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III. Progress in Methods of Breeding

III. Progress in Methods of Breeding

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226



WM. J. WHITE



pollination conditions a very high proportion of the seed set will be of

crossed origin. Thus high self-sterility may be utilized in a breeding

system to force crossing, in much the same way that detasseling is used

in hybrid corn breeding. The desirability of selecting for high selfsterility has been repeatedly stressed by Tysdal since 1942. Tysdal and

Kiesselbach (1944) compared the hay yield of open-pollinated progenies

of' nine highly self-fertile plants, nine medium self-fertile plants and

seven highly self-sterile plants, and found the yields to average 6.04,

6.35, and 6.59 tons per acre for the three groups. The lowest yielding

individual progeny was in the self-fertile group and the highest yielding

in the self-sterile group. Tysdal and Crandall (1948) report a significant

correlation coefficient of - 0 . 4 0 between the self-fertility of the parent

and open-pollinated progeny yield of 34 clones. While there is a wide

range in self-fertility in the crop a considerable mass of evidence has

accumulated demonstrating the desirability of selecting for high selfsterility.

The explanation for the lower yields obtained from open pollinated

progenies of self-fertile plants undoubtedly lies in the fact that selfing

(inbreeding) results in a marked loss of vigor. The work of Kirk (1927,

1933), Tysdal et aE. (1942), Tysdal (1942) and others has shown that

upon inbreeding, forage yield, and particularly seed yield, declines drastically and progressively with each advance in generation of selfing up

until a t least the seventh or eighth. A marked reduction in self-fertility

upon inbreeding has recently been demonstrated by Wilsie and Skory

(1948). Tysdal and Kiesselbach (1944) have shown that a population

can contain a certain proportion of inbred plants without depressing hay

yield, but there can be no doubt that above a certain level the proportion

of inbreds does have a depressional effect on yield.

A further breeding characteristic of alfalfa which has been established

in recent years is that a marked expression of hybrid vigor may be

secured by crossing certain plants or certain lines. Tysdal et al. (1942)

gave seed and forage yields of a number of hand-pollinated crosses some

of which yielded as high as 139 per cent of the average forage yield of

three st.andard varieties. Expressed in percentage of the checks the seed

yield of the hybrids ranged as high as 257 per cent. Tysdal (1947),

Tysdal and Crandall (1948), Bolton (1948) , Wilsie and Skory (1948)

all present further data demonstrating conclusively the occurrence of

hybrid vigor to rather marked degrees. Study of these data suggests

that heterosis is expressed more strongly in seed yield than in forage

yield. Nevertheless hay yield increases of 25 to 30 per cent over check

varieties have been obtained, and in improvement programs attainment

of such yield superiority is highly desirable.



ALFALFA IMPROVEMENT



227



Hybrid vigor. is, of course, not expressed in all crosses. In alfalfa, as

in corn and in nnimals, certain individuals cross with other individuals

to give superior yield or performance. Such individuals are said to combine or nick well. Ot.her individuals do not possess this capacity and

are said to be “poor combiners.” Data given by Tysdal and Kiesselbach

(1944) serves to illustrate the differences between plants in combining

ability. They showed that the F1 of one particular plant crossed as a

male with three female plants gave an average yield of 1003 g. of green

weight per plant. I n t.he case of three other male plants all crossed into

the same three female plants the FI yielded 483, 659 and 754 grams.

Bolton (1948) selected 13 plants and intercrossed them in all possible

combinations. The average seed yield of the F1progenies of one plant

crossed with each of the 12 others ranged between 309 and 166 lbs. per

acre. Further evidence of a similar natsure has been presented by Tysdal

ef al. 1942),Stevenson and Bolton (1947),and Wilsie and Skory (1948).

In a breeding program, in order to utilize the characteristic of

heterosis, it is essential to test for combining ability of the selected plants.

Methods of testing for this behavior will be covered in Section 111-3.

2. Utilizing Hybrid Vigor



Recognizing the close similarity between the breeding characteristics

of alfalfa and corn, Tysdal e t al. (1942) and Tysdal (1942) proposed

a breeding system for alfalfa similar to that employed in breeding hybrid

corn. Application of the hybrid corn breeding system, or variations of

it, represents in the writer’s opinion a most outstanding and promising

advance in methods of breeding alfalfa. By means of this system of

improvement it is possible to capitalize upon hybrid vigor in the crop

as utilized by the farmer grower, and at the same time to maintain a

degree of uniformity for such characteristics as disease resistance, insect

resistance, and quality that is not obtainable by any other method of

breeding.

Certain fundamental differences between corn and alfalfa necessitate

an alternative procedure in the application of the hybrid breeding system. Firstly, in corn the male and female organs are carried on different

part of the plant. This makes it possible to emasculate the female or

seed parent of a hybrid by the relatively simple process of detasseling.

I n alfalfa the male and female organs are contained in the same flower

and mechanical emasculation on any extensive scale is impossible. I n

Section 111-1, however, it was shown that about 15 per cent of alfalfa

plants are highly self-sterile, and that when such self-sterile plants are

in association with other plants under natural field conditions a very

high proportion of their seed is of crossed origin. Thus high self-sterility



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is the inherent characteristic which Tysdal et al. (1942) envisioned as

being used to force crossing as an alternative to detasseling in corn. The

degree of control of crossing is obviously not absolute in alfalfa as it is

in corn.

Inbreeding or brother-sister mating (sibbing) is necessary in the

annual corn in order to maintain and propagate the selected lines from

year to year. There are of course other reasons for selfing in corn. I n

alfalfa the perennial habit of the crop eliminates the necessity of selfing

or sibbing to maintain the selected unit. The selected alfalfa plants can

he readily propagated asexually by stem cuttings (Tysdal, 1942; Tysdal

et al., 1942; White, 1946; and Grandfield et al., 1948). Therefore, in the

system of breeding hybrid alfalfa proposed by Tysdal e t al. (1942)

selected single plants propagated clonally (by cuttings) become the basic

units used as parents in producing single crosses, in contrast to lines

maintained by inbreeding or sibbing in corn.

The breeding procedure evolved by Tysdal et al. (1942) entails rigid

selection of single plants for self-sterility, high combining ability, resistance to disease and insects, and any other characteristic desired. For

the production of single crosses two of the selected plants are propagated

clonally and clones of the two plants are established in an isolated crossing plot. Such a plot produces single cross seed. Because of the labor

required in clonal propagation and transplanting it is unlikely that it

will prove practical to produce single cross seed in sufficient volume to

supply the demand from farmers who wish to use the hybrid for hay

or pasture purposes. The exponents of this scheme of breeding thus

propose the prodiiction of double cross hybrid seed. This simply entails

the establishment of a second isolated single crossing plot comprised of

clones of two other selected plants. The seed from the two single crosses

is sown in alternate rows or mixed in a third isolated field, from which

the double crow seed is harvested.

In the production of the single crosses the high self-sterility of the

two clonally propagated parents insures that a high proportion of the

seed will be crowed. I n the production of the double cross, however, any

one plant may self, may cross with another plant or plants of the same

single cross (sib). or may cross with a plant or plants of the other single

cross. The latter type of cross is the desired one. The extent of selfing

in producing the double cross is not likely to be any or much greater than

in producing the single crosses. Wilsie and Skory (1948). have shown

that on crossing plants of low x low self-fertility the F, was low in selffertility. Nor is the proportion of sibbed seed likely to be high. Bolton

(1948) has shown that seed-setting upon sibbing is only 60 per cent of

that following out-crossing. Tysdal (1942) has indicated that it max



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be possible to select plants the F1 of which would be inter-sterile (sibsterile). Furthermore, the proportion of sibbed plants likely to occur in

the double cross progeny will probably not seriously reduce yield. Tysdal and Kiesselbach (1944) have shown that at, least 25 and possibly

50 per cent of selfed seed may be mixed with open-pollinated seed without

significantly reducing the yield of the open-pollinated variety.

As an alternative to the extensive use of clonally propagated plants

as parent.al units for the production of single crosses Bolton (1948) suggested the use of inbred lines. His plan would entail clonal propagation

of the parent selections and space isolation of each to produce inbred

seed, and would probably necessitate use of material somewhat more selffertile than would be the case in following the procedure outlined above.

While Bolton’s plan has not. been fully tested with alfalfa, essentially

the same procedure is practiced in the production of commercial hybrids

in sunflower, which is also an insect pollinated crop (Unrau and White,

1944; Unrau, 1947).

The breeding of synthetic varieties has also been suggested by Tysdal

et al. (1942) as a means of utilizing hybrid vigor. Tysdal (1947) has

described a synthetic variety as one “that is developed by crossing, composking or planting together two or more unrelated strains or clones, the

bulk seed being harvested and replanted in successive generations. B y

intercrossing the unrelated strains or clones are synthesized into a new

variety.’’ This breeding system demands that rigid selection for high

combining ability and other desirable characteristics be practiced just as

would be the cme if single or double crosses were to be produced. It

largely elminates the necessity of extensive vegetative propagation.

Experimentally produced synthetic varieties have demonstrated their

superiority over standard varieties. Tysdal and Crandall (1948) have

presented data showing that certain synthetics in their first generation of

synthesis yield as much as 16 per cent more hay than standard varieties,

and they point out t.liat in the second generation of synthesis the yield

was almost exactly the same as the first. These results provide grounds

for optimism for the successful use of this system of breeding.

While hybrid alfalfas and synthetics have been produced only experimentally as yet, the results have indicated that the evolution of breeding

systems which embody the utilization of hybrid vigor afford a means of

improvement not attainable in the breeding systems previously employed.

3. Methods of Testing for Combining



Ability



It has been shown in Section 111-1 that plants differ markedly in

capacity to combine in crosses with other plants to produce high yielding

progeny. It is impossible to assess combining ability by the appearance



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WM. J . WHITE



or the yield performance of a plant itself as it is dependent upon how

the genes of one plant complement those of another. It is thus necessary

to cross and test the crossed progeny to evaluate this characteristic. The

proportion of plants in any population possessing high combining ability

is likely to be m a l l and consequently to find such plants involves the

testing of relatively large numbers. To hand cross on an extensive scale

is a slow and expensive procedure, and, if not impossible, is impractical.

For example, to intercross 100 plants in all possible combinations would

require making and testing 4950 crosses, disregarding reciprocals. The

polycross procedure proposed by Tysdal et al. (1942) provides an alternative means of making crosses on an extensive scale. Tysdal and Crandall

(1948) state that “polycross seed is the seed produced on selected clones

inter-pollinated a t random in isolation.” The technique is comparable

to the top-cross met.hod used in corn (Hayes and Immer, 1942).

The polycross procedure simply consists of choosing plants for any

one or more major characteristic and low self-fertility and placing them

in an isolated nursery to intercross naturally among themselves. To

provide that each plant has an 0pportunit.y of crossing with several other

plants, the selections are cloned and replicated a t random several times

through the nursery. Seed is collected from each clone of each plant

and all the seed from a plant is bulked. Because the plants are relatively

self-sterile the seed produced is largely of crossed origin and has a number of different male parents.

Tysdal and Cranda!l (1948) have compared the yield, bacterial wilt

resistance, leaf hopper resistance, and cold resistance of polycrosses of a

number of plants with that of single crosses involving the same plants.

Their data show that the polycross progenies gave essentially the same

ranking as did the single crosses, thus demonstrating that the polycross

test provides a dependable means of determining general combining

ability.

It is of interest to note that Tysdal and Crandall (1948) found that

top-crosses of selected clones unto a standard variety, and also polycrosses unto a standard variety, gave progenies performing essentialIy

similarly to that of the conventional polycross and the single crosses.

This finding appears to the writer to throw doubt upon the necessity of

isolating the polycross nursery.

I n the conduct of a breeding program the polycross technique is used

to determine those plants having superior combining ability. It is then

necessary or highly desirable to test these superior plants in single cross

combinations. Having very materially reduced the numbers of plants

being worked with by the polycross test it becomes practical to make and

to test the single cross combinations.



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231



4. Selection Procedures for Certain Characteristics

In earlier breeding programs with alfalfa the general procedure was

to inbreed and select in inbred progenies (Kirk, 1927; Stewart, 1931;

Kirk, 1932; Tysdal and Clarke, 1934; and Dwyer, 1936). There has

been, however, a definite trend in recent years towards selection of openpollinated plants and utilization of them without inbreeding (Tysdal

et al., 1942; Tysdal and Kiesselbach, 1944; Bolton, 1948; and Reitz

et al., 1948). The results obtained to date in breeding for increased yield

of hay and seed, bacterial wilt resistance, and black stem resistance indicate the improvements which may be made without resorting to inbreeding. Reitz et nl. (1948), for instance, secured as high a level of resistance

for black stem disease in open-pollinated material as in selected selfed

lines. These latter authors point out that the high degree of self-sterility,

the low vigor of inbreds and t,he labor involved reduce the value of

inbreeding in an improvement program. In spite of the acknowledged

advances, however, which have been made by selection of open-pollinated

material without resorting to inbreeding, it would seem desirable to continue to explore the possibilities of increasing homozygosity for t.he particularly desired characteristic through inbreeding.

In breeding for improved seed yield, plants which trip automatically

to a high degree and are self-fertile occasionally may be selected.

Such plants set seed in the absence of tripping and cross-pollinating

insects. It is a strong temptation to utilize them in the breeding

program. Tysdal has repeatedly warned against the selection and use

of such material. Stevenson and Bolton (1947) have presented data on

the hand-crossed single cross performance of such plants showing that

certain F1combinations yielded four to six times as much seed as Grimm.

However, when open-pollinated progenies of eleven F1 plants were compared with the clones, selfed progenies, and with Grimm as a check, the

seed yield of the open pollinated progenies was only slightly more than

t.hat of the selfed progenies of the same plants. These data clearly

demonstrate that under open pollination such self-tripping self-fertile

plants self-pollinate rather than cross, and that the following generation

is decidedly inferior.

I n improving alfalfa for seed production Bolton (1948) has followed

t.he procedure of selecting on the basis of large pod size and heavy pod

production and in the field. This was followed by a test of cross- and

self-fertility und.er greenhouse conditions. B y selecting those plants

which were highly cross-fertile in the greenhouse test he found that the

average of all eingle crosses involving any one select,ed plant exceeded

the seed yield of Grimm and Ladak. The average of the single crosses



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from certain plmts outyielded the checks by over 100 per cent. This

procedure is obviously efficient for isolating superior seed yielding plants.

I n the selecting for disease resistance it is desirable to create controlled epidemics of the disease rather than to depend upon the often

sporadic natural infection. The methods of testing for bacterial wilt

resistance have now become fairly well standardized (Brink et al., 1934;

Jones, 1934; Peltier and Tysdal, 1934; Weimer and Madson, 1936; and

Jones, 1940). A procedure for use in selection for black stem resistance

has been described by Reitz et al. (1948). Cormack (1948) has worked

out. inoculation techniques in testing for winter crown rot. Laboratory

methods of testing for cold resistances have been developed by Peltier

and Tysdal (1932). The reliability of their method is indicated by results reported by Tysdal and Crandsll (1948). They found a significant

correlation of +.62 between resistance in laboratory cold tests at Lincoln, Nebraska and cold resistance under field conditions a t Saskatoon,

Saskatchewan.

IV. CONQUERING

SOMEDISEASES

Among the many factors restricting utilization and limiting production

of alfalfa certain diseases rank high in significance. The diseases of the

greatest obvious seriousness are those which cause killing of plants and

severe stand reductions either suddenly or over a period of time. Bacterial wilt causes damage of this nature. There are, however, many more

or less insidious diseases the effect of which on stand establishment or

maintenance, or on yield or quality is less apparent but none the less of

very considerable importance. Several leaf diseases and at least one

seedling disease belong in the latter category.

A mult.iplicity of organisms find the alfalfa plant a suitable host.

Chilton et al, (1943) have presented a lengthy list of fungi found on the

genus Medicago, to which could be added several diseases caused by

viruses and bacteria. To deal adequately with even the majority of the

major diseases would be beyond the scope of this review. Bacterial wilt

and black stem have been chosen for discussion as representative of

diseases upon which considerable work has been done and progress made

in control.

1. Bacterial W i l t

Of all the diseases attacking alfalfa on the North American continent

bacterial wilt is undoubtedly the most serious. The causative organism

now designated as Corynebacterium insidwsum (McCull) Jensen was

first identified by Jones in 1925, and the disease and organism was more

fully described by Jones and McCulloch (1926). Recognition of the

disease in many alfalfa growing areas soon followed its discovery.



ALFALFA IMPROVEMENT



233



According to Tysdal (1947) it has now been found in every major alfalfa

producing state in the United States. In addition it has been discovered

in each of the three prairie provinces of Canada. I n regions in which the

disease is prevalent it has been amply demonstrated that stands of

susceptible varieties survive only 3 to 5 years (Jones and McCulloch,

1926; Tysdal and Westover, 1937; Weihing et al., 1938; and Grandfield,

1945b), whereas prior to the advent of the disease longevity of st.ands

was much greater. Speaking of the United States, Tysdal and Westover

(1937) state that “Bacterial wilt annually destroys hundreds of thousands of acres,” and they point out that resistant strains which would

extend the life of stands even 2 years would save millions of dollars.

The disease usually does not manifest itself until plants are about 3

years old. The characteristic symptoms are dwarfing and profuse branching associated with yellowing and small leaves. The tap and larger

branch roots, when cross-sectioned, display a partial or complete ring

of yellowish or pale-brown discoloration immediately below the bark.

The discoloration and a slimy appearance are apparent when the bark

is peeled back. In the advanced stages plants wilt and die. The damage

is caused by the bacteria plugging the vasrular conductive tissue of the

plant (Jones and McCulloch, 1926).

Certain control measures were suggested by Jones and McCullocli

(1926). These mainly involved sanitary precautions. Tysdal and Westover (1937) , however, report that “Considerable preliminary work indicated that cultural practices in general would not control the disease.

The only avenue of approach that offered possibilities was a breeding

program.”

Recognition of the disease and its seriousness immediately touched

off an extensive search for resistant material. Hundreds of lots of seed

were collected from many parts of the world, and tested a t several points

in United States It was found that a reasonably high level of resistance

was present in some strains obtained from Turkestan or adjacent areas

(Wilkins and Westover, 1934; Weimer and Madson, 1936; Tysdal and

Westover, 1937; and Weihing et al., 1938). Seed secured from a Nebraska

farmer but tracing back probably to Turkestan origin was found to possess resistance, and was assigned the variety name of Hardistan by

Kiesselbach et al. (1930). Anot.her strain secured from France but

thought t o have originated from Turkestan was named Kaw by Salmon

(1932). According to Wilkins and Westover (1934) Turkestan alfalfa

strains in general were more susceptible t o leaf spot diseases and inferior

in yielding ability to commonly grown domestic varieties.

I n varieties and strains other than those of Turkestan origin various

levels of resistance have been found and breeding has yielded new



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WM. J . WHITE



varieties. From common alfalfa Grandfield (1945b) developed the resistant variety Buffalo which was released in 1943. Weimer and Madson

(1936) and Wilson (1947) have also shown that highly resistant lines

could be obtained from this type. Among the variegated varieties Ladak

has frequently been shown to possess a fairly high degree of resisthnce.

Even Grimm, which once was considered almost completely susceptible,

has been shown to be a source of some immune and resistant plants

(Jones and Smith, 1947). By compositing lines selected out of Cossack,

Turkestan, and Ladak, Tysdal developed the Ranger variety which was

released in 1942 (Hollowell, 1945).

I n the development of the Buffalo and Ranger varieties a high degree

of resistance to wilt has been attained, combined with a higher degree of

resistance to leaf spot diseases than was possessed by varieties of

Turkestan origin (Grandfield, 1945b; Hollowell, 1945; Tysdal, 1947). To

illustrate the sliperiority of these new varieties in respect to stand maintenance and yielding ability, data given by Grandfield (1945b) may be

cited. I n a tert a t Manhattan, Kansas comparing the varieties Buffalo,

Kansas Common, Grimm, Oklahoma Common, and Dakota Common the

stands ranged between 95 and 100 per rent in 1939 but by 1942 had been

reduced to 6 t o 25 per cent for the wilt-susceptible varieties while that

of Buffalo showed no reduction. The stand reduction was reflected in

hay yield. Buffalo was not superior in yield in 1939 but by 1942 it

yielded 3.26 ad compared to 2.53, 2.50, 2.46 and 2.85 tons per acre for

the varieties liuted in the order above. In the fourth year of a test at

Ames, Iowa, Buffalo yielded 2.54, Ranger 2.34, Kansas Common 0.60 and

Grimm 0.84 tons of hay per acre. These results serve to illustrate the

outstanding progress which has been made through development of resistant varieties.

Varieties even more resistant than those presently available will undoubtedly be forthcoming. Jones and Smith (1947) have described certain selected plants as immune to this disease. Wilson (1947) has

isolated one gene for high resistance, and plants possessing it in the

homozygous condition were found to be 72 per cent healthy in artificially

inoculated tests. Wilson (1947) has pointed out that in similar tests

conducted in Nebraska and Wisconsin Hardistan Rhowed 19 per cent and

Ranger 37 per cent. healthy plants.

The genetics o f resistance to bacterial wilt has been found to be complex. Brink et al. (1934) concluded that resistance behaved as an intergrading character and that a factorial interpretation of their data was

impossible. Wtiiner aiid Madson (1936) also found t.hat transmission of

resistance to selfed and open-pollinated progenies was complex. Wilson

(1947) , however, isolated “three and possibly four partially dominant



ALFALFA IMPROVEMENT



235



genes differing in strength of resistance.” As shown above, one of these

genes when homozygous affords a high level of resistance.

2. Black Stern



Black stem caused by Ascochyfa iinperfecta Peck is widely distributed

in North America and Europe (Toovey e t al., 1936; Remsberg and Hungerford, 1936; Peterson and Melchers, 1942; Cormack, 1945; and Reitz

e t al., 1948). I t s occurrence, distribution and intensity of attack is

favored by relatively cool humid conditions. It is thus of less economic

importance in t,he dry land agriculture of semi-arid or arid regions than

under irrigat.ion or in humid areas.

The appearance of small, dark brown or black spots on the leaves

and stems is tlit! common early symptoms of infection. As the disease

progresses the lesions on the leaves enlarge and coalesce, and the leaves

become chlorotk and die. The progress of infection on the stem is similar, and results in a smooth black discoloration often involving a considerable portion of the stem. Lesions may also occur on petioles,

racemes and pod6 Cormack (1945) showed that 50.5 per cent of the

seed samples he examined carried the disease organism although displaying no symptoms of disease. Death of axillary buds in early spring and

death of shoots in severe epidemics is a further, although not specific,

symptom of the disease.

The damage caused by the disease is more or less indicated by the

above described symptoms. Defoliation due to leaf and petiole infection

is probably thz commonest injury. Peterson and Melchers (1942) reported a loss of over 15 per cent of the leaves in some plots under conditions in which the infection was not particularly severe. Undoubtedly

the loss of Ieaves under certain circumstances is much higher. Since the

leaves are higher in protein and carotene (Tysdal, 1947) than the stems,

the injury and loss of leaves causes a reduction in forage quality and

nutritive value. It is likely that the lesions which also develop on the

stems also adversely affect forage quality. Under favorable conditions

for infection death of shoots and stems and whole plants occur (Johnson

and Valleau, 1933; Toovey e t d.,1936; Reitz et al., 1948). Richards

(1934) recorded that t.he yield of the first cut of severely attacked

varieties was reduced by 40 to 50 per cent. Cormack (1945) has shown

that the organism causes a reduction in seedling emergence.

Reduction in the incidence of the disease by management practices

has been suggested. Johnson and Valleau (1933) noted that early spring

grazing by sheep removed the dead growth and reduced the primary

infection in the new spring growt,h. It has frequently been observed that

in any one season t,he first crop is more severely infected than the second



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WM. J. WHITE



or third. Toovey et al. (1936)advised cutting the first crop early before

injury becomes severe. While management practices afford a means of

reducing the damage, control undoubtedly is contingent upon developing

resistant varieties. While no such varieties have been developed as vet,

progress has been made towards that end.

Johnson and Valleau (1933)noted that varieties differed in degree of

infection. Richards (1934) recorded marked intervarietal differences in

susceptibility. Toovey et al. (1936) recorded that a t Cambridge a strain

from Iraq was highly susceptible and that in Norfolk varieties were

observed to differ in susceptibility. Peterson and Melchers (1942) found

M . falcata and M . ruthenica more resistant than common alfalfa. Reitz

et al. (1948) reported that varieties, strains, and species differed significantly in resistance both to natural field infection and to artificial

inoculation in the greenhouse. The occurrence of interviarietal and

interspecific differences in disease reaction is evidence of inherent variation for resistance and also is suggestive of the possibilites of breeding

resistant strains and varieties.

Reitz et al. (1948)established that significant differences in resistance

existed between lines of Kansas Common and also between the selfed

progenies of reistant. and susceptible selections out of several different

varieties. They noted a “significant tendency for the inbred progeny to

react to black stem in the same manner as the parent had reacted.” A

significant correlat,ion of +.716 was found between the infection indices

of the first and second generation inbred progenies. Selection was shown

to be effective in progressively increasing the level of resistance. They

noted that the highest levels of resistance achieved by inbreeding with

selection was matched by selecting from open-pollinated varieties. Although immunity to the disease was not observed a few highly resistant

plants were isoleted.

Inheritance of resistance to the disease was examined by Reitz et al.

(1948). The F1of crosses between highly susceptible and highly resistant

inbred plants were found to be quite uniform for a level of resistance

nearly as low as that of the inbred progeny of the resistant parent. The

Fz of crosses involving two resistant plants was shown to be significantly

more resistant than the Fz of crosses of two susceptible plants or of one

resistant plant They concluded that “inheritance of resistance is definite

but not simple.”

While the breeding of resistant varieties is as yet in the more or less

preliminary stages, yet the work to date, particularly that of Reitz et al.,

(1948),has shown conclusively that the attainment of such an objective

is possible. It should be noted that control of black stem by the breeding

approach is complicated by the occurrence of physiologic races of the



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