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IV. Breeding for Disease Resistance

IV. Breeding for Disease Resistance

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SUGAR BEET IMPROVEMENT



101



Utah fields in which curly top was causing a decimating loss. Roots that

appeared to be definitely better than the general run of the field were

stored over winter and planted in a seed plot with other roots grown

from similar selections that had been made earlier by commercial companies. A total of 70 to 100 pounds of seed was produced in 1929. The

first test of this seed-later to be designated US l-was made in 1930



FIG.3. The initial test of US 1 sugar beet variety at Castleford, Idaho, in 1930.

Curly top exposure was severe, almost eliminating susceptible sorts. Plot 1703, left

foreground (to the right in front of the man), consists of four rows of US 1. Plot

1704, the four rows to the right, is a nonresistant European brand. In the background, other plots of US 1 and other curly-top-resistant varieties appear as green

“islands.” (Agronomic test by C. E. Corrnany; photographed September 19, 1930

by E. Carsner.)



under conditions of heavy curly top exposure (Fig. 3 ) . AS the photograph indicates, and in comparison with later achievements, the showing made by the variety was not impressive. But the situation of the

beet sugar factories, brought about by curly top, was so desperate that,

with this evidence of some superiority in US 1 over the European types,

steps were immediately taken to make a seed increase from the unused

seed stock as a source of a curly-top-resistant .variety. Details of the

increase and the subsequent multiplications have been given by Coons



102



G . H. COONS, F. V. OWEN, A N D DEWEY STEWART



(1936). In the summer of 1932,22,000 pounds of seed were harvested;

in 1933, beet sugar companies grew 200 acres of trial plantings of

US 1 in Idaho, Utah, and California. More than 800,000 pounds of

seed were produced in 1933, permitting over 35,000 acres to be planted

for sugar production in 1934. In 1935, nearly 103,000 acres were

planted with the new variety.

US 1 was successful under mild curly top exposures. About onefourth of the population within the variety was resistant enough to

produce fair-sized roots in spite of curly top. The other plants ranged

in disease reaction from extreme susceptibility to mediocre resistance.

The performance of US 1, however, in comparison with the almost

complete failure of the European brands when affected by curly top,

made the new variety outstanding. In 32 grower-test plantings in

1931, with curly top injury ranging from moderate to severe, US 1

averaged 5.5 tons better than the susceptible European brands.

The results with US 1 had far-reaching effects. Previously, the attitude of sugar beet breeders had been rather pessimistic as to the possibilities of incorporating in sugar beets the factors for disease resistance.

In these experiments, genes for resistance to a virus disease were shown

to be present within the sugar beet complex and, by mass selection

under severe curly top exposures, were segregated. In contrast to the

European practice in which a commercial variety of beets is never given

another increase, the original stock of seed of US 1 was multiplied by

repeated direct increases without selection, and no loss of vigor, no

change of resistance, and no change in other attributes that could be

measured statistically took place so long as the full population participated in the seed production. Of great significance for the American

economy was the acceptance by the sugar beet industry of its need for

varieties adapted to its beet-growing districts and its responsibility for

producing seed-a marked reversal from the former insistence upon almost exclusive utilization of European seed. A domestic sugar beet seed

enterprise was established to bring to farmers the seed of US 1 and the

other curly-topresistant varieties that were soon to follow. Fortunately,

in concomitant researches on methods of sugar beet seed production,

carried out as a federal-state cooperation with the New Mexico Agricultural Experiment Station, Overpeck (1928) had developed the field

overwintering method of seed production. In this method, plantings

made in early fall were left to overwinter in the field, the seed being

harvested by machine the following summer. It replaced the expensive

European method of producing seed from stecklings. The new variety,

US 1, was the first corpmercial variety increased by this method. The

sugar beet seed enterprise that was launched to produce the curly-top-



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SUGAR BEET IMPHOVEMENT



resistant variety grew rapidly. 11s coiitribution in the World War I1

crisis has been described by Coons (1943). Our entire sugar beet industry now uses domestically grown seed of adapted varieties, importation of sugar beet seed being completely discontinued.

The first curly-top-resistant variety was quickly replaced by US 34

and US 33, produced from the reselections made by Owen and Abegg

(Anonymous, 1936) in fields of US 1 severely injured by curly top



Old Type



Original U.S 1



U.S. 33



U.S. I0



U.S. oa



Improved



u s . 49



FIG.4. The susceptible European variety R and G Old Type is shown at the left,

in a four-row strip; then follows US 1, the first curly-top-resistant variety, and the

various improvements up to the second release of US 22. In the background is the

canyon of the Snake River a t Buhl, Idaho.



in 2931. Both varieties were more resistant to curly top than the parent

variety, the percentage of resistant individuals in the population having

been increased from about 25 per cent to 40 or 50 per cent. I n turn,

US 12 and US 22 were reselected by Owen and Abegg (Owen et al.,

1939) from the curly-topresistant breeding material comprising the

original stocks that had gone into US 1. Under conditions of moderate

exposure to curly top, it appeared that in US 12 about 75 per cent of the

plants were resistant; whereas, with US 22, introduced in 1949, about

85 to 90 per cent of the plants showed a very high degree of resistance.



104



G . H. COONS, F. V. OWEN, A N D DEWEY STEWART



US 22 has been repeatedly selected by Albert Murphy near Twin Falls,

Idaho. By midsummer field plantings made at right angles to earlier

infected strips, Murphy (1942) has been able to produce curly top exposures with a degree of severity far surpassing that which growers

experience in March, April, or early May plantings. After a genotype

has survived the severe curly top exposure to which these test plantings

are subjected, there is little likelihood that it will be seriously injured

in farmers' plantings. Sugar beet growers in Utah and Idaho are using

extensively the third reselection (US 2 2 / 3 ) , and this variety is also

used to a considerable extent in late March and early April plantings in

California. The degree of curly top resistance obtained by the repeated

mass selections is indeed remarkable (Fig. 4).I n the areas where curly

top was once so serious, the threat of crop failure from curly top has

been removed. In years of curly top epidemic, some depression of crop

yield still occurs. In extreme instances, growers may sustain a loss of as

much as 5 tons of sugar beets per acre, but when cultural conditions are

optimum and irrigation water adequate, yields of 30 tons of sugar beets

per acre have been obtained in spite of heavy beet leafhopper infestation. Good yields are obtained year after year in districts where European varieties once had failed completely. The industry has been able

to re-enter a number of districts from which it was once driven by

curly top. A successful factory now stands at Toppenish, Washington,

011 the foundation of one that was closed down after a brief run, eventually to be torn down-all because of curly top.

2. Cercospora Leaf Spot2



a. Resistant Znbreds. In most of the sugar beet districts east of the

Rocky Mountains, especially in Colorado, Nebraska, and the Great

Lakes region, the beet sugar industry had in Cercospora leaf spot a counterpart to curly top in the Far West. When climatic conditions favored

the development of the fungus, epidemics of leaf spot occurred. Ironically enough, in the irrigated regions of limited water supply, these

were the seasons in which, without diseases, the rainfall would have

given high yields of sugar beets. I n years when rains were frequent in

the first half of the season, the foliage of the sugar beet often blighted

several times, each new growth of leaves being at the expense of root

'The breeding of leaf-spot-resistant varieties was b y a team consisting of G. H.

Coons, Dewey Stewart, J. 0. Gaskill, F. G. Larmer, and H. W. Bockstahler, assisted

in the agronomic testing by J. G. Lill, G. W. Deming, S. B. Nuckols, J. 0. Culbertson,

R. W. Henderson, 0. E. Reece, and G. J. Hogaboam. The investigations were cooperative with the Colorado, Michigan, Minnesota, and New Mexico Agricultural

Experiment Stations and beet sugar companies operating east of the Rocky

Mountains.



SUGAR BEET IMPROVEMENT



105



growth and stored sugar (Fig. 5 ) . I n the period 1915 to 1940, blight

years recurred frequently, and in some districts the growing of sugar

beets was given up.

Mass selection, so effective with curly top, did not work with leaf

spot. The nature of leaf spot is such that in blighted fields resistant



FIG.5. Sugar beet leaf spot-blighting stage. The dried leaves in the outer whorls

that appear as if burned will be replaced by new growth of inner leaves, all at

expense of root growth and sugar storage.



individuals cannot readily be detected. As the beet leaves mature, they

become more subject to fungus attack, making the recognition of

potentially valuable individuals mast difficult. When the federal project

of breeding for leaf spot resistance was started in 1925, Coons, Stewart,

and their colleagues had available to them for experiments at Rocky

Ford, Colorado, 200 strains of sugar beet bred by W. W. Tracy, Jr., at

Fort Collins, Colorado. Using the Pritchard (1916b) material and certain commercial varieties as starting points, Tracy had, since 1915,



106



G. H .



COONS,



F. V. OWEN, A N D DEWEY STEWART



been engaged in isolating as many morphological types of sugar beet as

possible. Tracy depended chiefly on group increases of carefully rogued

material, but some strains had been obtained by a number of successive

selfings. From the 200 strains tested under severe leaf spot conditions

in Rocky Ford, Colorado, in 1925, 14 were found more resistant than

the others. They remained relatively green, whereas the bulk of the

other strains blighted severely. The same strains were found to be outstanding in resistance to leaf spot at Fort Collins, where an attack of

the disease also occurred. A study of the pedigree of the strains marked

as resistant showed that almost without exception the resistant ones

were the product of two or more selfings prior to a final grouping of

morphologically similar roots of a progeny to augment seed quantities.

From this experiment it was immediately evident that selection, along

with continued inbreeding and progeny testing, would be effective in

concentrating and stabilizing factors within a strain. Such work has

been carried out with the Tracy and other strains by the leaf spot resistance breeding team, as discussed by Coons (1953a, b). The inbreds

rapidly became stabilized for the characters governing resistance, but

as inbreeding proceeded, there frequently was a definite loss of vigor.

As will be discussed in more detail later, hybrids between two relatively

nonvigorous inbreds were found in 1932 to show strong heterosis

(Coons, 1936; Stewart et al., 1940). Breeding for leaf spot resistance

resolved itself into the job of producing as many distinct, leaf-spotresistant inbreds as possible, then finding, by a series of matings, those

pairs that would give a strong heterosis response, satisfactory resistance

being maintained.

b. Resistant Varieties. A number of leaf-spot-resistant varieties

have been introduced, each representing an improvement in resistance

and in productivity over its predecessor. US 217 (Coons, 1936), a

synthetic variety, was introduced in 1937 and grown to a limited extent

in Michigan. It was replaced by US 200 x 215 in 1940 (Coons and

Stewart, 1940). This variety was made by pooling the seed of two leafspot-resistant inbreds, US 200 and US 215, and using the mixture as a

planting stock for seed production. The variety consisted, therefore, of

the hybrid and the sibs of the parent variety. It was ready for introduction in 1939 when the European war cut off supplies of foreign-grown

sugar beet seed and forced American industry to move 100 per cent into

domestic seed production. US 200 x 215 was entirely satisfactory as a

replacement for the European varieties and, when leaf spot was a

factor, undoubtedly was greatly superior.

By continued breeding, new leaf-spot-resistant varieties have been

developed-notably US 215 X 216, as bred by Coons et al. (1941)-



S U G A R BEET IMPROVEMENT



107



which, in its current phase, i\ designated a s the third release of the

hybrid, i.e., US 215 x 216/3. Without leaf spot, this variety in comparative tests is equivalent in sugar production to the European varieties

previously grown. Under exposure to leaf spot, it is significantly better

than the susceptible European types.

Other introductions of leaf-spot-resistant varieties have been made,

including the US 216 X 226 and US 225 x 226. These varieties, in a

series of comparative tests, showed superior root yields to the US

215 x 216/3 and maintained the high sucrose percentage characteristic

of the latter. I n production of all three of these hybrid varieties, utilization was made of the male-sterility factor discussed later. By a series

of backcrossings, male-sterile equivalents of US 216 and of US 225 had

been produced. Seed stocks of US 216 were available which gave plants

showing about 70 per cent male-sterile individuals. US 225 male-sterile

(MS) equivalents, in which male-sterility occurred in about 90 per

cent of the seed stalks, were available. In the production 215 x 216/3,

the seed of the US 216 MS was mixed with seed of US 215 as a means

of promoting hybridization and reduction of the sib portions of the

population. A similar method was used in producing US 216 x 226. In

the production of seed of US 225 x US 226, the seed of pollen-fertile US

226 was planted in a series of strips in the seed field, the major portions

of the field being planted with the male-sterile phase of US 225. Presumptively, a high percentage of the seed from the US 225 portions of

the field would be hybrid, but it must be recognized that the presence

of pollen-fertile plants of US 225 within the main field brought about a n

appreciable amount of self-pollination.

Comparisons of other leaf-spot-resistant varieties, in which malesterile equivalents were utilized to increase the percentages of hybrids

in the populations, using the European variety as a check, indicate that

in these hybrids definite gains in root yields are obtained, leaf spot

resistance and high sucrose percentages being maintained. The hybrid

variety obtained by taking the commercial seed exclusively from the

male-sterile plants showed significant gains over US 215 x 216/3,

averaging nearly 10 per cent (Coons et al., 1954). Skuderna and Doxtator (1942) also have given an appraisal of the contributions of leafspot-resistant varieties to sugar beet production in southeastern Colorado.

A number of leaf-spot-resistant varieties have been produced by

breeding research of beet sugar companies to supply the requirements of

their respective districts-notably by Brewbaker and his associates, of

the Great Western Sugar Company (Brewbaker et al.,1950), who have

introduced GW 59, moderately leaf spot resistant, and GW 304 and



108



G.



H.



COONS,



F. V.



OWEN,



A N D DEWEY STEWART



GW 359, both highly leaf spot resistant; by Doxtator (1940) of the

American Crystal Sugar Company, who has introduced a series of leafspot-resistant varieties under the general designation of AMERICAN I;

and by C. E. Cormany and his associates (Peterson and Cormany,

1952), of the Holly Sugar Corporation, who have introduced leafspot-resistant varieties under the general designation of MIDWEST.

c. N e w Sources of Resistance. Italian varieties, CESENA, MEZZANO

71, and ROVIGO 581, have proved to be other excellent sources of leaf spot

resistance. These varieties trace to the classic work on sugar beet breeding begun in 1910 by Munerati (1920) at the Stazione Sperimentale di

Bieticultura, Rovigo, Italy. Among his many investigations, Munerati

studied Beta naritirna collected along the estuaries of the Po River and

made hybridizations of the perennial types with sugar beet. From these

matings and by repeated selections, Munerati (1932) obtained sugar

beet types that were characterized by a high degree of leaf spot resistance and by high sucrose. These plants, however, as seen by one of

us in 1925, had not been freed from certain undesirable characteristics

the tendency to be

derived from the B. rnaritima parent-notably,

multicrowned and to have sprangled roots, especially lateral roots

emerging from the taproot at about a 90-degree angle. By 1935, Munerati had greatly improved his breeding stocks and furnished his American colleagues his variety R 581 which, although not fully comparable

with sugar beets in root or crown conformation, was extremely resistant

to Cercospora leaf spot and high in sucrose. I n a personal communication in 1935, Munerati stated that he turned over his leaf-spot-resistant

stocks to sugar beet breeders at the commercial seed stations located at

Cesena and Mezzano, Italy, where the Munerati strains, by further

selections and by combinations with other breeding material, became

commercial varieties bearing names of the respective stations. The

Italian varieties tested in the United States were high-sucrose types

with high leaf spot resistance. Cesena contributed leaf spot resistance

to GW 304 and GW 359, and federal workers utilized Mezzano 71 in

the breeding of US 201, a strain that has shown a new level of resistance

among sugar beets in this country.



3 . Downy Mildew and Rust Resistance, Nonbolting Tendency,

Combined with Curly T o p Resistance

The curly-top-resistant varieties, US 1, US 33, US 34, US 12, and

US 22, only partially met the needs of California sugar beet districts,

since their use was limited to plantings of sugar beets made in late

March or April. If planted in late fa11 or winter months, these varieties

produced a very high percentage of seed stalks in the first or vegetative



SUGAR BEET IMPROVEMENT



109



year-a very objectionable and wasteful feature. In sharp contrast to

the easy bolting characteristics of these varieties was the nonbolting of

US i5.3 This variety had been selected in 1927 in cooperative work

with the New Mexico Agricultural Experiment Station (Coons et al.,

1950). Plants resistant to curly top were selected froni the high-sucrose

European variety, R & G Pioneer. Of the 14 roots selected, only 3

produced seed. T h e progeny of these roots was noted as showing curly

top resistance, and reselections were made; multiplication of the seed



FIG.6. US 15 in a cooperative variety test at State College, New Mexico. Curly

top exposure was severe enough to eliminate susceptible plants. Row 277a is US 15;

adjacent rows are planted with less resistant selections. (Photographed October 2,

1930, by H. A. Elcock.)



thus obtained was made in 1930 (Fig. 6 ) . The variety was about to be

discarded because of its susceptibility to Cercospora leaf spot in plantings made a t Fort Collins, Colorado, when its extremely low bolting

tendency was discovered by C. A. Lavis and F. G. Larmer in a series of

cooperative tests conducted at Davis, California. Here a number of

varieties were planted in October and November to find types that

The breeding of US 15 was by a team consisting of G. H. Coons, Dewey Stewart,

H. A. Elcock, F. G. Larmer, J. C. Overpeck, C. A. Lavis, and Charles Price. The

investigations have been conducted in cooperation with the Colorado, California,

and New Mexico Agricultural Experiment Stations and with beet sugar companies

operating i n California.



110



G. H. COONS, F. V.



OWEN,



A N D DEWEY STEWART



would not bolt when subjected to fairly cold weather in late fall or

winter in California. In these tests, US 33 and other curly-topresistant

introductions had about 40 to 50 per cent of their populations producing

seed stalks and European varieties bolted 10 to 20 per cent, but US 15

under these conditions bolted less than 1 per cent. The variety was also

found to be relatively resistant to downy mildew (Peronospora schachtii

Fckl.) and to rust (Urornyces betae (Pers.) Lev.)-both serious diseases in California coastal districts.

US 15 became the variety grown almost exclusively in winter plantings of sugar beets in southern and central California, but its greatest

achievement was to make possible sugar beet culture in the vast Imperial Valley of California. Here the growing seasons for sugar beets are

reversed, the seed being planted in September and October and the

crop grown over winter for harvest in May and June (Coons, 1941).

With ordinary sugar beet varieties, such a planting schedule cannot be

followed, because the onset of fairly cool weather shortly after the

seedlings emerge and the continued cold weather of November and

December give conditions that bring about abundant bolting, the sugar

beet plants changing from the vegetative to the fruiting phase. When

extensive trial plantings were being made in the fall of 1937 with US 33

and other varieties, to determine whether sugar beet culture could be

carried on in the Imperial Valley, the excessive bolting that occurred

in April and May with these types caused such difficulties in harvest

and so reduced yields that the proposal to grow sugar beets in the Imperial Valley was about to be written off as a failure, when the nonbolting characteristics of US 15, as seen in tests at the Meloland Substation of the California Agricultural Experiment Station, El Centro,

California, saved the day. Concurrently, with the first trial plantings

o i sugar beets, Charles Price of the Sugar Crops Section, Agricultural

Research Service, was conducting variety tests in cooperation with the

Meloland Station. Here it was found that US 15 planted in late September and early October was free from bolting, whereas other varieties

bolted 50 per cent or more. The industry therefore turned to US 15 as

the variety adapted to the Imperial Valley, and it was grown there

almost exclusively for over a decade. The Imperial Valley may have a

light exposure to curly top; hence for this region US 15 is well adapted.

For many years, over 30,000 acres of sugar beets were grown for shipment to factories located outside the valley, average performances over

the years being not far from 18 tons per acre with 18 per cent sucrose.

In 1948, a $7,000,000 factory was built at Carlton, near Brawley, to

process the Imperial Valley sugar beets.

After a ten-year period of dominance, US 15 was replaced by an-



SUGAR BEET IMPROVEMENT



111



other nonbolting variety, US 56, bred in California by Price et al.

(1948). This variety, by its superior curly top resistance, was better

than US 15 whenever curly top was serious. In its turn, and after four

years of use by growers, US 56 is being replaced by a variety bred by

McFarlane and Price (1952), US 75, which manifests a high curly top

resistance and a high resistance to downy mildew. Its curly top resistance about equals that of US 22/3.

US 15, US 56, and US 75 illustrate the possibility of combining

within one variety factors for resistance to several diseases, as well as

the genes for desirable physiological characters. Starting with a foundation stock of sugar beets having adequate curly top resistance, it has

been possible to maintain this resistance and select in other directions,

incorporating new characters in the variety that has the primary or

indispensable attribute-in this case, resistance to curly top.



4 . Black Root Resistance4

a. Resistance to Diseuse Complex Found. As noted, the humid area

has frequently had years of unprofitable sugar beet production because

seedling diseases so reduced stands that not enough sugar beet plants

remained in the fields to utilize the soil space efficiently or to produce

tonnages adequate to pay costs of culture (Coons, 1953b). With such

conditions occurring year after year, the beet sugar factories found it

difficult to contract adequate acreage for economical factory operation,

and the farmer, expecting the beet crop to yield at best only a small

margin of profit, tended to slight it with respect to fertilizer and care.

Thus, low yields started a vicious circle that led to closing of factories

and abandonment of the culture of the crop.

The seedling diseases commonly lumped under the term “black

root” have, more than any other factor, been responsible for the low average yields of sugar beets in the more eastern sugar beet districts whene+er rainfall has been normal o r above. The low yields of sugar beets in

the humid area in the past can definitely be attributed to poor, gappy

stands. A study made some years ago in Michigan (Lill, 1947) showed

for a typical sugar beet district that the average stand of beets over a

five-year period ranged, for the various row widths, from 63 to 69 per

cent, so low that root yields could not reach half of normal production.

Studies by Coons et al. (1946) showed that in the seedling disease com* The breeding of black-root-resistant varieties was conducted by the same group

that worked on leaf-spot-resistant varieties, with the names of the late J. E. Kotila,

and of C. L. Schneider to be added. The investigations were carried on in cooperation

with the Colorado, Michigan, Minnesota, New Mexico, and Oregon Agricultural

Experiment Stations and the beet sugar companies comprising the Farmers & Manufacturers Beet Sugar Association.



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