CHECK FOR UNBOUND
UNIVERSITY OF ILLINOIS
Agricultural Experiment Station
BULLETIN No. 265
TULIP BLOSSOM BLIGHT
BY FRANK LINCOLN STEVENS AND O. A. PLUNKETT
A BLIGHTED TULIP SHOWING WITHERING OF STALK
AND WOOLLY GROWTH OF MYCELIUM
ON DISEASED PART
URBANA, ILLINOIS, APRIL, 1925
FIG. 1. HEALTHY TULIP (CENTER) AND Two BLIGHTED TULIPS SHOWING
BLOSSOM INFECTION AND INFECTION AT LEAF AXIL
TULIP BLOSSOM BLIGHT
By FRANK LINCOLN STEVENS and 0. A. PLUNKETT, in Cooperation with the
DEPARTMENT OF HORTICULTURE 1
Tulip blossom blight is a disease which manifests itself by a
blighting and wilting of the flower stalk just below the blossoms, fol-
lowed by a dropping over and collapse of the floral parts (cover illus-
tration). In extremely damp weather and where there is an abundance
of shade, the petals decay rapidly, becoming covered with a cottony
mycelial growth, while the flower stalk undergoes a watery .decay.
The disease is caused by a fungus heretofore unreported in connection
with this host.
The disease was first brought to the attention of the plant pathol-
ogy laboratory at the University of Illinois in May, 1924, by gardeners
in charge of the University grounds, who had found it present and
destructive in a number of tulip beds. A survey of the tulip beds on
the campus and in the surrounding territory has revealed the disease
in only six beds on the campus and as prevalent only in tulips of the
double varieties. In one bed, infection to the extent of 32 percent of
the plants was found; in another bed, 26 percent. The causal fungus has
been isolated from rotting stems and leaves of single varieties, but the
typical blossom blight has not been observed on these varieties in
GENERAL SYMPTOMS OF THE DISEASE
The first outward manifestation of this blossom blight of tulips is
a shriveling of the flower stalk just below the flower, followed by a
falling over of the blossom (cover illustration). The blossom then
withers and dries up, or, if conditions are favorable for the disease,
the petals become covered with the mycelium of the fungus and a
rapid soft decay follows. The lesion at the base of the blossom is found
to be shrunken and withered and of a yellowish-white color in contrast
to the green of the normal flower stalk. There is also a tendency for the
stalk or blossoms to twist at the lesions.
A second and less common method of attack of the fungus occurs
lower down on the stalk, near the base of the leaf (Fig. 1). The
symptoms here are similar to those described above. When infection
*Frank Lincoln Stevens, Professor of Plant Pathology, University of Illinois; O. A.
Plunkett, Research Assistant.
BULLETIN No. 265
takes place in the leaf axil, the leaves sometimes rot off at the base.
If the tulips are growing in a very moist situation, and conditions are
favorable for the disease, it may progress downward and rot off the
entire stem. In most cases, however, the disease occurs as a lesion
just below the blossom or at the base of the upper leaves.
FIG. 2. TULIP BLIGHTED BY INOCULATING IN BLOSSOM
Study of a number of plants in various stages of infection showed
that the fungus usually gains entrance thru the blossom, probably at
the point of attachment of the petals, and advances into the stalk,
causing decay and withering. In cases where infection occurs lower on
the stalk, the fungus apparently penetrates thru the epidermis in the
7925] TULIP BLOSSOM BLIGHT 301
INFLUENCE OF ENVIRONMENT ON THE DISEASE
From the observations made by the authors it appears that the
prevalence of this disease is almost entirely dependent upon environ-
mental conditions. The beds in which the disease occurred in abund-
ance were all in damp, shaded situations. Beds in open and well-lighted
and ventilated areas showed no evidence, or only slight evidence, of the
disease. The mode of infection bears out the assumption that moisture
is a necessary factor for prevalence of the disease. The double-blossom
varieties hold moisture for forty-eight hours or more under usual con-
ditions, while the single varieties dry out quickly. This, then, appears
to be the reason that the double varieties are susceptible, while the
single varieties, even in the same bed, are not. The unusual amount of
rain during May, 1924, gave ideal conditions for the development of the
THE CAUSAL ORGANISM
A practically pure culture of fungus was noted in the material first
examined and in all newly collected material, and was found constantly
associated with the typical disease. A large number of isolations con-
sistently gave this fungus, either in pure culture or in great preponder-
ance over any other growth. The isolations were readily made by
planting bits of diseased tissue directly on poured corn-meal agar plates.
PROOF OF PATHOGENICITY
Inoculation No. 1. Recently opened, double tulip blossoms, grow-
ing outside under normal conditions, were inoculated by placing at the
base of the petals and also on top of the pistil pieces of corn-meal agar
containing the fungus. At the end of six days the normal type of blos-
som blight resulted (Fig. 2).
Inoculation No. 2. Tulips of a single variety were inoculated in
a similar manner without positive results.
Inoculation No. 3. Double and single tulips were inoculated
under bell jars in a saturated atmosphere. All of these showed typical
blossom blight at the end of three days.
Inoculation No. 4. Blossoms of Tradescantia spp. were inoculated
in an inoculation case, in a saturated atmosphere. None of the blossoms
of this host became infected.
Inoculation No. 5. Green tomato fruits were inoculated with the
fungus by puncturing the epidermis. Ten days later there was a small
rotten spot around the point of inoculation, but the remainder of the
tissue remained firm.
Inoculation No. 6. Growing stems of beans and growing radishes
were inoculated in a moist chamber. Seven days later no signs of infec-
tion were visible.
BULLETIN No. 265
Inoculation No. 7. Iris blossoms were inoculated by placing
among the petals chunks of corn-meal agar containing the fungus. The
blossoms were placed in an inoculating chamber and kept in a saturated
atmosphere for forty-eight hours. Five days later the blossoms had
fallen over and the peduncles showed lesions similar to those on tulips.
FIG. 3. BLIGHTED AND HEALTHY IRIS BLOSSOMS; BLIGHT
CAUSED BY INOCULATION WITH A PURE CULTURE
The petals were soft and rotten. Uninoculated checks remained upright
and healthy (Fig. 3).
Inoculation No. 8. Growing gladioli bulbs were inoculated by
puncturing the epidermis and by placing agar bearing the fungus on
uninjured surfaces and on roots. The plants were kept under bell jars
for forty-eight hours. There were no signs of infection either within the
bulbs or on the roots.
TULIP BLOSSOM BLIGHT
Inoculation No. 9. Seeds of cotton, flax, sugar beets, and onions
were planted in six-inch pots. Corn-meal agar containing the fungus
was mixed well with soil, and a covering one inch in depth was placed
over the seeds. The soil was kept well moistened under bell jars.
Damping-off occurred in pots containing sugar beets and flax. Check
pots remained healthy.
In all cases where positive results were obtained from the inocula-
tions, reisolations have been made and in all cases the fungus isolated
proved to be identical with the fungus used in the inoculum.
DESCRIPTION OF THE FUNGUS
The mycelium is of the true Phycomycete type, being a long tube,
irregularly branched, with few or no cross walls. Within the tulip
tissue and in artificial media, the mycelium is, for the most part, thick
and profusely branched, measur-
ing from 2.5 to 11 /x in diameter.
The hyphae are granular and
slightly yellowish in color. In cul-
ture and on the normal host, the
aerial mycelium is much finer and
less branched than that within the
host tissue or within artificial
media. In culture, the mycelium
sometimes becomes prominently
warty, owing to the development
of numerous small protuberances
Conidia are abundant in- cul-
tures on oat-meal agar and are
usually borne above the sub-
stratum, apically, on thin hyphae
which branch off from the larger
mycelium with the agar. The
conidia are slightly variable in
size and shape, but are compara-
tively large and short-ovate in
form. They measure from 22 to
32 by 21 to 29 /,. Distinct papil-
lae are always present on these conidia and are mostly apical (Fig. 5),
tho occasionally a lateral one may be observed. They are about 3.6 /x in
height by 5.4 /x wide. In rare instances two papillae occurred on one
FIG. 4. MYCELIUM FROM WITHIN THE
MEDIUM, SHOWING COARSE AND
BULLETIN No. 265
conidium. No intercalary conidia have been ob-
served, and the conidia are never borne on the
heavy mycelium but on thin branches. When
placed in sterile, distilled water these conidia
germinate by an ordinary germ tube which gives
rise to the normal mycelium (Fig. 6). Repeated
attempts to obtain zoospores have failed.
Oospores are abundant in pure culture and
within the host tissues. They are spherical, rang-
ing from 22 to 32 /A in diameter. The oospores are
thick walled (1 to 2 /A), mostly hyaline, tho some-
times distinctly yellowish brown, depending upon
the substratum. They are borne within oogonia, on
short side branches of the thick, heavy mycelium inside the medium or
host tissue. Antheridia are always present and are persistent and pre-
ponderantly paragynous. They may be borne on the same or on differ-
ent mycelium from that of the oogonia. The antheridia are variable in
shape, ranging from long curved tubes to irregular and spherical bodies
Chlamydospores have not been seen either in culture or in the
normal host tissues.
FIG. 5. A SINGLE
FIG. 6. GERMINATION OF THE CONIDIA
TULIP BLOSSOM BLIGHT
Oogonia are borne on short side branches on the large mycelium
within or on the surface of the medium. In nature the oogonia are borne
within the host tissue.
FIG. 7. DRAWINGS SHOWING OOGONIA, OOSPORES, AND ANTHERIDIA
Growth on Corn-Meal Agar. On corn-meal agar there is a trans-
lucent growth of the fungus, giving a granular appearance to agar.
Here and there over the surface, flakes of white appear where aerial
hyphae are sent up bearing conidia. The fungus penetrates up to 4 mm.
in this agar and produces abundant oogonia at this depth. The
mycelium shows more irregularity on this medium than on any other.
Oogonia and conidia are not so abundant as on oat-meal agar. There is
no aerial mycelial growth excepting the very short conidiophores which
are sent up from within the medium. The conidiophores are much
shorter on this medium than on oat-meal agar. Growth is very rapid, a
colony occupying a plate within four to six days after inoculation.
Growth on Tulip-Stern Agar. On tulip-stem agar the growth is
similar to that on corn-meal agar, but not so abundant. There is no
aerial mycelium and the mycelial development is entirely within the
medium. The rate of growth is rapid, similar to that on corn-meal agar.
No conidia are produced* on this medium and the development of
oogonia is not so abundant as on corn-meal and oat-meal agars.
Growth on Oat-Meal Agar. On oat-meal agar the growth of the
fungus, tho slower in spreading over an agar plate, is much thicker and
the mycelium more abundant than on- corn-meal agar. A thick weft of
white, aerial mycelium is present. It is on this thin, white, aerial
mycelium that conidia are produced in abundance. The aerial mycelium
is produced in quantity only when the cultures are several days old and
306 BULLETIN No. 265 [April,
are grown in the light. When grown in darkness, only a slight aerial
mycelium is produced. Small, white, irregular spots that are granular
are visible within the agar. .These spots are masses of oogonia which
are produced in more abundance on this agar than on corn-meal or
Growth on Sterilized Vegetable Plugs. An abundant growth of the
fungus is produced on sterilized potato plugs, the mycelium forming a
weft completely covering the plug. The growth is white and aerial
mycelium is very abundant. Scattered conidia may be found within the
aerial mycelium. Oogonia are produced in abundance on the mat of
mycelium lying on and within the outer surface of the plug. Oogonial
development is not so rapid as on carrot or bean plugs.
Growth of the fungus is more rapid on carrot plugs than on potato,
but no aerial mycelium is produced. The surface of the plugs appears
gray and powdered, as tho sprinkled over with meal. This powdered
appearance is caused by the abundant clumps of oogonia. The mycelium
forms a mat around the plug, penetrating the outer cells for a milli-
meter or so.
On green bean plugs the fungus grows rapidly, but only a small
amount of mycelium is formed. Oogonia are produced earlier on these
plugs than on potato or carrot but are not so abundant. Slight aerial
mycelium is present at first, but as growth continues the bean tissue
becomes watery and rots. Conidia have not been observed on this
TAXONOMIC POSITION OF THE PARASITE
The morphology of the oogonia, oospores, and conidia clearly
places this fungus as a Phytophthora. In this genus the preponderance
of paragynous antheridia, the oospore measurements, and the pre-
ponderance of oospores in the early growth of the cultures show that it
is closely allied to, if not identical with, P. cactorum. The characters
given above make it impossible to regard the fungus as cospecific with
any other known species. In view of its close similarity with P. cactorum
it has been referred to that species.
.7925] TULIP BLOSSOM BLIGHT 307
1. An hitherto unreported tulip disease is described.
2. The causal fungus is a Phytophthora which is referred to the
species P. cactorum.
3. Its etiological relation to the disease was indicated by (a) its
constant presence in the diseased tissue; (b) the absence of any other
constant parasite; (c) proof of its pathogenicity by inoculation;
(d) reisolation of the organism.
4. The disease is more common in humid, ill-ventilated situations.
5. It is more common on double than on single blossoms, appar-
ently because of humidity differences.
6. Iris was susceptible to inoculation.
The only suggestion which can be made as to the prevention of
the fungus is the avoidance of situations that are especially humid.
UNIVERSITY OF ILLINOIS-URBANA