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AWHT DISEASE OF THE
CULTWMED SNAPDRAGDN

WEEKS HER THE DEGREE OF M. S.

‘ o. B. HOWELL
19:32-

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‘1 WILE DISEASE OF THE CULTIVATED SNAPDRAGON

Thesis presented in partial fulfillment
of the requirements for the Degree of
Master of Science

Michigan State College

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of“ BS Howell

1932

KNEE-«:3.

Acknowledgements

The writer is grateful to Dr. Ray Nelson tor guidance in the
conduct or this investigation and to Dr. E. A. Bessey for advice and
suggestions and for criticism and correction or the manuscript.

‘Acknowledgement is also made to Dr. Nelson for photographs

and photographic work.

101735

Table of Contents

I. Introduction................. ...... ...... ........ .........
11. History and Economic Importance. ................ ..........
III. Symptoms and Signs........................................
Iv. Emperimental Work...... ....... ............................
(a) Material and Mbthods................................
(b) Proof of Pathogenicity..............................
(c) Plants Experimentally Infected......................
V. The Causal Fungus............ ....... ......................
(a) Physiology..........................................
(b) Morphology of the Fungus. ......
(c) Biometric Studies...................................
((1) Taxonomy of the Fungus..............................
(e) Comparison with Other Wilt Diseases.. .......... .....
VI. Control............... ........... .........................
(a) Exclusion...........................................
(b) Eradication.........................................
(0) Protection..........................................
(d) Resistance..........................................
VII. Discussion................................................
VIII. Summary................ ......... ..........................
IX. Literature Cited............... ........... . ..... ..........
.X. Legends...................................................

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26

A WILT DISEASE OF THE CULTIVATED SNAPDRAGON-

Introduction

The cultivated snapdragon, Antirrhinum majus, L., is one of
the most popular plants grown by the florist during the spring and
winter*months. ‘Under Michigan conditions this plant responds well
to the low light intensities that prevail from.N0vember until March
and is also one of the most profitable flowers grown for the Easter
trade-

Close planting in the benches and reduced light intensity
during the winter mmnths results in a soft succulent type of growth
which renders the snapdragon susceptible to certain diseases that
are practically restricted to greenhouse-grown plants. Fer some
time’s wilt disease has been observed in various sections of the
state, occurring mostly from.January to March and at times causing
serious losses. These investigations were begun in the winter of
1931 following an unusually destructive outbreak in some of the
larger greenhouses in the central and southern parts of the state.
This paper presents results of the investigations which establish
the fungous nature of the disease and defines the conditions con-

cerned in its occurrence.

History and Economic Importance
Wilt of the cultivated snapdragon is a disease only recently
recognized as a distinct disease of importance. Literature up to
the present time contains no description.of this particular type of

tilt on the snapdragon in the United States, although a wilting of

this plant had been observed in 1924 at the Wisley Laboratory in
England occurring on the flower parts and upper stems but not found
on the basal portions of the stems as in Michigan.

Attention was first directed to this disease in 1929 when it
occurred at Mount Clemens and other parts of the state and caused
severe losses. Later in 1930 and 1931 plants with similar symptoms
were found in Lansing and the surrounding vicinity, although not in
such numbers as to constitute an epiphytatic of the disease. It is
probable, however, that this disease has been present for many years

without attracting attention.

Symptoms and Signs of the Disease

The first symptoms of this disease are usually seen in the
snapdragon bench or bed when the first spikes are cut, or perhaps
at pinching or disbudding time. The diseased plant can be detected
by a pronounced wilting of the leaves with no recovery, such as
occurs in wilting due to lack of water (Plate 1)- The wilted leaves
soon take on a dried appearance with a brown color, the adjacent
lower leaves down the stem.becoming wilted within ten to fifteen
hours after the first observation. The stems next attract attention
by their pallid, light yellow color, assuming gradually a dried out
appearance accompanied by a shriveling of the upper portions of the
affected part. The next noticeable symptom is a dark green water-
soaked ring around the stem in advance of the pallid colored zone,
marking a boundary between the diseased and healthy portions of the
stem. Under humid conditions in the greenhouse, a white cottony

growth may be detected on the yellow dying.tissue, especially if the

-3-

stem is broken over, which is frequently the case, as a result of the
infection. The flower parts may become infected and show a drooping
of the unexpanded buds and a bending over of the ends of the spikes
(Plate II). The flowers at the base of the infected inflorescence
wither, the remains of the corollas hang down, and the peduncles and
the axes from.which they sprang shrivel and assume a striking white
color, giving the aspect of being girdled at acme point lower down
the stan.

Upon examination of the side branches near the point of attach-
ment to the main stem, small light to dark brown cankers may be noticed
on all parts of the branch, and in sane cases, nearly encircling the
stem (Plate III & IV). The progressive desiccation of the affected
tissue is arrested at the soil level where the fungus grows laterally
around the stan completely girdling it.

The most common points of infection are at the base of the
plant where the flower spike has been removed, or where the epidermis
has been injured through cultivation. Following infection through
wounds or the cut ends of the flower stems, a water soaked appearance
of the invaded tissue is the first noticeable symptom.

Following the death of the plant, an examination of longitu-
dinally split infected stems reveals small, hard, black sclerotia,
sometimes as large as a kernel of corn, surrounded by dried white
mycelium and imbedded in the pithy portions. The presence of the
sclerotia in the pith furnishes evidence as to the cause of the wilt-
ing of the plant.

On inoculated plants the first observed symptom is a pro-

nounced wilting of the foliage within five to ten hours followed
rapidly by a light yellow area with a dark green water-soaked zone
separating the diseased from the light green healthy tissue (Plate V,
A 8c B). Within Wenty-four hours a cottony mycelial mass can be
detected on the upper portions of the discolored stem. The point of
inoculation, if high on the stern, is subsequently weakened by the
fungus and a breaking-over of the upper part of the spike generally
occurs. it this point a copious mat of white mycelimn may be found
on the surface of the injured stun. The next symptom to appear is a
marked shriveling of the yellowed diseased portion within twenty-four
to thirty hours after the initial infection. As the fungus travels
down the stan, a wilting of the leaves and lateral branches occurs
until the basal portions of the side branches are reached where small
cankers are formed, sometimes in such proportions as to nearly girdle
the lateral stems. The fungus, after reaching the main stem, usually
forms a large canker at the soil level and sometimes completely
girdles the main stem. When this occurs, death of the whole plant
ensues within twenty-four hours.

Upon sectioning the stuns longitudinally down which the fun-
gus has progressed, hard black elongated sclerotia are sanetimes
found imbedded in the pith. In no cases are they fbund on the ex-
terior of the plant.

In some cases small clumps of mycelium form in the exile of

the leaves, where moisture collects due to condensation or to water-

ing.

-5-

Experimental Work

Material and Methods.

Diseased plants showing stem cankers were collected from the
greenhouse and a fungus isolated from the affected portions. The
infected stems were first washed thoroughly with tap water, soaked
in a l to 1000 aqueous solution of corrosive sublimate five minutes,
and then washed in six changes of sterile water. Small bits of the
diseased tissue were cut out with a flamed scapel and placed in
petri dishes containing potato dextrose agar. Within twenty-four
hours after plating, a fungous growth was evident as a white felt
of'mycelium one-half inch wide surrounding each piece of plated
tissue-

From these dishes single hyphal tips were cut off under
binoculars with a sharp sterile needle and grown in pure culture on
various media such as steamed carrot plugs, corn meal agar, potato
dextrose agar, malt dextrose agar (Leonian's formula), Coon's
synthetic agar, Melilotus stems, Baxter's malt agar, moist blotting
paper, and whole steamed oats.

After four days the surfaces of these media were covered with
a matted pure white growth.of mycelium, which.after the fifth day
became very dense in spots and began to fenn solid sclerotia. These
at first were white to cream colored but when mature were black and
bore large drops of liquid on their surfaces until the moisture con-
tent of the media was exhausted. The sclerotia then became dry and
hard, corresponding in size and appearance to those produced in stuns

of diseased plants collected by Dr. Ray Nelson in 1950. No other

fructifications of any sort, such as micro—conidia, were ever observed
in these or any subsequent pure cultures originating either from asco-
spores or from mycelium.

Snapdragon seeds of comercial varieties were sown in pots
of clean sterilized composted soil, and the seedlings were later
transplanted one inch apart in flats containing soil sterilized by
steams Five hundred and sixty plants of six popular varieties, namely,
Jennie Schneider (light pink), Ceylon Court (yellow), Philadelphia
Pink (pink), Sun Tan (light yellow bronze), Roman Gold (deep bronze),
and White Rock (pure white) were selected. These plants were pinched
back to six "breaks”, or lateral branches, and shifted to sterilized

soil in six inch pots where they were grown to maturity.

Proof of Pathogenicity.

' Sterilized whole oats were inoculated with sub-cultures of
the fungus and after two weeks time this material was used to inoculate
sterilized soil in thirty six-inch pots. In similar pots, five plants
of each variety were grown on sterilized soil under the same condi-
tions. After fifteen days the plants in inoculated soil showed no
symptoms of the disease. Twenty of those plants grown in inoculated
soil, as well as the checks, were wounded at the soil level with a
sterilized knife to simulate the breaking of the epidenmis by culti-
vation in the snapdragon bench. Five plants so treated showed signs
of the disease and wilted within three days after wounding. Within
two weeks eighteen.of the twenty wounded plants had become infected

and showed typical wilt symptoms. The check plants were perfectly

-7-

normal at the end of the experiment and showed no symptoms or signs
of the disease.

Isolations were then made from.the diseased plants and the
fungus again obtained in pure culture. The re-isolated fungus was
identified as that of the original pure culture. mycelium of the re-
isolated fungus was then introduced into a new series of plants which
gave the characteristic symptoms and signs in all cases. Diseased
tissue was again plated and allowed to form.sclerotia to prove Kech's
postulates. Cultures of both the original and the re-isolated fungus
were used for biometric studies discussed later in this paper.

Experimental work of inoculating plants by introducing bits
of mycelium into incisions, some slight and others deep, on various
parts of the plants (Table I) was started on February 15, using 24
plants for each group of experiments. Twelve plants were used for
inoculation and the same number as checks. The inoculated plants
and the checks were grown under like conditions at various tempera-
tures and humidities in separate inoculation chambers.

The inoculated plants consisted of the six varieties to deter-
mine whether color was correlated with immunity to the disease.

Inoculated plants were allowed to remain in the inoculation
chamber from.three to six days in all but three series, which were
allowed to remain until the plants were dead in order to study
sclerotial formation in the stems.

All the plants used at the beginning of the experiment had
just begun to bloom and were in a succulent condition and susceptible

to infection.

-8-

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-9-

Plants that were inoculated at the base were easily infected
until April 13, when the stem.tissue became hardened as a result of
growing conditions. During this period, from the middle of April to
the last of.May, eighteen.of’the inoculated plants failed to become
infected although 100 per cent infection resulted from earlier inocu-
lations with the fungus.

The flower spikes were cut off near the base of the plant
leaving two or three "breaks" near the soil level to produce the next
crop of flowers, and mycelium.was placed directly on the exposed cut
surfaces. Additional plants also were inoculated in this manner as
the entrance of the fungus generally occurs at this point.

The axils of leaves, where the lateral branches arise, were
inoculated with the fungus when these side branches were removed by
disbudding. Only eighteen of the 42 plants inoculated became in-
fected, and these only wilted when the plants were syringed to lower
the temperature and increase humidity in the inoculation chamber.

The fungus usually died before infection occurred in the leaf exile,
and less than one-half the inoculated plants became diseased.

In a few cases, entire tops of plants were cut off and inocu-
lated where the growth was soft and succulent. By using this method
100 per cent of the plants so treated became infected. Twenty-three
plants were wounded slightly within three inches of the tops, and
twelve of these were inoculated in the wounds. Infection here resulted
in five of the inoculated plants. The eleven check plants were then
wounded deeply at the tops and inoculated. Evidence of infection was

observed within five hours after inoculation regardless of the tempera-

-10-

ture and woodiness of the tissues. Breaking over of the tops was
prevalent in these deep wounds, and copious growths of mycelium were
readily observed in every instance.

In all these tests the check plants were treated exactly the
same as the inoculated plants as regards temperature, light, and
humidity. The checks were first wounded at the various places and
the plants for inoculation wounded in.similar locations. The checks
in.all cases were free from infection and showed no signs or symptoms

of the disease.

Plants Experimentally Infected.

Inoculations were made on various other plants and the fungus
found to be infectious on tulip, Regal lily, hyacinth, gladiolus,
Delphinium, stock, Schizanthus, tomato, gourd, chrysanthemum, and
lettuce.

The tulip, hyacinth, lily, and gladiolus plants were inocu-
lated on freshly cut flower stems. The fungus grew down the stems
causing the characteristic light yellow color on the shriveled stems.
The tips of the leaves began to turn yellow and shrivel when the
fungus reached the base of the plant. Examination of’the bulbs and
corms showed a soft odorless rot at the point of attachment of the
leaves.

Lettuce plants were inoculated in the leaf cluster at the
base of the plant. The first symptom of infection was a wilting of
the lower leaves, which was immediately followed by the drooping of
the upper ones until the entire plant was involved, giving the

appearance as if the whole plant had been dipped in boiling water.

-11-

Near the moist portions of the stem end of the dead plant the white
cottony growth of the fungus was quite evident on the under sides of
the leaves. When the plant began to decay, sclerotia which varied in
size from 0.5 x 5 to 0.5 x 9 mm. were formed in the decomposed tissue.

Gourd and tomato plants were very susceptible to the disease,
and healthy plants became infected when they came in contact with the
leaves or stems of diseased plants. The fungus grew more rapidly in
the tomato and gourd than it did in the snapdragon, due to a greater
succulence of these plants when grown at cool temperatures. When
inoculated on bruised leaves, the fungus grew rapidly and caused
death of the blooming plants within a week. Symptoms of the disease
were similar to those on the snapdragon with the exception that the
fungus grew more rapidly and formed sclerotia more freely in the
stems.

When Schizanthus, stock, Delphinium, and chrysanthemum were
inoculated in the tops, the development of the fungus was slower than
any of the previously infected plants, but the leaves and the stems
gave the characteristic symptoms in wilted, drooping vegetative
parts. There was a lighter tint to the diseased stems and fewer
sclerotia were formed in the pith.

0n hyacinths, when the fungus had completely killed the
foliage, the bulbs were dug, rested six weeks, and again planted in
bulb pans. The bulbs were solid at the time of planting but soon
decayed in the ground as the result of the renewed activity at the
time of root fermation of the mycelium which had remained dormant in

the bulb tissue.

-12-

The Causal Fungus.
Physiology.

The fungus grew readily on.all types of media used, but best
growth occurred on potato dextrose agar at 17° to 21° C. The fungus
grew, however, at temperatures ranging from 7° to 30° C., but growth
was slow at both extremes. High humidity gave the best results in
the culture chamber stimulating rapid growth of the fungus. Asco—
spores were readily discharged when mature apothecia were subjected
to a moist atmosphere following comparatively dry conditions in small
flasks.

Growth rate measurements were taken by inoculating the tOps
of snapdragon plants with the fungus and marking the stems of the
plants with india ink at quarter—inch intervals. When the water-
soaked area coincided with the first marked interval on the stem,
the time was recorded and observations were made intermittently as
the fungus traveling down the stem reached the marked graduations.
The fungus was found to progress 1.18 cm. a day on mature flower
spikes, and as rapidly as 3.58 cm. a day on young succulent growth.

The fungus was grown on agar slants upon which were placed
six inch lengths of number 40 white thread previously soaked in
dilute agar. When the mycelium had completely grown into the threads
they were removed with flamed forceps and cut into quarter-inch
pieces with sterile scissors and placed on potato dextrose agar in
petri dishes. These were put in the differential thermastat where

the temperatures ranged from 7° to 33° c. to determine at which

temperatures the best growth took place. This was accomplished by

-13-

measuring the growth of the mycelium extending from the thread in the

petri dish at 9:00 A- M. and 5:00 P. M. daily.

the fungus was noted between the temperatures of 17° and 21° C-

The best progress of

On January 13, 70 sclerotia collected in April, 1931 were

dipped in alcohol, flamed, and placed in 250 cc. sterilized flasks

containing sand previously baked in an electric oven for thirty-six

hours.

moistened with sterilized water.

Six to ten sclerotia were placed in the flasks and the sand

The flasks were stored at various

temperatures and light intensities to record time of genmination and

production of apothecia (Table II).

Table II.

Condition of Formation of Apothecia from Sclerotia.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Location of , No. of TemperaturesLight intensity. Date of , Date of
flasks sclerotia. stipe apothecia
formation formation
1. Electric 8 7.2° c. Dark March 2 March 16
ice box
1EW‘Greenhouse "7""T575°T~msed feb. 16 March 1
sunlight
#5 Greenhouse IO ‘IC°'CT"“T§EEJEEZIEif:”"'Feb. 9 "Feb. 23
1 ’fused sunlight “
i. Greenhouse 6 29.I°fC. Diffiégd’ e . "'“'eb. 8
sunlight
Ffieflc room 6 22.2 C. Total darkness FEB. 24 March 13
F‘Oflflfi on v 4.1" -- Full sunlight Feb. 27 March 23
window sill 71§3 C. ___ __
.‘Inside on 6 22i§6_C. fiiffused Feb. 6 f;b:7f7..
window sill _* sunlight
Flce box " ‘10. 9.4!5 C. Dark Feb. 20 March 2

 

 

 

 

 

 

 

-14-

The sclerotia of lot 1 were placed in an electric ice box
and were subjected to daylight once a day for observation. Lot 2 was
placed in the rose range of the greenhouse with diffused sunlight and
partial shade from 7:00 to 12:00 A, M. throughout the experiment.

Lot 3 was placed in the snapdragon and carnation house under a bench
with shaded conditions and diffused light at all times. Lot 4 was
placed in the cucumber house in full diffused sunlight during the
whole day. Lot 5 was placed in the photographic dark room with no
light except when observations were made with the aid of a dim.e1ec-
tric bulb. Lot 6 was placed on an east window sill in full morning
sunlight outside the Botany Building and subjected to freezing and
thawing at day and night temperature varying from.4.l° to 7.2° C.

It was brought back into the laboratory when the first signs of
germination were observed on February 24. Lot 7 was placed on an
east window sill inside the Botany laboratory and subjected to dif~
fused light conditions and constant room temperature. Lot 8 was
placed in an ice box in semi-darkness with temperatures varying from
9.40 to 12.70 C. until germination occurred, when it was brought out
and kept at room temperature.

The first germination and production of stipes occurred on
February 6 in flask number 7. These sclerotia produced from one to
four stipes which bore apothecia eleven days later. The second lot
to germinate was in the cucumber house at a temperature of 29.4° C.
One day later sclerotia began to germinate at 10° C. in the carnation
and snapdragon house. Apothecial formation occurred eleven days

after first germination signs were observed in the above three cases.

-15-

The remaining lots of sclerotia germinated within 25 days after lot 7.
The sclerotia in the electric ice box kept approximately at 7.20 C.
were the last to germinate but formed apothecia within fourteen days
after stipe formation.

Small apothecia in a few cases in four of the lots were pro-
duced on multi-branched stipes, this abnormality apparently being due
to environmental conditions.

A study of Table II shows that sclerotia will germinate and
produce apothecia in a wide range of temperature and light conditions.
The sclerotia in lots 1, 6, and 8 were subjected to temperature com»
parable to those conditions occurring in the compost pile.

sclerotia in lots 2, 3, 4, 5, and 7 were subjected to tempera-
ture fluctuations from 100 C. to 29.4° C., which correspond to con-
ditions in the greenhouse in midsummer and early fall when such
plants as chrysanthemums and carnations are benched. It is evident
that the develOpment of the fungus through sclerotia germinating in

the greenhouse may be in progress throughout the year.

lbrphology.

The resting stage of the fungus causing snapdragon wilt is
a black tuberiform sclerotium.averaging 0.5 - 5 x 0.5 - 9 mm. The
sclerotia germinate to form.scattered, stipitate, naked, goblet-like
apothecia (Plate VI) which vary in color from pale salmon to cinna-
mon (Ridgway). The apothecial cups are 4 - 8 mm. wide, with a slender
sub-flexuose more or less elongated stipe averaging 3 cm. (4). The
asci are cylindrical, 115 - 144 x 8-4 - 12 microns, with ascospores

arranged in a single row, ellipsoidal and generally guttulate,

-l6-

7.2 - 12 x 5.2 - 9.6 microns (Plate VI, 1). Paraphyses are few and
elevate. The mycelium is pure white and consists of long branching
cells, the hyphal threads becoming more septate and branched within
the host. When a sufficient amount of food has been taken by the
mycelium, the latter begins to form dense masses which ultimately
became the sclerotia. No conidia were observed either in culture or
on the host.

The characteristics of this fungus identify it as a member

of the genus Sclerotinia.

 

Biometric Studies.

Three hundred and twenty-six sclerotia formed in pure cul-
tures were measured and found to range in size from 0.5 - 5 x 0.5 - 9
mm. with an average size of 2.75 x 2.125 mm.

From.fifteen apothecia hand microtome sections were cut and
six hundred asci and ascospores were measured. These measurements
coincide closely with ascus and ascospore measurements of Sclerotinia
sclerotiormm (Table III) by Stevens (6:141-142) and by Dowson (2),
although they are a trifle smaller than those usually accepted for
the species. The following table gives a comparison of ascus and
ascospore measurements by the author and two other workers on this

fungus.

Table III.

Comparative Measurements of Asci and Ascospores.

 

 

 

 

 

 

 

 

asci (microns) V ascospores (microns)
Stevens (6) . 130 - 135 x 8 - 10 9 - 13 x 4 - 6.5_#
Dowson (2) 121 - 130 x 6.6 - 8.8 8.8 - 11 x 3.5 - 5
Author 110 - 135 x 7.9 - 11.5 7.2 - 11 x 5.2 - 9.6

 

In most cases the author's measurements corresponded closely
to those by Stevens and Dowson. Dowson working on the fungus from
Schizanthus recorded spore measurements of 10 - 13 x 5 - 6 microns
in contrast with 8.8 - 11 x 3.5 - 5 on snapdragons. However, the
measurements still fell within the limits as defined by Stevens.

The hyphae varied in width from 10 to 24 microns (Plate VI, 2).
The smallest hyphae observed were those formed on Coon's synthetic
agar and on malt dextrose agar. The largest hyphae were found on
potato dextrose agar, showing a possible correlation of’growth with

the carbohydrate content of the medium.

Taxonomy.

According to the spore measurements and other characters, the
fungus causing the wilt disease of snapdragons in Michigan is
Sclerotinia sclerotiorum (Libert) masses (=Sclerotinia libertiana
Fuckel). madame Libert in.1837 first gave this organism.the name of
Peziza sclerotiorume In 1870 Fuckel transferred this to his genus

Sclerotinia and changed the name to S. libertiana. Later in 1895

 

 

 

"33300. following the recognized rules of nomenclature, used the name

-18-

.gclerotinia scleroticrume The question as to the correct name for

this species is discussed fully by Miss Wakefieldf.

Comparison With Other Wilt Diseases.
Several other diseases which cause wilting affect the snap-
dragon in America, the most important and troublesome being anthrac-

nose (5 and 7:109-110), Phyllosticta antirrhini Stew., which affects

 

the leaves, stems, and shoots. This disease, however, can be dis-
tinguished from.Sc1erotinia wilt by the characteristic spots that
occur on the stems and leaves. The spots When numerous cause the
affected foliage to shriVel, cling to the stem, and die. All parts
above ground are attacked by the parasite, but there is no sudden
wilting of the foliage such as characterized the sclerotinial disease.
Another disease of snapdragon causing a wilt and drooping of
the foliage is Verticillium.wilt (1), caused by the fungus Verticillium

albo-atrum Rein. and Bert. This is a soil inhabiting parasite and

 

causes a typical wilt of the whole plant. It is distinguished from
sclerotinial wilt by a number of characteristics, viz. (1) the wilt
is not sudden; (2) the vascular tissues of the plant are discolored;
(3) no cankers are formed on the stems; (4) no sclerotia are formed;
(5) Verticillium albo-atrum produces conidia; and (6) affected stars
do not have the characteristic color seen in plants attacked by

S. sclerotiorum.

 

”Wakefield, E0 Me Phytopath. £2126-1270 1924.

-19-

Control

Since snapdragon wilt occurs in the greenhouse in late winter
and early spring, the control measures advised by Pethybridge (3) to
combat diseases of other plants caused by the same organism in the
field cannot be used.

The plants are attacked by two fonns of the fungus, (1)
mycelium in the soil, and (2) by ascospores.

Since infection in the greenhouse results from these two
sources, the most effective method is to exclude the parasite from

the soil.

Exclusion.

Soil in benches, pots, ground, and seed beds, as well as pots
and flats, should be thoroughly sterilized by steam. If steam is
not available, formaldehyde or Semesan may be used to free the soil
of mycelium.

A.large grower of snapdragons in Michigan experienced a severe
loss in 1929 due to an epiphytatic of this disease in his greenhouse.
In 1930 the soil in the benches was sterilized, but the disease was
not completely controlled. In 1931 not only the bench soil but the
pots, flats, benches, and seed were sterilized, and as a result not
a single plant was attacked by the disease.

Sclerotia in the soil of the snapdragon'bench are carried out
to the compost pile at the end of the growing season where they freeze
and thaw from one to three years before being brought back into the
house with the soil for use in snapdragon benches or for other suscep-

tible crops. S. sclerotiorum being an omnivorous parasite and sapro-

 

-20-

phyte is in all probability often present in the soil used primarily
for preparing greenhouse compost either in the form.of growing my-
celium or of sclerotia. This makes it imperative to sterilize every
bit of soil before attempting to grow this crep. If the parasite
gains entrance, other measures must be taken to eradicate the disease

before it has progressed very far.

Eradication.

Plants found in the greenhouse infected high up on the stems
can be saved by excision of the diseased branch two or three inches
below the darkened area. .As a diseased plant bears no spores, it is
innocuous to the neighboring plant if its diseased leaves or stems do
not come in contact with parts of the other.

If dead or diseased plants are allowed to remain on the sur-
face of the bench soil or are thrown out on the refuse pile, there
is an excellent Opportunity for sclerotia to form and be freed by
decomposing stems. All diseased stems and plants should be destroyed
by burning immediately after they are collected.

When the disease has been observed and all infected plants
eradicated, steps should be taken to protect the crOp from further

infection.

Protection.

Care in watering, temperature, ventilation, and spacing may
protect the plants from.being attacked by the parasite. Spores and
mycelium on the surface of the soil are easily splashed by water to

freshly cut or injured stems. A layer of sterilized sand or gravel

-21-

spread over the surface of the soil in the bench will prevent distri-
bution of the fungus by water.

Wind blown ascospores may be a source of infection on freshly
cut stems. As a protective measure, cut flower stubs may be daubed
with a Bordeaux mixture paste.

In cloudy weather water should be withheld and the plants
given an abundance of ventilation in order to keep the foliage as
dry as possible at all tunes. Snapdragon wilt is favored by high
humidity, overwatering, poorly drained benches, and insufficient ven-
tilation. Spacing the plants about one fect apart will facilitate
ventilation and assure quick drying. In view of the thermal require-
ments of the plant and the fungus, a night temperature of 100 C.

should be maintained.

Resistance.

Since all varieties and colors of snapdragons used in the
experiment were susceptible, there is no indication that resistant
varieties are now available among the important commercial kinds.
However, in view of the results obtained in control of disease in
other'plants, there is a possibility that if large numbers of
varieties were studied, less susceptible ones might be found, parti-
cularly in view of the fact that older plants with harder tissues
were much more resistant. If certain varieties are characterized
by harder tissues or by earlier maturity of cortical tissues, they

would be less susceptible.

-32-

Discussion

The pathogen S. sclerotiorum is a very common fungus attack-
ing a great variety of plants, especially in rainy seasons in the
field or in humid conditions under glass. Because of the various
symptoms on a large number of host plants, diseases caused by the
fungus have become known under various names, such as ”drOp", "wilt",
"Sclerotinia rot", ”stan rot”, and "soft rot".

Whether or not this organism is distributed by prOpagators
cannot be said; although, if the greenhouse man has had a previous
epidemic of wilt, sclerotia or mycelium could easily be transported
through the soil adhering to the seedling roots or in pots and flats.
This is possibly not the case, but it behooves the grower of plants
for sale to supply strong and vigorous plants grown in sterilized
soil, and the buyer should select and accept only good healthy stock.

The practice of using old composted soil without sterili-
zation is not recommended, and satisfactory control of this disease
must depend upon the use of sterilized soil with attention to the

temperature and humidity relationships of the plant and the fungus.

Summary
1. A wilt disease of the cultivated snapdragon occurs throughout
central and southern Michigan causing great losses in the crOp
each year in the greenhouses. The host range of the parasite on
the snapdragon is not known, but it is logical to assume, because
of the omnivorous nature of the parasite, that it is present

wherever the snapdragon is grown.under glass.

2.

5.

6.

7.

-23-

Diseased plants were obtained from Lansing, East Lansing, Mount
Clemens, and elsewhere, and the fungus was isolated and proved to
be pathogenic to snapdragons, hyacinth, gladiolus, Delphinium,
stock, Schizanthus, tomato, chrysanthemum, gourd, lettuce, and
many other greenhouse crops.

Plants are usually attacked by mycelium or by ascospores through
cut and wounded stems, and the fungus causing the disease works
with great rapidity causing the loss of the entire plant.

The pregress of the disease was most rapid between the tempera-
tures of 17° - 21° C. It was most destructive under conditions
of high humidity.

EYidence is presented of the fungous nature of the disease and

the causal organism identified as Sclerotinia sclerotiorum.

 

A technical description of the parasite is given.
Recommendations for control are: (l) Sterilization of soil, pots,
and all material with which the plants come in caitact, (2) Care
in watering, (3) Regulation of temperature and humidity, (4) Air
circulation by spacing of plants, (5) Covering the soil with
sterilized sand, (6) Excision of diseased stems and eradication

of diseased plants, and (7) Destruction of refuse.

1.

2.

3.

6.

-24-

Literature Cited
Carpenter, C. w.
The Verticillium wilt problem.
Phytopath. 4:393. 1914.
Dawson, W. J.
A blossom wilt and step rot of Antirrhinum and Schizanthus
due to Sclerotinia sclerotiorum.

Journ. of the Royal Hort. Soc. §15252-265. Figs. 61-71. 1926.

 

Pethybridge, G. H.
The "stalk" or "Sclerotium" disease. Investigations cn potato
diseases. Dept. Agr. and Tech. Instr. Ireland Journ. 1959-12.
Figs. 3-6. 1910. 115429-435. Figs. 1-3. 1911.

Saccardo, P. A.
Sylloge Fungorum1§3196. 1889.

Smiley, E. M.
The Phyllosticta blight of snapdragons.
Phytopath. lgzzi32-248. Figs. 1-8. 1920.

Stevens, F. L.

The fungi which cause plant disease. i-ix. 1-754. Figs. 1-449.

 

Macmillan. New'York. 1913.

Stewart, F. C.
An anthracnose and a stem.rot of the cultivated snapdragon.l
New York (Geneva) Agr. Exp. Sta. Bull. 179:105-111. Pls. 1-3.

1900.

Plate

Plate

Plate

Plate

Plate

Plate

II.

-25-

Legends
Snapdragon plant affected with wilt (natural infection).
The wilting is due to canker formation.which completely
girdled the stem near the soil level.

Snapdragon plant attacked by Sclerotinia sclerotiorum.

 

The blossom spikes and young succulent shoots were first
attacked by the fungus which gradually spread and affected

the entire upper portion of the plant.

III. A.mature snapdragon plant girdled Just above the soil level

V.

VI.

 

by a canker formed by the fungus S. sclerotiorum. Infection
occurred at the cut end of the flower spike a few inches
above the soil level.

Large roughened canker fanned by S. sclerotiorum at the

 

crown of a flowering plant. Infection occurred through the
stub of the cut flower stem.
A. Snapdragon plant inoculated through the stub of the

excised flower stem with a pure culture of S. sclerotiorum.

 

The fungus progressed rapidly downward until it girdled a
lateral stem.following which a rapid wilting occurred.
B. Snapdragon plants inoculated in the tops through wounds

with a pure culture of S. sclerotiorum.

 

A. Asci, ascospores, and paraphyses of S. sclerotiorum

 

(camera lucida drawings 1 600).

B. Byphae of S. sclerotiorum showing characteristic habit

 

of branching (x 600).
0-3. Germinating sclerotia from pure cultures of s. sclero-

tiorum.(x 8).

Plate I

 

Plate II

 

Plate III

 

Plate IV

 

Plate V

 

 

 

 

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