States but only builds up to the point of affecting strawberries following plantings of tomatoes, potatoes, and other solanaceous plants in northern and eastern states. The amount of inoculum from such plants is so great in northern states that susceptible varieties should not be grown for about two years after such crops. It persists for many years in the rocky soils of Dade County, Florida. The inoculum is so great along the Pacific Coast that for at least ten to fourteen years after such crops, as well as after cotton, melon, and many others, the disease may still be serious. However, since the disease is so serious, soil fumigation for Verticillium and other troubles is general for strawberries in California, consequently the problem is no longer acute.
Wilhelm (1955) has reported that many plants may be infected and survive and even show no injury, but no such tolerant seedlings were obtained from susceptible parents. Several varieties are resistant and have owed some of their popularity to this resistance, notably Howard 17, Catskill, Blakemore, Robinson, the Siletz in Oregon and Washington, and the Marshall. Sierra also is highly resistant. The Senga Sengana of Germany is also resistant, but many new varieties, such as Earlidawn, Dixieland, Jerseybelle, Northwest, Lassen, and Shasta are susceptible. The Florida Ninety, grown almost exclusively in Florida, is very susceptible. See page 391 for resistant varieties.
Breeding for resistant varieties is under way in California, New Jersey, and Florida in the United States, and has been successful in Oregon with the introduction of the Siletz. In British Columbia, Climax, Marshall, Northwest, Oberschlesien, Perle de Prague, Red Crop, Sierra, and Temple gave a high percentage of resistant seedlings. Chiloensis seedlings had the highest percentage of resistant seedlings of the species (Newton and Adrichem, 1958).
OTHER CROWN Rots. Sclerotina crown and Rhizoctonia bud rot are serious under some conditions in the South, but no varietal differences in resistance are known. Brooks (1931) reported an anthracnose disease of strawberry runners and Horn and Carver (1963) reported that the same fungus, a Colletotrichum, caused a crown rot of the strawberry in Louisiana. Varietal differences in resistance may occur.
Virus-Free and Virus-Tolerant Strawberries
In the United States in the 1920's, proof was obtained that strawberries were affected by virus diseases. By 1930 in Pacific Coast states and by 1940 in the East, it was discovered that viruses were serious there. By 1946 "virus-free" plants were found for three varieties in the East. By 1955 "virus-free" plants were found for the 42 varieties that represented 95 percent of the commercial production in the United States. Because virus-free plants propagate more freely than affected ones, the virus-free stocks were quickly propagated and have rapidly replaced diseased stocks. The greater part of the stocks in nurseries are now grown either under virus-free certification or have been grown from virus-free plants.
Because tests have shown that many wild plants have virus and could reinfect stocks in nurseries, regulations are in effect to ensure clean stocks by growing the plants in isolation from sources of reinfestation and by controlling insects, chiefly aphids and leaf hoppers, that spread the viruses.
We know that otherwise splendid varieties have been so weakened by viruses that they have disappeared and have been replaced by less affected varieties, and it is likely that many fine varieties of the past century have been replaced for the same reason.
Symptoms vary with the variety, the particular virus or viruses present, with the stage of plant growth, and with weather conditions. Infected plants of some varieties show no obvious symptoms, though their yields may be greatly reduced by virus. Some varieties, like Marshall, show marked symptoms and are quickly weakened (Figs. 21-10 and 21-11). Others, like Ettersburg 80 (Huxley), Howard 17, and Blakemore, do not show symptoms plainly though they seem to be just as readily infected, and they are called tolerant varieties. It is considered best to originate tolerant varieties, for it is impossible for a grower in some areas to keep his planting virus-free for even one crop. Moreover, non-tolerant varieties planted near infected stock of tolerant varieties can be severely injured by spread of virus from the tolerant ones. It is therefore desirable for nurseries to keep all varieties virus-free and for breeders to breed for tolerant varieties. On page 391 are listed some varieties showing marked tolerance.
Variegation or June Yellows or Non-infectious Chlorosis
This trouble of the strawberry, the leaves becoming variegated or yellowed (Fig. 21-12), was noted as early as 1719 in F. vesca, the native woodland strawberry of Europe, in 1778 in F. virginiana and F. moschata, and in 1832 in the cultivated strawberry. It has also been reported in native F. ovalis and F. chiloensis in Oregon. The greatest possible range of incidence of the trouble occurs, from tiny to large seedlings in their first year yellowing and dying outright; to varieties all of whose plants turn yellow, so that such varieties are lost many years after introduction; to varieties in which no plants ever turn yellow or to those in which only a few plants turn yellow through all the years that a variety may be grown. One phase of the trouble called June Yellows is temperature controlled. It appears in early spring but largely disappears in the hot summer. All runner plants from an affected one are variegated and no recovery is known (Fig. 21-13).
The seriousness of this trouble is evident today. The Auchincruive Climax variety raised in 1939 and introduced in 1947 quickly became the most important variety in Great Britain and other parts of northern Europe, supplanting many of her varieties because of its many superior qualities. Variegation was first noted in it in 1950, and by 1954 this variety, which otherwise contained perhaps one of the finest combinations of desirable qualities yet known, had to be given up. Blakemore, for about twenty-five years the most important variety of the United States, was raised from seed in 1923 and introduced in 1929. In 1933 the major stocks of it were variegating. However, some stocks did not show variegation, and the variety is still extensively grown. Other varieties have been saved by such selection of non-variegating stocks. At present, Dixieland, introduced in 1953, the best shipping variety of eastern United States, is variegating and it is not known whether the variety can be saved by selecting non-variegating stocks. Many varieties have shown variegation and consequently have been lost.
Another serious, though indirect, loss due to variegation has been the elimination of seedlings and selections of certain crosses which incorporated high proportions of desirable genes. Thus, from the cross Blakemore x Dorsett many superior selections were made, but year after year additional selections became variegated and none was saved.
Because variegation has not been transmitted by grafting, it is not classed as a virus. A similar variegation has been obtained by irradiating seeds of the diploid Alpine (Fig. 21-14). It is, therefore, classed as an inherited character due to an unstable gene. Different varieties have different rates of mutating to this variegated condition. No sure test to determine whether a variety will mutate, is known. Demaree and Darrow (1937) suggested that it might be helpful to grow a large population of selfed seedlings and observe whether variegation appears in the small seedlings. Fortunately it is possible to utilize variegating varieties in breeding; and most present varieties of the United States have Howard 17, which variegates, in their ancestry. Blakemore is also represented in many leading varieties. Variegation has never been found in Marshall, Klondike, and Lassen. Neither has it been found in Suwannee of the same parentage as Blakemore.
Variegation in any plant is thought to be due to a rogue plastid or plasma-gene carried in the cytoplasm of the cells (Darlington, 1951). In breeding with a variegated plant, or with a plant of a variety subject to variegation, the variegational plant should always be used as the pollen parent. Morrow and Darrow (1952) obtained non-variegating varieties where varieties like Howard 17 and Blakemore were in their ancestry.
Fasciation of the Strawberry
This abnormal flattening and enlargement of the fruit stem and fruit and the witches-broom-like appearance (Fig. 21-15) of the plants is a trouble which is apparent in the Middle and South Atlantic States. Affected produce little marketable fruit and no runners. Because the principal varieties grown in the South are immune or highly resistant to fasciation, it usually is not considered important. However, when promising new varieties are tested, their culture may be limited, for many northern varieties are affected by fasciation when grown in the South, as is the case with Fairfax, Howard 17, Sparkle, and Catskill. For example, in Fairfax fasciation is serious in New Jersey and Maryland and rarely seen in New York and New England. Susceptibility to this trouble is inherited.
Fasciation has never been found in Blakemore and Missionary, and only in three plants of millions of Klondike; yet, at the end of one year, 100 percent of the mother, 29 percent of the June-rooted, and 8 percent of the August rooted plants of Fairfax at Willard, North Carolina, were affected. Fasciation is initiated mostly in the fall when the days shorten and flower buds begin to develop. When tests were made by growing plants under different light periods, Howard 17 had no fasciated plants with twenty hours of light, 5 percent with seventeen hours, and 95 percent with fourteen, eleven, and eight hours (Darrow and Borthwick, 1954). Missionary, Blakemore, and Massey had no fasciated plants under any day length. Thus, when growing conditions in the fall continue later than usual, fasciation can be serious farther north than usual. This happened with the Sparkle variety in New Jersey in the warm fall of 1952, during days which were as short as eleven hours.
Gray mold or botrytis rot (in all United States and in all the world), tan rot (in southern United States), hard rot (in all United States), leather rot (in Mississippi Valley, Maryland, and Virginia), and stem-end rot (in Michigan and surrounding area) are common field rots of the strawberry in the United States. No real resistance is known, although observation indicates that the Solana in California, Mollala in Oregon, Cascade & Columbia in Washington, Valentine in Sweden, and Tashkent in U.S.S.R. are less subject to gray mold than other varieties grown there. Firm-fruited varieties, like Blakemore and Dixieland, under some conditions, are much less injured than softer varieties. Because berries that stay moist are most affected, varieties that have dense leaf growth that prevents rapid drying after rain and dew are more likely to be affected than those with more open growth (Fig. 21-16). Gray mold may develop in frosted flowers and in injured green berries, so that frost-resistant varieties and varieties with flowers protected by leaves may have less rot than others (Plate 20b).
Running out of Strawberry Varieties
"Running out of strawberry varieties" to use the common expression, refers to the idea that varieties have a definite length of life or usefulness after which they decrease in vigor and productiveness. Jucunda lasted for more than a hundred years as a major variety, but others last for only a few years. Research has shown that the loss of vigor in many varieties is due to several virus diseases, some of which are transmitted by aphids, while others are transmitted by nematodes and possibly still others by leaf hoppers. Still other viruses may be transmitted other ways. Plants free of known viruses have been far more vigorous and productive than ordinary stocks. Infestation of plants by nematodes, both bud and root, weaken them as do also root and leaf fungi, but stocks free of these pests can be obtained by heat treatment, or by rooting runners of plants infested with root nematodes in sterile soil.
When the strawberry is grown in subtropical regions, stocks of most varieties become weak and grow feebly, or not at all, the second year. Now we know that they need a cold rest period or chilling, as do most temperate climate plants. Thus, planting stocks may be obtained annually from the North to set in Florida, or Florida-grown plants may be refrigerated for a month and then reset. Varieties succeeding in such regions are those which require the least chilling. Varieties have been originated that are adapted to each particular length of day and chilling requirement. When grown continuously under shorter days, they are dwarfed and lack vigor; under longer days than where best adapted, they are usually too vegetative.
Plants freed of known viruses, fungi, and nematodes, and grown in the proper latitude for the variety, have for the most part been as vigorous and productive as when first fruited as seedlings. The question arises, do any varieties decrease in vigor just because the variety is older? It does not seem likely. In searching for causes for weak planting stock, first, winter injury to crowns and roots must be eliminated as a cause, for the variety must be hardy where grown; second, eliminate red stele root rot; third, eliminate bud and root nematodes; fourth, get stock free of all known viruses and keep them clean; fifth, get stock free of leaf and crown diseases and insects; and sixth, get stock that has had a cold rest period.
If all the above conditions are met most difficulties are eliminated. There are cases where it is not possible to explain the reason for loss of vigor, and unrecognized viruses may be the cause, but first the conditions listed above should be considered.
Through the past one hundred and fifty years there has been expressed a more or less decided opinion that some varieties are somewhat biennial: they fruit so heavily that they use up stored food reserves as well as current food supplies, and may die or may be severely weakened as a consequence; even without fungi and other pests that affect weakened plants, they do not recover enough to give a good crop the following year. The effect of breeding, especially the use of the hardy native Virginian strawberry, has probably largely eliminated this, were it ever really a factor. Before 1850 growers of the Virginian varieties were known to keep their bearing fields for as long as ten to twelve years; and present varieties protected against weeds, fungi, virus, and animal pests may, without replanting, still fruit well in a bearing field that long. Another factor in making some of the old varieties die after fruiting was their lack of resistance to high temperatures and to diseases after producing a heavy crop that weakened the plants.