Foreign plant quarantines in-service training series

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14 Urocystls tritici

United States Departmnent of Agriculture
Bureau of Entomology and Plant Quarantine
Apr. 1, 1940
Foreign Plant Quarantines In-service Training Series. No. 7
(No. 1 is an introductory and explanatory number.)
Prepared by N. Rex Hunt

Name of disease: Flag smut of wheat.

Name of pathogen: Urocystis tritici Koern.
Syn. (For a time it was called U. occult)

Hostsi Triticum spp.

Part attacked: Coleoptile, leaves, sheaths, stems.

Place of origin:

Country of first report: Australia, 1868. (7)

First report from U. S.: Collected May 11, 1918, in St. Louis
County, Mo., but not reported until after it was found
May 5, 1919, in Madison County, Ill. (5, 17)

Present distribution: Australia, Bulgaria, China, Cyprus, Egypt,
India, Italy, Japan, South Africa, Spain, Tasmania, Trans-
caucasia, Tunis, United States (the Plant Disease Survey
received no reports of the occurrence of flag smut in 1957
or 1958 but had reports of slight local infections in
Kansas as late as 1956, in Illinois in 19534 and in Missouri
in 1935.) (2, 7, 10, 1, 14, 18) Washington, 1940

Factors affecting severity: Susceptibility of host, and moisture
and temperature factors related to the germination period
of the host are important. Grain sowed before the fall
rains is more heavily infected than if sown late in the
season. If the soil is rather dry when the wheat germinates
and for a few days thereafter, infection may be relatively
light and if the soil is wet there is little infection.
Infection is reduced if the soil temperature gets too high
or too low, the optimum varying according to different
workers but apparently not far from 200 C. The number of
spores overwintering and present in the soil. in condition to
germinate readily is important. 1
It is quite possible that, as in other fungi, the parti-
cular physiologic form or forms present in abundance in any
given year and locality are important factors in determining
the losses incurred. (4 7, 8, 9, 10o, 15, 18, 19)

Methods of spread: Local spread is largely by air currents, in
manure, in straw, in surface water, and on farm implements.
Both local and long distance spread is accomplished by use





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of seed wheat with adhering spores. Infected straw might
carry the disease long distances if baled and shipped for
use as bedding for animals and afterward sold for fertilizer
or if carried down stream in flood waters. (2, L, 7, 12, 15,
1i)

Losses incurred: Losses are extremely variable on the same host
varieties, in the same localities during different seasons.

Comparative losses: It has been estimated that losses from flag
smut average 3 to L% of the crop in Australia; in China, up
to 90% or more of the plants may be infected; in lower
Egypt, losses are appreciable but the disease is less pre-
valent in upper Egypt; in India the losses are slight; in
Italy losses were usually slight but in 1931-2 reached
approximately 20% in some places; in South Africa yields were
reduced as much as 20% in some fields; in Transcaucasia
losses were slight when the disease was first noted but
increased rapidly in succeeding years; in the United States
losses are negligible in general owing to the growing of
resistant varieties but losses in portions of fields have
been as high as 50%. (2, 6, 7, 13, 14, 16, 18, 19)

Control methods: Quarantines are used to prevent spread to new
areas. Where the disease is established the most effective
control is the use of resistant varieties. Seed disinfection,
destruction of infected straw and manure, crop rotation and
late fall planting are recommended as supplementary control
measures. While some of the spores may remain viable in the
soil for several years, most of them lose their vitality
within a year or two apparently so a relatively short rota-
tion should prevent serious losses. (2, 5, 7, 12, 17, 18)

Quarantine action: The quarantine on account of flag smut and take-
all diseases, No. 39, was promulgated July 2, 1919, effective
Aug. 15, 1919. Australia, India, and Japan were the only
countries in which flag smut was known to occur. The flag
smut disease quarantine, No. 59, was promulgated Dec. 51,
1925, effective Feb. 1, 1926. This quarantine superseded
quarantine No. 59, omits the take-all diseases, and extends
the list of countries covered to include India, Japan, China,
Australia, Union of South Africa, Italy and Spain. Wheat
(Triticum spp.) and wheat products, except such as have been
so milled or so processed as to have destroyed all flag smut
spores, are covered.
On July 15, 1919, a public hearing was held to consider
the desirability of promulgating a domestic quarantine to
prevent spread of flag smut from the known area of infection
in Illinois. Imposition of a state quarantine by Illinois






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made it appear unnecessary to establish a federal domestic
quarantine on account of flag smut. Hope was expressed
that the disease would be eradicated.
While flag smut has not been eradicated in this
country, it is extremely difficult to find specimens of it
in the field. New and better varieties of wheE't which are
resistant to flag smut are being grown in the states where
the disease gained a foothold.
Important wheat varieties grown on the Pacific Coast
are not resistant, and the wheat growing areas there are
said to have conditions approximating those of regions in
Australia and elsewhere in which flag smut damage is
greatest. Until satisfactory resistant varieties for this
Pacific Coast area have been provided, it is deemed
necessary to protect it by maintaining quarantine No. 59.
It is expected that such resistant varieties will be
available soon. At that time consideration may be given
to the question of recinding the flag smut quarantine.
Before recinding the quarantine, however, it will be
desirable to determine the status of physiologic strains
of the smut. Since known strains vary markedly in virulence
and some of them may be destructive to the wheats found to
be resistant to the form or forms introduced into this
country, the need for the quarantine may be as great as it
ever was. (17, 19)

Description: The symptoms of flag smut infection are characteristic
but their time of appearance and distribution on infected
plants is variable. Symptoms sometimes show as early as on
the fourth leaf to form; at other times no symptoms show
until the plant is nearly grown and then on the upper parts
of the plant only. The earliest symptoms are more or less
elongated stripes on the young leaves or flags. These
stripes are white at their base, becoming whitish gray
further up and lead-gray toward the tip of the leaf. As the
spore masses mature the stripes become grayish-black, or
in some cases dark brown. Finally the stripes become very
dark and rupture exposing the masses of black sooty spores.
Until the spores reach full size infected plants may appear
to be as vigorous as healthy ones, but later the leaves and
upper part of the culms are likely to become twisted and
curled. Infected plants may have only part of the culms
infected. If heads are produced on infected culms, they
may be small and obviously sterile or they may appear normal
but contain few or no grains. Where heads are produced
there may be no striping except on the glumes at the base
of the head and on the culm just below the head.
It seems possible that the number of culms showing the
disease in a single stool depends on the number and vigor
of the hyphae present in the plant, these in turn being






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dependent on the number of spores producing infections in
that plant, and on the susceptibility of the plant.
Noble reports the production of spores on the fifth
leaf of a plant only 29 days after inoculation, but the
incubation period is usually longer, approximately six
weeks in Noble's experiments.
When the stripes on the leaves rupture exposing the
masses of spores, these are readily scattered by any move-
ment of the plant, fall to the ground, are blown about by
the wind or adhere to anything that comes in contact with
them. During threshing the grain may become contaminated
with numerous adhering spores. Spores may occur singly or
two to several in a spore ball and are surrounded by a
single layer of sterile pale brown cells.
Spore sizes as given by Butler are 9-16 mu in diameter;
by Noble, 12-16 x 9-12 mu; and by Verwoerd, 12.6-18 x 8-9.6
mu. The size of the sterile peripheral cells is given by
Noble as 7-10 x 5-9 mu.
The spores seem to need a rest before germinating, but
the length of the rest period depends on the conditions
under which it occurs and the conditions existing or pro-
vided when germinability is tested. Spores may germinate
within a few weeks after they reach maturity or they may
remain viable in the soil for years before germinating and
causing infection. If conditions are otherwise favorable,
the presence of seedlings of wheat, or of any one of several
other plants that are not hosts of the fungus, may stimulate
the germination of many of the spores present. Thus the
growing of non-susceptible plants may greatly reduce the
number of viable spores present in the soil. It was found
that resistant wheat varieties were infected as readily as
susceptible varieties but the fungus was unable to make
rapid growth after it gained entrance. The fungus is able
to produce spores on some resistant varieties. In order to
eradicate the fungus, varieties on which spores are not
produced should be grown.
Infection normally takes place in the coleoptile only.
The coleoptile may be infected at any time after it breaks
through the seed coat until it is broken to permit the first
true leaf to emerge. While the time required for the
coleoptile to make this development varies considerably, it
is normally only a few days. It appears that the smut
spores do not germinate readily unless previous temperature
and moisture conditions have been such as to prepare them
for germination. Hence the coleoptile may emerge and break
under conditions that appear to be favorable for infection
but without much infection taking place because the spores
were not in condition to germinate at that particular time.
Age of spores as well as temperatures, moistures, and length
of duration of the temperature and moisture conditions, and






-5-


their variations, and perhaps soil acidity, and other
conditions are involved in determining whether or not
spores will germinate at a given time under the conditions
then prevailing. When all factors are favorable for the
fungus, heavy losses result.
The role of physiologic races in the flag smut problem
has received little or no attention from most investigators.
Verwoerd states that he found no physiologic strains not-
withstanding the fact that in his experiments the South
African form appeared to be more virulent than the American
form. Yu, Hwang, and Tsiang demonstrated 5 physiologic
strains in Chinese material tested over a four-year period.
A study of this phase of the problem is important to wheat
breeders and to quarantine officials particularly. (1, 2,
4# 6. 7 8# 910, 11, 12. 15, 18, 19)





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Bibliography:

(1) Angell, H. R., Hely, F. W., and Allan, F. E.
1937 The effect of Uroeystis tritici Koern. on the extent
of development of the roots and aerial parts of the wheat
plant. I, II., J. Coun. Sci. & Indus. Res. 10: 136-112,
May 1937 and 11: 256-257, Aug. 1938.
The reduction in the root system of infected plants
is influenced by environmental conditions, being more
pronounced in winter grown plants than in spring grown
plants.
The root system of Federation, a susceptible variety,
was less affected than that of Ford, moderately resistant,
and Nabawa, resistant. In Federation the weight of tops
was reduced, in Nabawa it was increased, while in Ford
there was a reduction in one of two experiments.

(2) Butler, E. J.
1918 Fungi and disease in plants. 547 PP. 1918 (Flag smut
on pp. 171-173.)
Flag smut (Urocystis tritici Koern.) is confined to
the Punjab in India and does relatively little damage there.
The spores are formed in tiny balls consisting of 1 to 4
fertile bright broih spores, surrounding which is a layer
of light-colored sterile cells. Spores spherical or oval,
9 to 16 mu in diameter, spore balls up to 40 mu in diameter.
Sterile cells are smaller, more elliptical.
Old smutted straw will cause infection at sowing time.
Manure from horses fed infected straw carries infection
also. Flag smut persists in the soil.
Crop rotation important.

(3) Faris, J. A., Tapke, V. F., and Rodenhiser, H. A.
1933 Wheat smuts and their control. U. S. Dept. Agr.
Farmers' Bul. No. 1711.
Quarantine and sanitation are recommended to prevent
introduction of flag smut into new areas. Treatment of seed
as for stinking smuts or bunts will kill adhering spores but
will not affect spores in- the soil. The use of resistant
varieties is recommended in infected areas.

(4) Griffiths, Marion A.
1924 Experiments with flag smut of wheat and the causal
fungus, Urocystis tritici Kcke. Journ. Agr. Res. 27:
425-449. Feb. 16, 1924.
Infestation of soil rather than of seed was the most
important factor in the annual occurrence of the disease in
Australia. Flag smut reported to occur in the United States,
Australia, Japan, China, India, South Africa, Italy, and
Spain.
Symptoms are detailed.






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Urosti tritici is not known to infect plants of
any genus other than Tritioum.
Spores solitary or two to several, invested by a layer
of small sterile cells. Each spore of a spore ball may
germinate but usually only one or two do so. On germina-
tion the spore sends out a promycelium with a whorl of one
to several sporidia at the apex. The sporidia do not
separate from the promycelium but elongate to form "infection
threads." Sizes vary with conditions of germination. Spores
collected in Australia in 1919 gave 100% infection when used
in 1921-22. Spores buried two and six inches in soil out of
doors remained viable until early spring. Studies are
detailed on relation of date of sowing, temperature, and
stage of growth to infection. The highest percentage of
infection occurred at 21.5 to 23.5 C. The most favorable
stage of growth for infection was in the seedling stage
before the coleoptile was broken and before the seedling
emerged from the soil.

(5) Humphrey, H. B., and Johnson, A. G.
1919 Take-all and flag smut, two wheat diseases new to the
United States. U. S. Dept. Agr. Farmers' Bul. 1063. Aug.
1919.
Flag smut was found May 5, 1919, in Madison Co.,
Illinois, when pathologists assembled to get acquainted with
the take-all disease. Of 121 fields covered in the first
preliminary survey 23 had flag smut, some spots showing as
high as 2% infection. Within a few days the disease had
been found in three Indiana counties, La Porte, Porter, and
Tippecanoe. Later it was found in Sangamon and Mason
counties, Illinois, also. --Source of disease unknown.
Symptoms and control discussed.

(6) Jarrett, Phyllis H.
1932 Investigations of flag smut of wheat. Journ. Australia
Council Sci. & Indus. Res. 3: 165-169. 1932.
Flag smut has never assumed epidemic proportions but
takes a steady annual toll.
Use of resistant varieties is the best control method.
Entrance of the fungus into the host only occurs through
the young entire coleoptile, i.e., before this white sheath
has been broken by the first true leaf. Germ tubes enter
resistant and susceptible varieties alike. Unable to keep
up with host growth in resistant varieties, but may show in
small smutted shoots from the lower nodes of the main tiller.
In susceptible varieties the growth rate of hyphae corres-
ponds to that of the meristematic tissues of the host but
no outward symptoms show until the fourth or fifth leaf
stage in artificially grown plants and much later usually in





-8-


the field.
Different varieties react differently to flag smut
under similar conditions and individual plants of a variety
react differently. Plants of Aussie, Canberra or Federa-
tion may die at the fourth to the sixth leaf stage or may
live to produce one or two heads with few or no grains.
Plants of Bunyip, Geeralying or Nabawa may show infection
in one or two tillers only or appear quite healthy, micro-
scopic examination being necessary to show the presence
of the hyphae in the base of the plant. Effect on yeild
is the best way to measure effects of flag smut. Experi-
ments along this line were carried out but results are not
given.
A method for determining the degree of resistance of
varieties for comparative purposes is presented and
resistance of varieties tested is tabulated.

(7) MoAlpine, D.
1910 The smuts of Australia. 285 pp. 1910 Urocystis
tritici Koern. pp. 88-102.
U. tritici and U. occult compared, latter does not
cause distortion or twisting of the flag or blade of the
rye leaf and the streaks are principally noticeable on the
stems instead of on the leaves. Plants with flag smut of
wheat do not usually form heads while plants with stem smut
of rye usually form skeleton ears which are likely to droop.
A report of "Black Rust" in South Australia in 1868 is
believed to be the earliest record of U. tritici in
Australia, although it was an old established disease at
that time apparently.
Early and self sown crops are said to suffer most.
Dry sowing tends to increase the disease.
Dry spores germinated in water in 24 hrs. Pronycelium
1-6 usually 5 or 6-celled, up to 100 mu long by 3-5 mu,
with 2-6 usually 3-4 conidia at the apex.
Mycelium was obtained in young leaf tissue 10 days
after wheat was sown. Infection was obtained by using straw,
chaff, horse manure.
Infection does not occur after the plant is above
ground.

(8) Miller, W. B. and Millikan, C. R.
1934 Investigations on flag smut of wheat caused by Urocystis
tritici Koern. Journ. Dept. Agric. Victoria, 32t 365-380,
July 1934. And under same title and authors, II 418-432.
Aug. 1934.
In susceptible varieties there are usually more plants
totally infected than partially infected while the reverse
is true of resistant varieties.
Germination of spores and infection of host are favored
by soil temperatures above 50 F. and only moderate moisture,






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or by conditions prolonging the period of susceptibility
of the host.
Severe infection occurred in soils ranging in PH from
5.5 to 8.7.
The ratio of reduction in yield to percentage of smut
was greatest in resistant varieties, apparently due to the
fact that infections present on resistant varieties did
not always show.
Deep sowing tended to increase infection.
p. 366 "Unlike rust, this disease seldom assumes
epidemic proportions, but takes a steady annual toll of the
wheat crop."
P. 373 "The amount of inoculum applied determined to
a large degree the percentage of smut which developed."
P. 377 "It is found that in general the number of
"partial" infections greatly exceeds "whole" infections in
resistant varieties, but the reverse is not always the case
in susceptible varieties.....0t
p. 378 Counts made ..."indicate that there is a
marked tendency for the symptoms to appear much later in
resistant varieties, in contrast with those more susceptible
to the disease."
p. 380 "If the soil is very wet during the suscept-
ible period, infection is low, even though the temperature
is favorable.
The minimum temperature for germination depends upon
the moisture present."

(9) Noble, R. J.
1923 Studies on Urocystis tritici Koern., the organism
causing flag smut of wheat. Fhytopath. 13: 127-139. March,
1923.
A study of the conditions under which U. tritici spores
germinate, in the laboratory primarily, p. 137. "Soil
moisture, soil temperature, soil aeration, and the presence
of a stimulatory substance at any given time may not be
necessarily conducive to infection. It would appear that
these factors must operate in the proper order and must be
correlated chronologically. Sometimes the factors may
operate together in nature in such a manner as to cause the
development of flag smut epidemics."
Tissues from any part of seedlings of cereals and other
grasses and uninjured wheat seedlings stimulated spore
germination. The minimum temperature for spore germination
was about 5 C., maximum about 32 C., optimum apparently
between 18 to 240 C.
The optimum temperature for germination depends on
previous moisture conditions of the spores. The promycelium
produced by a germinating spore usually becomes 20 to 50 x





- 10 -


5 mu before forming 1 to 6, usually 2 to 4 sporidia.
Sporidia are more or less cylindrical and when fully formed
are about 30 x 5 mu. Sporidia sometimes fuse.

(10) Noble, R. J.
1924 Studies on the parasitism of Urocystis tritici Koern.,
the organism causing flag smut of wheat. Journ. Agr. Res.
27: 451-t489. Feb. 16, 1924.
Occurrence reported in Australia (1868), Japan (1895),
India (1906), Southern Europe, Italy (1922), Transvaal
(1920), China and United States, Symptoms given. Pathogen
called Urocystis occulta (Wallr.) Rab. by Wolff in 1873 but
Koernicke described it as a separate species in 1877.
Spores may occur singly or in spore balls of two or
three spores or sometimes more. Spore balls are dark brown,
usually globose but variable, usually 18-52 x 18-45 mu.
Individual spores spherical or oval, 12-16 x 9-12 mu. The
spores whether single or in spore balls are usually
completely invested with a layer of sterile peripheral cells,
pale brown, globose or ellipsoid, 7-10 x 5-9 mu. On
germination a promycelium and sporidia are produced.
Spore germination was studied, gemination being
obtained by a period of soaking following a rest period or
maturation period, and stimulating by use of seedlings or
wheat seedling sap. Certain chemicals stimulated germina-
tion also. Relative humidity of 50-75% was found most
favorable for retention of viability, spores kept at such
humidities and temperatures of 5 to 26.5 C. germinating
readily in distilled water.
Infection did not depend on amount of inoculum but on
suitable conditions for infection, moisture, soil tempera-
ture, condition of host. Optimum soil temperatures for
infection ranged from l4 to 210 C.
The mycelium is intercellular.
Disease lesions may first appear at any stage in the
growth of the host up to heading, the earliest noted being
on the fifth leaf 29 days after inoculation, although the
fourth leaf is sometimes first affected.
Losses from the disease are about 3% in Australia
although losses in individual fields may be 70%.
Bibliography of 58 titles.

(11) Noble, R. J.
19534 Note on the longevity of spores of the fungus Urocystis
tritici Koern. Journ. & Proc. Roy. Soc. New South Wales 67:
4o3-410. 1934.
Spores collected in New South Wales in 1923 and kept
relatively dry in the laboratory still germinated vigorously
in 1953. None of the spores kept at relative humidities of
72.5 and 89% were found to germinate at any time during the






- 11 -


ten-year period (Perhaps owing to contamination, mainly
Penicillium.)
All lots kept at relative humidities of 33.5 or less
gave more than 50% germination at the end of ten years.

(12) Reed, G. M. and Dungan, Geo. H.
1920 Flag smut and take-all. Ill. Agr. Exp. Sta. Circ.
No. 2142. July, 1920.
Report appearance of the diseases in Illinois. Discuss
symptoms, distribution methods, control and susceptibility
of wheat varieties.

(13) Rivera, V. and Corneli, E.
1933 Progressivo estendersi di epidemic da 'Urocystis' su
Frumento, Riv. Pat. Veg. 253: 171-176, 19533.
(Abstract in R.A.M. 12: 681-2, 1933.)
Flag smut has been present in Italy for years but since
1928 has been getting increasingly severe, perhaps as much
as a 20% loss in individual fields. The disease was equally
severe on wheat sown early in dry weather and sown late
after very wet weather. Spores left in the soil make the
disease worse on successive crops.

(14) Szembel, S. I.
1934 Threat to wheat in Transcaucasia. Crop Protection
1934, Moscow, pp. 22-23. Translated title and citation in
Rev. Appl. Mycol. 14: 23. Jan. 1935.
Urocystis tritici was recorded in 1930 as being present
in insignificant amount in the southwestern littoral of the
Caspian Sea. It spread rapidly until in 1954 up to 20% of
the wheat fields in the Republic were infected, affected
plants remaining unproductive. Steps are being taken to
determine the extent of infection with a view to putting on
a strict quarantine.

(15) Tisdale, W. H., Dungan, G. H., and Leighty, C. E.
1923 Flag smut of wheat, with special reference to varietal
resistance. Ill. Agr. Exp. Sta. Bul. No. 242. April, 1923.
The infected area in Illinois is about 50 miles long
and 5 to 15 miles wide, an adjacent infected area in
Missouri including four fields only. Disease said to be
spreading. Losses not severe as yet but parts of fields
showed infections as high as 30%, indicating that the
disease may become serious, as it has elsewhere.
A chief source of infection is spore balls adhering to
the seed and contaminating threshers, wagons, bins, etc.
Another is spores in the soil from straw or manure or
carried by wind, streams, animals, or vehicles which have
passed through infected areas.
Longevity of the spores is unknown, but some over-
winter.





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Wheat should not be planted in the fall following
infected wheat. Spring wheat and late fall sown wheat are
not so likely to be infected as that sown in early fall.
Susceptibility of some varieties is indicated.

(16) Tisdale, W. H., Dungan, G. H., and Leighty, C. E.
1923 Flag smut of wheat. U. S. Dept. Agr. Dept. Circ. 273,
6pp. June, 1923
Flag smut found in 5 more contiguous counties in
Illinois since 1919, in 5 adjacent counties in Missouri
near St. Louis, and in several counties in northwestern
Missouri and northeastern Kansas near Kansas City. Losses
small, usually not over 1% of the plants infected except in
small spots in fields. Symptoms, control, resistant
varieties. The principal varieties grown in the flag smut
area are susceptible.

(17) Tisdale, W. H., Leighty, C*. E., and Koehler, Benjamin
1927 Further studies on flag smut. U. S. Dept. Agr. Dept.
Circ. 424, 11 pp., July, 1924.
Disease first discovered in this country by S. M.
Zeller on May 11, 1918, in St. Louis Co., Mo.
Apparently not getting any worse in infected area.
Perhaps the greatest danger from flag smut in the United
States in the future lies in its possible spread to the
Pacific Coast States, where the climatic conditions are
similar to those in Australia, and where wheats susceptible
to flag smut are grown.
Seed treatments and varietal resistance are reported.

(18) Verwoerd, Len.
1929 The biology, parasitism, and control of Urocystis
tritici Koern., the causal organism of flag smut of wheat
(Triticum spp.) and recording the occurrence of Urocystis
occult (Wallr.) Rab., in South Africa as the cause of 'stem
smut' in rye. (Translation.) So. African Dept. Agr,, Sci.
Bul. 76, 52 pp. 1929.
Flag smut occurs in the Western Province and Transvaal,
up to 24.5% of the plants being infected by actual counts.
Yields from diseased plants varied, the reduction being 90%
or more in some varieties. The optimum temperature for
spore germination is 22 to 240 C., the optimum temperature
for infection is 22 C. Presoaking for three days and
dessicating with sulphuric acid hastens germination. Spores
remain viable 5 years, in soil they remain infective at
least 4 years. The disease is more prevalent in soils with
a high moisture content. No physiological strains were
found but the South African form seems more virulent than
the American form, producing 66.7 and 45% infection on
Michikoff and Illini Chief varieties as compared to 32.6






- 13 -


and 21%.
Spores are 8-9.6x12.6-18 mu; spore balls 27-45 x 253.4-
56 mu; promycelium 253.4-28.9 x 4.5 mu, sometimes 45 to 65
mu long; sporidia averaging about 25.2 x 4.5 mu when mature.
Thirty-seven percent of the spores tested after 5
years germinated.
Results show that the susceptible period is between
the time at which the coleoptile penetrates the epidermis
up to the time the coleoptile breaks, after which period
no further infection occurs.
A high percentage of moisture interferes with spore
germination and hence with infection. The disease is worse
in fields sown before the first rains.

(19) Yu, T. F., Hwang, L., and Tsiang, C. T.
1936 Varietal resistance and susceptibility of wheats to
flag smut (Urocystis tritici Koern.) III. Physiologic
specialization in Urocystis tritici Koern. Bull. Chin. Bot.
Soc. 2: 111-113. 1936.
Urocystis tritici attacked at least 90% of the crops
in Siao Hsien, Kiangsu and caused from a trace up to 91%
infection in 37 localities in 8 provinces.
Four years inoculation tests with collections from
different parts of the country on ten wheat strains enable
the authors to differentiate 5 physiologic forms of U.
tritici. Nanking wheat No. 716 was resistant to forms 1,
2, and susceptible to 4 and 5. No. 799 was resistant to
1 and 2 but susceptible to 3. No. 795 was resistant to 1
but susceptible to 2 and 3. No. 795 was resistant to 4 but
susceptible to 5. No. 796 was resistant to all five forms,
and No. 800 and H. 1102 were susceptible to all five forms.
The percentage of smut obtained on the different
wheats (which were uniform selections specially grown)
varied somewhat from year to year and with the wheat strain
used.
Detailed infection percentages are given for 10 wheats
for each of 4 years using smut strains from 7 localities.
These are said to be only a part of the numerous tests on
which the conclusions are based.








Urocystis tritlol


Plate 1


'I
4
I.t


*
< Iflrol

*L'^ ^
1^ ^
BE: -L i .T


r~ ~E
.4


A*--Upper leaves of a
wheat oulm infected with
Urooystis tritioi, show.
ing the long stripes, or
sori.
B.--An enlarged portion
of a wheat leaf infected
with Uroystis tritioi,
show ; thne -;i;t Sorn.
Ce--Flag smut lesions on
the lower glumes of an
infected wheat head (a),
and on the leaf sheath
(b).
Do-Lesions of Urooystis
tritioi on the lower
glie (a), the raohis
(b), and the upper part
of the oulm.(o).


(From Jour. Agr. Res.
27: 425-450. Feb. 1924.
Plate 1)








Urocystis tritioi


Plate 2

Germination of spores of
-Urooystis tritici, photo-
.^^R ~graphed usin 4nnn
objective and No. 10
Qocular by Miss Ruth Colvin.
A.--Spore balls.
B.--Showing promycelium.
A C.-Sporidia forming at
1_ apex of promyoelium.
1D, E, & G.-Germinating
Spores with sporidia.
S. F.--Sporidia elongating,
INV %" ~ forming so-oalled in-
feotion threads (a)*
Two sporidia (b) did
not elongate.
r' H.--Spore ball with four
S, spores in different
stages of germination.
I.--hree sporidia, each
^ ^with a secondary
sporidium.


\(From Jour. Agr. Res. 27:
h25-450,. Feb. 1924.
F G Plate 2.)


H.6



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