The Many Ailments of Clover

12 Pages·2010·0.85 MB·English

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grasses and
I es um es
the roots, crowns, stems, leaves, and
The Many
inflorescences. Fungi, bacteria, and the
viruses all can damage the clovers.
Some of the pathogens infect only
Ailments
specific organs of the plant, such as the
leaves or roots. Others attack several
or all parts of a plant. The pathogens
of Clover
diner also in parasitism. Some infect
only certain species of clover. Others
have a broad range of hosts and can
Earle W. Hanson, Kermit W. Kreillow
attack nearly all clovers and many
other hosts as well.
There are some 250 described species
in produc-
of true clovers {Trijolium) but only four
AN IMPORTANT PROBLEM
ing clovers, the establishment and
species—red, alsike, white (including
maintenance of stand, involves several
Ladino), and the crimson—are widely
factors. One is the root and crown
grown and of great importance.
disease complex, which includes the
The sweetclovers {Melilotus) are not
seedling blights, root rots, and crown
true clovers. Twenty-two species of
rots.
sweetclover are recognized. Three
Those diseases probably are the most
species—white, yellow, and sour clo-
important of all clover diseases. They
ver—are of importance in agriculture.
occur wherever clovers are grown.
All clovers are subject to injury from
They are caused by a complex of
diseases. All parts of the plant are
soil inhabiting fungi. The fungi may
attacked and sometimes destroyed—
21 7
2l8 YEARBOOK OF AGRICULTURE 1953
be widely distributed or occur only lo- rapidly during cool—55° to 65° F.—
cally. Some are virulent pathogens that wet weather, but the fungus that causes
can attack vigorous plants. Others are it can grow and infect plants at
weak pathogens that cause damage temperatures ranging from below
only after the plants have been weak- freezing to 75°.
ened by winter injury, nematodes, in- Sclerotinia trifoliorum has a broad
sects, drought, unfavorable soil con- range of hosts, which include all
ditions, or improper management. important true clovers and the sweet-
Some are primarily seedling pathogens. clovers, alfalfa (Medicago sativa), black
Others attack clover plants of any age. medic {M. lupulina), birdsfoot trefoil
Some are primarily root pathogens. {Lotus corniculatus), sainfoin {Onobrychis
Others are primarily crown pathogens. viciaejolia), and many other legumes
Several organisms may attack a plant and nonlegumes, including numerous
simultaneously, or one may follow weeds. Red clover, crimson clover
another in sequence. Thus the difficult ( Trifolium incarnatum), and alsike clover
problem of root and crown disease is {T, hyhridum) are all very susceptible.
one that must receive greater atten- White clover is generally considered
tion if productive stands are to be to be less susceptible but not immune.
maintained. Some other species of Sclerotinia may
also occasionally infect clovers.
CROWN WART of clover, caused by The disease is commonly referred
Urophlyctis trifolii, occurs in central to as a crown and stem rot, but it can
Europe on red clover {Trifolium pra- attack all parts of the plant. Symptoms
tense), white clover (T. repens), and first appear in the fall as small, brown
some others. In the United States the spots on the leaves and petioles. The
disease is of minor importance. It heavily infected leaves turn grayish
occurs mainly in the South Central brown, wither, and become overrun
States and on excessively wet soils. It with white mycelium, w^hich spreads
is similar to the more important crown to the crowns and roots. By late
wart of alfalfa. Its characteristic symp- winter or early spring the crowns and
tom is the formation of irregularly basal parts of the young stems show
shaped galls around the crown of the a brown, soft rot, which extends down-
plant, at and just below the soil level. ward into the roots. Consequently
The galls first become noticeable in part or all of the new growth of the
late spring and increase in size as infected plants wilts and dies. Stolons
summer advances. Infected plants wilt of Ladino clover may become soft and
in hot weather. Leaves of white clover flaccid over their entire length or only
are sometimes distorted. small areas may be afí'ected.
Sclerotinia crown and stem rot is As the stems and petioles are killed,
caused by Sclerotinia trijoliorum, and is a mass of white mycelium grows over
widely distributed, especially in the them. Some of the masses of mycelium
regions of mild winters or heavy snow then change into small, hard, black,
cover. It has long been recognized as cartilaginous bodies—the sclerotia.
one of the most destructive diseases They are attached to the surface of
of clover in northern Europe. It occurs (or imbedded in) the dead stems,
also in the Soviet Union and Canada. crowns, and roots or in the soil near
In the United States it is of consider- the roots. Some are as small as a
able economic importance in the clover seed. Some are larger than a
southern and central clover belts and pea seed.
causes extensive damage in the Pacific When the affected plant parts decay,
Northwest and in the Northeast. the sclerotia remain in the soil as a
Rarely does it occur in the north central future source of infection. Sclerotia
part of the northern clover belt. The are the chief means by which the
disease spreads and develops most fungus survives from year to year.
THE MANY AILMENTS OF CLOVER 219
They can remain viable in soil for Scientists have learned a lot about
several years. In the fall, if conditions the problem, but relatively little is
are right, the sclerotia germinate and understood concerning many of its
produce one or more small, disk- phases. The field symptoms of common
shaped, pinkish-buff, mushroomlike root rot of red clover are well known,
fruiting bodies called apothecia, which for example, but attempts to repro-
are borne individually on slender duce them under controlled conditions
stalks. The apothecia are one-sixteenth often are unsuccessful. Research men
to one-fourth of an inch in diameter. have demonstrated that some isolates
They produce millions of spores, which of the fungi can attack clover seedlings.
spread to the leaves and petioles of Species of Fusarium have been most
nearby plants, causing infection and frequently reported as causing root rot.
repeating the cycle. Symptoms of the disease are a local-
The greatest reduction in stands ized or general rotting of any part of
occurs in late winter and early spring. the root system. Taproot, secondary
The damage therefore is sometimes roots, and even the crown may be
confused with winter killing. The dis- attacked. The color of the diseased
ease usually occurs in patches through- areas ranges from light brown to black.
out a field, but when conditions are The rotting may be limited to the cor-
very favorable the patches may become tical areas around the exterior of the
so numerous as to merge and cause root, the vascular core may be dis-
extensive damage to a stand. In the colored, or the entire root may be
South a few days of warm weather afí'ected. Secondary roots are con-
sometimes checks the disease, and the stantly pruned away by the rots and
plants recover. new secondary roots are formed to
Control is difficult. Clean cultivation, compensate, but the replacement proc-
deep plowing to bury the sclerotia be- ess is usually the slower, so that by
yond their capacity to send up apothe- the end of the second year most plants
cia, and long rotations are helpful. have left only a few short secondary
Care should be taken not to distribute roots. The lower part of the taproot
the sclerotia with clover seed. Grazing often is destroyed completely. Such
or clipping in late fall som.etimes re- destruction causes wilting and a grad-
moves infected leaves and reduces the ual dying of the plant.
amount of foliage that may become Common root rot kills plants in all
infected and mat down on the crowns stages of development. Efi'^ects on stand
during the winter. Adapted varieties are most conspicuous during the second
are more resistant than nonadapted year, but losses up to 45 percent during
strains. The most promising method of the first year are not uncommon. Stand
control appears to be the breeding of losses occasionally occur in the spring
resistant strains. when the plants are weak because of
Common root rot is a group of root low food reserves or winter injury.
diseases caused by species of Fusarium Diseased stands frequently produce a
and several other soil fungi that pro- fair first crop of hay but fail to recover
duce similar symptom.s and frequently and to produce a second crop. Most
attack plants simultaneously. The rela- clovers and sweetclovers are susceptible
tive prevalence and importance of the to root rot.
fungi vary with the locality, kind of Besides Fusarium^ Rhizoctonia, Phoma,
clover, age of plant, season of the year, and other organisms may be associated
soil type, and management practices. with root rots. Plenodomus meliloti and
Mostly they are weak pathogens and Cylindrocarpon ehrenhergi arc of primary
cause damage after the plants have importance following the winter dor-
been weakened or injured. Most of mancy period on sweetclover in Al-
them are widely distributed and cause berta, Canada.
damage wherever clovers arc grown. Control is difficult, but any practice
220 YEARBOOK OF AGRICULTURE 1953
that improves the general vigor of the causes varying degrees of stunting, in
plant is helpful. Proper liming, ferti- which plants survive the early seed-
lization, and crop rotation are impor- ling stages, after which some recover
tant. Only adapted varieties should be and some die.
grown. No varieties available in 1953 Seedling blights are caused by a
had high resistance when conditions complex of fungi, including several
favor the disease. Plant breeders have species of Pythium, Rhizocionia, Fusa-
under way a project to develop resist- rium, Gliocladiurriy Phoma, and others.
ant strains of red clover. One of the most virulent is Pythium
Phytophthora root rot, caused by debaryanum. It would seem that seed
Phytophthora cactorum, is a widespread treatment might help control this
disease of sweetclover in North Amer- group of diseases, but the results of
ica, notably Ohio, Indiana, Illinois, field tests have given limited encour-
and Missouri. It occurs in Alberta and agement for this method of control.
Ontario. The fungus attacks individ-
ual plants or small groups of plants in THE STEM diseases attack the support-
fields or along roadsides. It is most ing and conducting systems of the
abundant in low, wet parts of fields, plant. Often they cause serious losses.
where in seasons of heavy spring rain- Usual symptoms are stem discolora-
fall and cool temperatures it may kill tion, withering and dying of attached
most of the plants. leaves and petioles, and general wilt-
Its presence is first noted in the ing and stunting of the plant. Fre-
spring, when infected plants wilt, die, quently stems break off or crack open
or are generally unthrifty. When their at the site of infection. Several of the
roots are examined, the upper portions major diseases of clover are included
usually are found to be rotted. The in this group.
decay generally is limited to the upper Northern anthracnose, caused by
3 or 4 inches but may extend as much Kahatiella caidivora, is a major disease
as 8 inches below the crown. The of red clover in the cooler areas of
decayed places usually are soft and North America, Europe, and Asia. It
watery. The color changes but little at develops best at 68^ to 77° F. and is
first. Later they may become dis- checked by continuous hot dry w^eather.
colored and shrunken. In the United States it is important
Crop rotation and the use of well- only in the northern clover regions but
drained fields are helpful control meas- there it frequently causes damage—
ures. It should also be possible to occasionally exceeding 50 percent of
develop resistant varieties because the crop in some fields. Losses as high
resistant plants are known to exist. as 50 to 60 percent have been reported
Seedling blights, caused by Pythium^ in Germany. Complete crop failures
Rkizoctonia, and other fungi, are the have been observed in the Nether-
most destructive seedling diseases. lands. Seed production and hay yield
They occur wherever clovers are and quality are greatly reduced in
grown. Sometimes they seriously re- badly infected fields.
duce the stands. Three types of injury The disease is serious only on red
occur. Preemergence killing starts clover. It may occur on alsike, white,
shortly after the seed is sown and de- crimson, and Persian {Trifolium resu-
velops rapidly, so that the seedlings are pinatum) clovers and possibly others.
destroyed before they emerge from It has never been found on alfalfa, but
the soil. In postemergence damping- has been reported on black medic and
oflf, infection commonly occurs before on sainfoin in the Netherlands. The
emergence, although the rate of disease species of fungus inciting the disease
development is slower and the seed- consists of a large number of physio-
lings emerge only to be killed soon logic races, which dififer in their capac-
thereafter. Root and hypocotyl rotting ity to infect different species of clover
THE MANY AILMENTS OF CLOVER 221
and different strains of a particular lotus alba). It has not been observed on
species. No red clover strain yet de- white clover. Alsike clover is prac-
veloped is immune to the disease, but tically immune.
wide differences in resistance exist Southern anthracnose has been re-
among European and American garded as the most destructive disease
strains. Varieties developed in the of red clover in the Southern States.
southern part of the United States are It reduces yields of hay and seed and
more susceptible than those developed can destroy stands of clover. A re-
in the northern clover areas. sistant variety, Kenland, is available.
Symptoms are confined mostly to the Most European and American strains
petioles and stems. Infection also developed in regions where the disease
occurs on the petiolules—small stalks does not occur are susceptible; hence
connecting the leaflets to the petiole— it is important to grow only locally
and occasionally on the leaflets them- adapted strains or strains known to be
selves. The first symptoms noticed in resistant.
the field are usually dark-brown or Symptoms resemble those of north-
black spots on the petioles. The spots ern anthracnose; in fact, a positive
soon cut off translocation to the parts identification in the field is frequently
above them—the upper part of the difficult and sometimes impossible.
petiole and the leaf—causing them to Dark tufts of setae in the older lesions
wilt, turn grayish brown, and die. The indicate that the disease is southern
petiole bends downward at the site anthracnose. But there are other dis-
of the lesion to form the familiar tinguishing features. Southern an-
"shepherd's crook." Stem lesions are thracnose commonly attacks the upper
most characteristic. They develop first part of the taproot; that has not been
as small, dark spots, which soon observed for northern anthracnose.
lengthen to form lesions with dark Southern anthracnose usually pro-
margins and light-colored centers. As duces more spotting of the leaves, but
the stem grows, a crack often appears that is not an infallible characteristic
in the center of the lesion. Stems because of the frequent presence of
finally may be girdled and killed. similar leaf spots incited by other
Plants in a badly infected field look as pathogens. Like northern anthracnose,
if they were scorched with fire, because it may occur on plants at any stage of
of the abundance of blackened and development. It most commonly de-
broken stems, withered petioles, and velops on the young, succulent parts
brown, dead leaves. The name scorch of stems and petioles but is not limited
has been aptly used in Britain to de- to them.
note the disease. The disease occurs on the leaves as
Southern anthracnose, caused by dark-brown spots of irregular shape,
Colletotrichum trifolii, is a major disease which vary from pin-point lesions to a
of red clover in the southern clover general infection over most of the sur-
belt of the United States. It has been face. Petioles are very susceptible.
recorded as far north as southern They become dark brown, and the
Canada, but is primarily a high-tem- attached leaflets droop. First symp-
perature disease that flourishes at toms on the stems and petioles are
about 82^ F. It is of little economic small, water-soaked spots, which usu-
importance in the northern clover ally lengthen to form long, depressed,
areas. It is confined mostly to North dark-brown or black lesions, many of
America, although it has been re- which develop gray or light-brown
ported on alfalfa in South Africa and centers. Lesions near the base of a
in Europe. It occurs occasionally on stem often cause death and browning
crimson clover, sub clover {Trifolium of the entire stem.
subterraneum), bur-clover {Medicago his- The most destructive effect of south-
pida)^ and white sweetclover {Meli- ern anthracnose is on the taproot and
201500°—5IÎ KÎ
222 YEARBOOK OF AGRICULTURE 1953
crown. Dark lesions develop on the small, dark-brown or black spots,
upper part of the taproot, gradually which increase slowly in size and
girdle it, and cause the plant to wilt eventually kill the affected parts.
and die. This crown rot is closely Infection occurs the first summer on
associated with taproot decay and may spring-sown clover but becomes more
result from spread of the fungus upward destructive the following late winter
from the roots or downward from the and spring. On unadapted clovers,
stems and petioles. Diseased crow^ns new leaves may be killed as rapidly as
Ijecome brittle so that the stems are they are formed. On sweetclover the
readily broken off at the soil level. disease appears most commonly in the
Crown and root rot caused by southern spring of the second year. The spots
anthracnose kills some plants and at first are dark. As they enlarge they
weakens others so that they cannot change to light brown. Leaf spotting
survive long drought, adverse winter is increased by frost injury, which
conditions, and attacks of other dis- seems to provide an avenue of entrance
eases. for the fungus. Heavy stands may be
Black stem, caused by Phoma^ Myco- greatly injured. The disease is more
sphaerella^ and Ascockyta, is a major severe on plants that have been clipped
disease of clovers. It is widely distrib- or grazed.
uted and may cause extensive damage Crop rotation and burning of dead
during cool, wet weather in the fall, leaves and stems before new growth
late winter, and spring. It causes the develops in the spring are helpful con-
familiar stem blackening and repeated trol measures. Breeding for resistance
defoliation, which weakens and some- has l^een started.
tim.es destroys stands. The disease was Stem canker, or gooseneck, is caused
so severe in Kentucky in 1933 on by Ascockyta caulícola. It was first
some of the un adapted red clovers that reported in Germany in 1903 as a new
plots that had had perfect stands the disease of sweetclover. It is now known
previous December were bare by April. to occur in most areas of the world
Among the fungi that cause black where svv^eetclover is grown. It has not
stem of clovers are Ascockyta imperfecta, been observed on other legumes.
w^hich occurs mostly on alfalfa but It produces silvery-white cankers on
sometimes on clovers; Phoma trifolii, the stems, petioles, and occasionally
the organism most frequently attacking the midribs of the leaves. The cankers
red clover; and Mycosphaerella lethalis, vary in size. They are stippled with
the cause of black stem of sweetclover. numerous tiny black dots and have
Little is known about the host range of brown margins. On the lower parts of
these pathogens except that each of the stems the cankers may be so large
them can infect alfalfa, red clover, and and numerous as to girdle the stems.
sweetclover and each is primarily the On the upper parts they are less abun-
cause of the disease on its own crop. dant, smaller, and more isolated. Heav-
During midsummer and fall, another ily infected stems often appear swollen,
pathogen, Cercospora, also causes black are retarded in development, and have
stem. fewer and smaller leaves. They also
The most conspicuous symptom is tend to twist and bend at the top.
stem blackening, w^hich may involve
all or any part of the stem. Blackening LEAF DISEASES usually do not kill
increases when clover is not cut at the plants, but they interfere with the nor-
proper time or when the crop is left for mal functions of the leaf. Sometimes
seed. Frequently young shoots or they cause defoliation, which reduces
petioles are girdled and killed. This, yield, quality, and palatability of the
as well as leaf infection, may result in forage. If the defoliation is extensive
severe defoliation. and continuous, the plants lose vigor,
On red clover the disease produces are less able to survive unfavorable
THE MANY AILMENTS OF CLOVER 223
conditions, and are more readily at- crop in individual fields. It is known to
tacked by pathogens that cause root occur in nature only on red clover.
rots. L. J. Krakover, working at the Michi-
Pseudopeziza leaf spot, caused by gan Agricultural Experiment Station
Pseudopeziza trifolii, is widespread in the at East Lansing, inoculated sweetclo-
cooler, humid clover regions of the ver, alsike, white, and crimson clovers,
United States and Europe. It has been as well as alfalfa, vetch, and several
reported also from Canada and Russia. other legumes, but he was not able to
It usually is of minor importance, but infect them. James G. Horsfall, at the
occasionally severe local outbreaks New York Agricultural Experiment
cause extensive defoliation. Serious Station at Ithaca, however, reported
outbreaks have occurred in northern infection from artificial inoculations on
Indiana, Ohio, and the Northeastern alsike and white clovers, sweetclover,
States. It has been called the most seri- and alfalfa. That difference in ability
ous leaf disease of red clover in New of the isolates to cause disease suggests
York. that the fungus may have more than
It resembles the pseudopeziza leaf one race.
spot of alfalfa, but it does not attack Symptoms are limited almost exclu-
alfalfa and the disease of alfalfa does sively to the leaflets. Minute, light-
not attack the clovers. It occurs on red, brown spots, similar to the early symp-
alsike, white, crimson, zigzag (Trifol- toms of some of the other leaf spot
ium medium)^ and strawberry (T.jragi- diseases, appear first. Fully developed
ferum) clovers and several others. A spots are mostly oval or round and
similar disease of sweetclover is caused about one-fifth inch across. They may
by P, meliloti. be larger when only a few occur on a
Dark spots that may be olive to red- leaf. The most characteristic symptom
dish brown, purple, or black develop is the occurrence of concentric rings
on either leaf surface. The spots are within the lesion, suggesting the name
tiny, angular, or round and commonly target spot. The center of a typical
have dendritic margins. A minute, am- spot is dark and distinct. It is sur-
ber, jelly like globule occurs in the cen- rounded by alternately light and dark
ter of the older spots. The globules, or rings. The darker rings are sepia to
fruiting bodies, more frequently are dark brown. The lighter ones are
found on the lower side of the leaf but ocher to light brown. The darker
occasionally occur on both sides. They rings near the center of the lesion are
are most abundant in wet weather. narrow and ridged. The color con-
Later they dry up, shrink, become al- trast between the two outermost rings
most black, and are not readily de- is very sharp. Spots are most abundant
tected. Positive diagnosis of the disease near the margin of a leaf but may
in its early stages in the field is difficult occur anywhere. Frequently they coa-
because at first the minute pin-point lesce, killing large areas of the leaf
lesions are not markedly different from and causing defoliation. Symptoms on
those of other leaf spots. The disease is the stems and petioles are uncommon
almost entirely limited to the leaves, but when they do occur they appear as
but has been reported to produce small, dark-brown to black linear streaks.
long, dark streaks on the petioles. A related fungus, ^S*. botryosum, at-
Stemphylium leaf spot, or target spot, tacks alfalfa and sometimes occurs on
is caused by Stemphylium sarciniforme. It red clover.
is com.mon on red clover in the United Blackpatch, caused by an uniden-
States and Europe. It is not considered tified fungus, was first recognized in
a major disease of red clover, but its Kentucky as a disease of red and white
importance may have been underesti- clovers. It has also been reported from
mated. It can cause serious defoliation Wisconsin, West Virginia, and Geor-
and losses of 15 to 40 percent of the gia. It has generally been considered
224 YEARBOOK OF AGRICULTURE 1953
of little economic importance but clover may be more susceptible than
occasionally causes losses in local areas. common white clover. It is possible to
In heavily infected fields of red clover infect other species of clover in the
the seed yield may be reduced at least laboratory, but the disease has not
50 percent. been found attacking them in the
In addition to red and white clovers, field. Infected leaves usually have a
the disease has been found on soybean large yellowed area, which soon turns
{Glycine max), cowpea (Vigna sinensis), watery gray and translucent, then
kudzu {Plieraria thunhergiana), and blue light brown. A yellowish band usually
lupine {Lupinus angustifolius). It has not outlines the advancing edge of the
yet been reported as occurring in na- infected part of a leaf. Diseased areas
ture on alsike clover, crimson clover, that originate at a leaf tip sometimes
alfalfa, and the sweetclovcrs, but all become V-shaped. Infected leaves wilt,
of those crops have been infected by then shrivel and die. Sometimes the
artificial inoculation. dead V-shaped part of a leaf curls.
Blackpatch attacks the leaves, stems, The fungus can invade the entire
flowers, and seeds. Under normal con- leaflet and grow down the petiole,
ditions it occurs in patches. Otherwise causing complete wilting of the leaf.
it appears only on scattered plants. Apparently it does not attack stolons.
Leaf lesions are similar in size and The disease develops most rapidly
color to those caused l)y Stemphylium during warm, wet weather. A temper-
sarciniforme. They vary from brown to ature of 75°-8o^ F. is most favorable.
grayish black and usually have con- Cercospora leaf and stem spot, also
centric rings. Large areas of a leaf may called summer black stem, is widely
be affected. Sometimes all the lower distributed in the United States and
leaves are killed. Greatest damage Europe. It is commonly found on
results from the girdling of the stems most of the true clovers, including red,
beneath the flower head or from direct alsike, white, crimson, zigzag, hop,
infection of the flowers before the seeds {Trifolium agrarium), and many others.
are fully developed. The fungus is A similar disease occurs on alfalfa,
seed-transmitted. It also causes seed- sweetclover, black medic, and related
ling blight. Examination of diseased species. Cercospora zebrina is the most
plant parts with a hand lens usually important pathogen on all of those
reveals the presence of coarse, dark, hosts except sweetclover. C. davisii is
aerial mycelium, a characteristic that the most important (or the only)
helps in diagnosing the disease. species infecting sweetclover. Damage
Treating the seed with a fungicide varies from year to year, depending on
should aid in preventing initial infec- weather conditions, but the disease is
tion. In hayfieJds, losses can be reduced always present and frequently causes
by early harvesting. Crop rotation and excessive premature defoliation.
sanitation should also be helpful. Symptoms vary somewhat on the
Curvularia leaf spot is caused by difí"erent host plants. Leaves, stems,
Curvularia trifolii. It sometimes causes petioles, petiolules, and seeds may be
considerable wilting and premature attacked. Leaf lesions are usually
dying of leaves of Ladino clover in the angular and more or less confined by
eastern United States. It was first dis- the veins. The size and shape of the
covered in 1919, when it caused minor lesions vary from rather small, linear
damage to white clover near Wash- spots on red clover to large, almost
ington, D. C Since 1940 it has oc- circular ones on sw^eetclover. Appar-
curred more frequently, presumably ently atmospheric conditions and the
because of the widespread use of kind of tissue influence the size and
Ladino clover along the Atlantic sea- shape of the lesions. Color of the spots
board. Up to 25 percent of the leaves also varies considerably. On the true
may be attacked and damaged. Ladino clovers the general tone is reddish or
THE MANY AILMENTS OF CLOVER 225
smoky brown. On sweetclover it is collapse as a dead mass. Flower stalks
ashy gray. When conditions favor and floral parts may also become
sporulation of the pathogens, a silvery- infected and be killed.
gray down develops on mature lesions. Infection occurs from spores that
Lesions on the stems and petioles are develop on dead, overwintered leaves
somewhat sunken but with colors like and petioles. Spots can usually be
those of the leaf spots. Stem infections found on the first new leaves that
are serious as they cause the distal emerge in the spring.
parts to wilt and die. Seeds may also be No practical control measures are
infected. The disease may be dis- known, but plants difí'er in suscep-
tributed on the seed. tibility, and breeding for resistance is
Infection may occur at any time possible.
during the growing season on plants of Bacterial leaf spot, caused by Pseu-
any age, but the disease is usually most domonas syringae, does not usually
abundant in late summer and autumn. cause serious damage, although it is
On sweetclover the disease is most widespread in the United States and
conspicuous on second-year plants has been reported in Italy and
after they have started to bloom and is England. Wet weather favors its
more severe on plants that have been rapid spread and development. Hot,
cut or grazed. dry weather checks it.
Little is known concerning the con- The disease may appear at any time
trol of this leaf and stem spot. Remov- during the growing season. It is most
ing old crop residues and crop rotation conspicuous on the leaflets. It also
help reduce damage. Infected seed affects the stems, petioles, petiolules,
should be treated with a fungicide stipules, and flower pedicels. First
to reduce seedling infection. The symptoms are tiny, translucent dots
development of resistant varieties of on the lower leaf surface. The spots
legumes seems to hold out the most enlarge and fill the angles between
hope for controlling it. the veins. They are tiny and black
Pseudoplea leaf spot, or pepper spot, except for the margins, which retain
is due to Pseudoplea trifolii^ a fungus a water-soaked appearance. In wet
that attacks the true clovers and less weather a milky-white bacterial ex-
commonly alfalfa. It occurs through- údate may develop as a thin film or
out the United States, Canada, Eu- as droplets. On drying, the exúdate
rope, and Asia, The disease has not becomes a thin, incrusting film, which
been reported on sweetclover. It is glistens in the light. Tissues surround-
important on Ladino and white clover ing the spots are yellowish green.
in Northeastern and Southern States, Infection may be so abundant that
where severe infection frequently whole sections of a leaflet are killed.
causes premature yellowing and defo- Mature leaves are often perforated
liation of lower leaves. It occurs and frayed because of the drying and
throughout the growing season but shattering of parts of the diseased
develops most abundantly in cool, wet tissues. Lesions on the petioles and
weather. stems are dark, elongated, and slightly
Tiny, sunken black flecks develop sunken.
on both surfaces of leaves and on Several clovers, including red, alsike,
petioles. The flecks rarely reach a white, crimson, zigzag, and Berseem
diameter of more than a few milli- (Trifolium alexandrinum), are known
meters but frequently are very numer- hosts of this pathogen. Isolates from
ous—hence the name pepper spot. difí*erent areas difl^er in pathogenicity.
Later they turn gray with a dark, Several other bacterial diseases have
reddish-brown margin. Heavily in- been found on clovers, but none of
fected leaves and petioles become them is important in America.
yellow, wither, turn brown, and Sooty blotch, caused by Cymadothea
226 YEARBOOK OF AGRICULTURE 1953
trifolii, is one of the most conspicuous varieties, but American strains are
and easily identified leaf spots of now available that are highly re-
clover. It is prevalent throughout sistant. Wisconsin Mildew Resistant is
North America, particularly in the one of them. Most varieties contain a
southern part of the United States, few resistant plants.
and in Europe. It is most common in . At first barely perceptible patches of
alsike, red, and white clovers. Fre- fine, white, cobwebby mycelium de-
quently it reaches epidemic propor- velop on the upper surface of the
tions on crimson clover. It also has leaves. The patches enlarge and merge,
been reported as occurring on some 24 the fungus sporulates, and the leaf
other true clovers. surface appears as if it had been dusted
In the Southern States the disease with white flour. Symptoms may also
appears in the spring. In Northern occur on the lower surface of leaves.
States it is more prevalent in late Severe attacks can make whole fields
summer and fall. The earliest symp- appear white.
tom is the development of minute,
olive-green dots mostly on the lower RUSTS OF GLOVERS are widely dis-
leaf surface. The dots enlarge and tributed in the humid and subhumid
become thicker and darker until they areas of the world. Damage is difficult
acquire the appearance of velvety to assess because heavy infection usu-
black, angular, elevated patches or ally does not occur until late in the
warts. In the fall the warts are re- summer. Occasionally heavy infection
placed by other black areas, which occurs, and severe loss results when a
have a shiny surface. Chlorotic and grower attempts to produce two seed
later necrotic spots appear on the crops in a season and leaves the ac-
upper surface of the leaf immediately cumulated rust-infected old growth to
above the warts. When spots are infect the new growth.
abundant the entire leaf may turn Three common varieties of rust
brown, die, and fall off. Sooty blotch attack clovers. They cannot be dis-
is of considerable economic impor- tinguished on the basis of symptoms
tance on crimson clover, causing re- but can be differentiated by differ-
duction in seed yield. ences in their capacity to infect the
Powdery mildew, caused by Ery- various clovers. For example, the
siphe polygoniy is a common and wide- variety of rust on alsike clover (Uro-
spread disease of clovers. It probably myces trifolii hyhridi) infects only alsike,
occurs wherever they are grown. It while the rust on red clover {U. tri-
can cause reductions in the yield and Join fallens) infects red, zigzag, crimson,
quality of hay. Ordinarily it is of little Berseem, and several other clovers.
consequence on the first hay crop but The rust on white clover {U. trifolii
is more abundant on the second. It ¿rifolii-repentis) does not infect red or
can attack plants at any stage of ma- zigzag clovers but does infect crimson
turity but develops best during the and Berseem, besides white clover.
cool nights and warm days of the latter These rust fungi differ from those
half of summer and fall. Long spells of causing the cereal rusts in that they
dry weather favor its development. can complete their entire life cycle on
The pathogen has been recorded on a clover species and do not require an
some 359 species belonging to 154 alternate host.
genera. It consists of a number of The most conspicuous symptom of
physiologic races, which differ in their clover rust is the uredial, or brown rust,
ability to attack difí'erent genera and stage, in which round or irregular,
species of hosts and different varieties pale-brown pustules, surrounded by
of a species. European varieties of red the torn epidermis, appear on the
clover have been reported generally lower surface of leaves and on the
to be more resistant than American petioles and the stems. Sometimes in