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THE UNIVERSITY

OF ILLINOIS

LIBRARY



A6RICDLTURAL
LIBRARY



UNIVERSITY OF ILLINOIS

Agricultural Experiment Station



BULLETIN No. 213



TWO ILLINOIS RHUBARB DISEASES

BY FRANK LINCOLN STEVENS




URBANA, ILLINOIS, JANUARY, 1919



CONTENTS OF BULLETIN No. 213

PAGE
RHUBARB AKTHRACNOSE 299

General Character 299

The Fungus 300

Isolation 301

Culture Characters 301

Inoculations 305

Germination of Conidia 305

Taxonomy 305

RHUBARB LEAF SPOT 308

The Fungus 310

Isolation 312

Culture Characters 312

Taxonomy 312



TWO ILLINOIS RHUBARB DISEASES

By FRANK LINCOLN STEVENS, PROFESSOR OF PLANT PATHOLOGY, UNIVERSITY

OF ILLINOIS 1

RHUBARB ANTHRACNOSE

(Due to Collet otriclium erumpens)

Attention was first called to this disease on market rhubarb in the
stores of Champaign during the spring of 1918. It has since been
repeatedly seen in the grocery stores and has been collected in the field
in numerous localities, in both cases in such abundance as to show
that it constitutes a real factor in rhubarb culture.

GENERAL CHARACTER

The disease, which is of fungous origin, consists of a soft rot of the
petioles. As yet no natural field infection of other parts of the plant
has been seen. The decayed spots usually are soft, watery, and
translucent, oval in outline, with the long axis lengthwise of the
petiole (Fig. 1). When they attain a length of somewhat more than
a centimeter, the black, sporulating bodies (acervuli), smaller than fly
specks, appear abundantly in the centers of the spots (Fig. 2). In
advanced stages the whole petiole is covered with acervuli and is
entirely soft and rotten (Fig. 3).

In the market usually only the milder cases of disease are found ;
petioles with numerous small rotten spots which have escaped the
notice of the grower when preparing his product for sale. In the
field, often all old petioles lying dead on the ground are thickly cov-
ered with acervuli, and older petioles slightly wilted are heavily
infected. In general this disease appears much more abundantly on
old plant parts than on fresh, vigorous parts, tho many vigorous,
salable petioles are attacked, as is clearly evident from the abundance
of the disease in the market as well as in the field.

This disease has been collected at Champaign and Urbana, Cham-
paign county ; at Kankakee and Bourboiinaise, Kankakee county ; and
near Anna, Union county; and is apparently of quite general oc-
currence.

The injury done is threefold. Even a small amount of the dis-
ease in a bunch of market rhubarb practically destroys the possibility
of selling the bunch to a careful purchaser. In the field many petioles



Assisted in the laboratory and in illustrating by Nora E. Dalbey.

299



300



BULLETIN No. 213



[January,



otherwise salable must be discarded on account of the rot. The pre-
mature death of infected petioles and consequently of the leaf blades
lessens the general strength of the plant.

THE FUNGUS

The rotten spots are uniformly occupied by a species of Colletot-
richum, and in early stages of decay by this fungus alone. Young,
watery, translucent spots which have not yet developed acervuli show
the diseased tissue quite fully occupied by the mycelium. This
mycelium is hyaline and of somewhat characteristic appearance in that





FIG. 1. PETIOLES OF EHUBARB SHOWING VARIOUS STAGES OF DECAY EESULTING

FROM COLLETOTRICHUM

the older parts are rather coarse (4 p.), conspicuously septate, and
constricted at the septa (Fig. 4). Younger threads of mycelium are
smaller, tapering to 2 /u at the extremities.

The acervuli. begin subcuticularly as an aggregate of hyphae,
Fig. 5 (1), which soon ruptures the cuticle. When mature they are
circular or oval and of an average diameter of about 200 p.. Soon
after rupturing the cuticle, one or more setae appear, and in the
mature acervulus the setae are numerous, usually from fifteen to



1919]



Two ILLINOIS EHUBARB DISEASES



301




Fig. 2. A single spot,
enlarged. The minute
pustules are the acer-
vuli



twenty or more Fig. 5 (2, 3). Spores begin
to form as soon as the cuticle ruptures, and
the mature, undisturbed acervulus is covered
with a mound of spores, often so deep as to
cover even the tips of the setae.

The setae are black, somewhat pale at the
tip, usually 3- or 4-septate, upward of 150 p.
long, and about 7 p. thick at base, and with an
acute tip, Fig 6 (1, 2).

The conidia are falcate (Fig. 7), quite uni-
form in size, measuring 22 to 29 /* by 3.5 p.,
hyaline and continuous, and acute at each
end. The conidiophores are shorter than the
conidia, simple and continuous.

Isolation

The fungus was readily isolated by sev-
eral of the usual methods:

1. By dilution platings of spores. In
many cases the plates were entirely pure,
with hundreds of colonies of the Colletot-
richum.

2. By differential growth, by direct



planting of bits of infected tissue upon
poured corn-meal agar plates. When
this method was employed, the Colletot-
richum outgrew its contaminators, and
could thus be easily isolated.

3. By direct planting of spores from
drops of sterile water upon poured agar
plates. The colonies so secured on corn-
meal agar did not produce normal acer-
vuli, but they did eventually produce
characteristic conidia, and w r hen trans-
ferred to sterile rhubarb gave abundant
normal acervuli.

Culture Characters

On corn-meal agar plates the myce-
lium showed a tendency to darken in
some cases, producing many filaments
almost black, and giving to all the col-
onies a dark appearance. Analysis
shows that the darkening of the colonies
is due to an excessive production of ap-



Fig. 3. Advanced stages of de-
cay of the rhubarb petioles



302



BULLETIN No. 213



[January,



pressoria 1 and of structures which are morphologically appressoria.
Often these are produced in large numbers in clumps which appeal-
as black sclerotia-like bodies, several millimeters in diameter. This
was particularly true where two colonies approached each other in
the plate (Fig. 8). The structure of the component parts of such
a spot is represented in Fig. 6 (3). A general view of several col-
onies on corn-meal agar is shown in Fig. 8. On this medium the
acervuli are produced somewhat sparsely, after an interval of about
a week.




FIG. 4. CHARACTERISTIC BRANCHING AND SEPTATION OF THE COLLETOTRICHUM

MYCELIUM



When sterile (autoclaved) rhubarb was laid upon poured plates
of corn-meal agar and then inoculated with the fungus, acervuli
formed much more quickly than upon corn-meal agar alone, and spores
were produced in much greater quantity. Thus abundant mature
acervuli were present upon the bits of rhubarb at the end of three
days.

Bits of rhubarb petioles were cut aseptically, and placed on poured
corn-meal agar plates. Upon these growth was as upon the auto-
claved rhubarb but decidedly more vigorous.

Corn-meal agar was poured into plates, then inoculated. When
the colonies attained a diameter of about a centimeter, bits of raw,
aseptic rhubarb were laid on the agar about a centimeter from the
edge of the colony, in order to see whether the growth rate in this
sector of the colony would be altered by the rhubarb. No such effect
was noticeable, but it was found that tho the fungus made acervuli



'Hasselbring, H. Bot. Gaz. 42, 135, 1906; and Halsted, B.D., N.J. Agr.
Exp. Sta. Ept., 1892, 303.



1919]



Two ILLINOIS RHUBARB DISEASES



303



but very sparsely on the agar it did make them abundantly upon
the bits of raw rhubarb upon the agar.

On various autoclaved plugs the fungus grew well : e.g., on Bumex
crispus, with typical acervuli but growth rather slow; on Rumex
acetosella, similarly but growth more vigorous ; on Polygonum erectum
growth was scant and aeervuli small; on Polygonum lapatliifolium
growth was similar to that on Polygonum erectum but slightly more
vigorous; on Tropoeolum stems, growth was very rapid and acervuli




FIG. 5. (1) A VERY YOUNG ACERVULUS; AN AGGREGATE OP MYCELIUM BELOW
THE CUTICLE WITH INDICATIONS OF BEGINNINGS OF SETAE. (2) TYPICAL
GROUP OF ACERVULI. (3) A TYPICAL ACERVULUS

numerous, showing these steins to be more favorable than any other
medium.

On cabbage plugs the whole culture was less dark than on rhu-
barb plugs, the floccose aerial mycelium was white, and there was
almost entire absence of appressoria, and of sclerotia, or other dark
structures except those of the acervuli, which were very abundant
and normal.



304



BULLETIN No. 213



[January,



On rice there was an unusually large production of appressoria
and sclerotia, giving the culture a striking black character (Fig. 9).
In tubes on autoclaved rhubarb plugs, growth was vigorous, the sur-
face of the plugs becoming closely covered with acervuli which pro-
duced spores about a week after inoculation. In the older portions




FIG. 6. (1) DETAIL OF SETAE AND CONIDIOPHORES. (2) A SINGLE SETUM. (3)
COMPONENT PARTS OF THE SCLEROTIA-LIKE BODIES PRODUCED IN AGAR PLATES

the plugs were covered by a thin growth of aerial mycelium. On
corn-meal agar in slant tubes the surface of the slant soon became
covered with black sclerotia-like structures, and a ring of these struc-
tures was formed wherever the fungus touched the glass.



1919]



Two ILLINOIS EHUBARB DISEASES



305



Inoculations

When spores from diseased tissue, or fragments of diseased tissue,
or mycelium from a pure culture were inoculated into rhubarb

petioles, the typical rot rapidly followed.
Tho the disease was not found in the
field upon leaf blades, laying the spores
upon leaf blades in a moist chamber in the
laboratory resulted, in about seven days,
in large, rotten, leaf spots bearing numer-
ous typical acervuli. The disease also often
appeared thru natural infection on leaf
blades in the laboratory.




Fig. 7. Spores of Colletot-
richum



Germination of Conidia

Germination in water in hanging drop
gave the usual characters of the anthrac-
nose fungi; i.e., the spores usually became

two-celled, developed a germ tube from one or both cells, and often
from the germ tubes one or more appressoria (Fig. 10).

TAXONOMY

The fungus is a typical Colletotrichum. Never were setae absent
from mature acervuli, nor is there any basal tubercular structure
which would throw it into the genus Volutella. Colletotrichum as




FIG. 8. TYPICAL COLONIES ox CORX-MEAL AGAR
LEFT: FOUR COLONIES EIGHT: MANY COLOXIES

A dark line due to sclerotia-like bodies is evident in all cases when one
colony meets another.



306



BULLETIN No. 213



[January,



given by Saccardo 1 contains 113 form species.
Of these, however, only 33 have falcate spores.
There are 83 species of Volutella listed by
Saccardo. The falcate-spored Colletotrichums
are given in the accompanying table, together
with such of the closely related genus Volu-
tella as have falcate spores.

From this tabulation have been omitted
such forms as are described as having spores
"straight or slightly curved," "straight or
curved," or similarly designated, on the as-
sumption that such are not typically falcate,
a character which were it present would be
sufficiently obvious to be mentioned.

Accepting the descriptions as correct, Nos.
1 to 27 inclusive and Nos. 29, 38, and 43
do not agree in spore size with the fungus
under consideration. Nos. 44 to 47 inclusive
need not be considered. Nos. 28, 30 to 37,
and 39 to 42 are so close in spore measure-
ments to the species causing the rhubarb rot,

that this might indeed, on that basis alone, be considered as co-specific
with any one of them ; nor is there any evidence from measurements
of setae, except in the case of No. 37, that would bar such an assump-
tion. Close inspection of the specific descriptions eliminates Nos.
40 and 41, which are clearly tubercular.

There then remains a group consisting of ten species which from
the descriptions are indistinguishable from each other and from the




Fig. 9. Growth of Col-
letotrichum on auto-
claved rice




FIG. 10. GERMINATING CONIDIA AND APPRESSORIA



1 Saceardo. Sylloge Fungorum.



1910]



Two ILLINOIS EHUBARB DISEASES



307



FALCATE-SFORED FORMS OF COLLETOTRICHUM AND VOLUTELLA
Arranged in order of maximum spore length (/*)



No.


Saccardian


Name


Spore


Setae


Host




Vol. No.




Length X Breadth


Length X Thickness




1

2
3


4 3234
11 3676
10 6851


V. chalybea
C. sanguisorbae
C. sphaeriaeforme


7-8X4-5
9-10X2-2.5
10 X3 1


115-140X2.3
80-10.0X3-6


"fimo"
Sanguisorbia
Menispermum


4
5

6

7
8


4 3237
14 4026

10 6846
14 4019
4 3227


V. buxi
C. yuccae

C. pisi
C. solitarium
V. comata


10-11X3^
13X3.5

11-13X3-4
12-14X2.5-3
12-14 X


100-120X4
60X6

60- 90X6
65- 75X2-3


Buxus
Yucca

Pisum
Solidago
Robinia


9
10


18 5034
16 3885


C. liliacearum
C. piri


12-17X2.5-3.5
13-18X3.5-5.5


70X5-5.5


Hemerocallus
Pirus


11
12
13


3 3843
4 3263
18 5014


C. volutella
V. therryana
C. theobromicolum


14-18X2.5-3.5
15-18X3
15-18X4-5


300X5-6
150X1


Ulmaria
Grasses
Theobroma


14
15

16
17

18


16 3882
18 5772

10 6852
16 3886
10 6854


C. violae-tricoloris
V. tristis

C. spinaciae
C. elasticae
C. gloeosporioides


20X5
11-20X2

14-20X2.5-3
16-20X4.5-5
18-20X5-6


20- 70 X

400X7-8

60- 75X4-4.5
200-250 X


Viola
Erica

Spinacia
Ficus
Hedera


19


16 4258


V. violae


15-21X3-4




Viola


20

21
22
8


4 3244

16 4262
10 6847
22 7733


V. fusarioides

V. allii
C. carpophilum
C. (impelinum


18-21X2-3.5

18-21X3-4
16-22X2.5-4
17-22X4-5


60- 72X5-7

60-175X6
60-100X5-6


Citrus

Allium

Astragalus
Vitus


24
25

26
27

*8


16 3883
22 7767

11 3679
22 7741

10- 6845


C. rhoinum
C. vermicularia

C. ailanthi
C. fructus
(V. fructus}
C. brassicae


20-22X4-4.5
20-22X4

22X4-5
17-23X2.5-3.5

19-24 X


150-180
80- 90X4

90-135X5-9
100-4"OOX5-8


Rhus
Bromus

Ailanthus
Pirus

Brassica


29

30

31
32
33
34
35


22 7763
22 7744

3 3842
11 3683
22 7732
22 7768
11 3681


C. sublineola
C. hedericola

C. erumpens
C. falcatum
C. vitis
C. cereale
C. omnivoriim


15-25X4-5
17-25X3.5

25X5
25X4
21-25X2.5
18-26X3-4
20-28x3-5


'30- 50X7
140X7

150X4
100-200X4
140-160X6-9
60-120X6-8


Sorghum
Hedera

Ruscus
Saccharum
Vitis
Grasses
Funkia, Aspedis-


36
37

38
39
40

41
42
43
44


22 7753
3 3844

22 7765
18 5025
14 4493

14 4494
18 5036
22 7766
4 3245


C. platani
C. lineola

C. antarcticum
C. fici-elasticae
V. acalyphae

V . oxyspora
C. andro'pogonis
C. janczewslcii
V . isabcllina


25-28X3-5
25-28X3.5-4

15-30X3-4
20-30X3-4
25-30X2-3

25-30X2-3
30X4
24-34X3-6


200
60X3

30-100X4-6
200X6
150-210X4-7

100-150X4-5
100-150X8
70-150X4


tra, etc.
Platinus
Andropogon,
Umbellifers
Poa
Ficus
Acalypha

Beta

Andropogon
Poa
Salix


45


4 3232


V ' . pulchella






Branches


40


4 3252


V ' . melaloma






Carica


47


4 3226


V . georginae






Georgina



'Spuriously 2 to 3 septate.



308



BULLETIN No. 213



species described in this paper. These are designated as follows,
f rom their oldest species, as the

Colletotrichum erumpens group:

No. 31. Colletotrichum erumpens Sacc.
28. C. brassicae Schulz and Sace.

35. C. omnivorum Hals.

32. (7. falcatum Went.
39. G. fici-elasticae Zimm.

33. C. vitis 1st.

42. C. andropogonis Zimm.

30. C. hedcricola Laub.

34. C. cereale Manns

36. C. platani Da Camara

In the present condition of knowledge concerning these forms,
and until monographic work is completed regarding them, it seems
wise to assume that the rhubarb Colletotrichum belongs to this group,
and tentatively to refer to it as Colletotriclmm erumpens.

RHUBARB LEAF SPOT

(Due to Phyllosticta straminella)

This disease was first collected at Bourbonnaise, Kankakee county,
Illinois, where it was in great abundance in one field. Nearly every
leaf in the plantation was affected with many spots, often occupying
considerably more than half of the leaf area. A general idea of the




FIG. 11. A SINGLE DISEASED SPOT OF THE PHYLLOSTICTA DISEASE ox RHUBARB



1919]



Two ILLINOIS RHUBARB DISEASES



309



nature of the effect
upon the leaf is
given in the front-
cover illustration.
More detail of a
single spot is
shown in Fig. 11.
Unlike the preced-
ing disease this is
not preeminently a
disease of old
leaves but even the
c o mp aratively
young leaves may
be seriously spot-
ted. While mainly
affecting the leaf
blade, it has also
been found upon
the petioles. Upon
the leaf blade the

chief character is the irregularly circular dead spot, varying from
a few millimeters up to several centimeters in diameter. The dead
area is tan color and often dry and cracked or torn (see front cover).
The margin of the spot is definite, with a rather sharp limitation
between diseased and healthy tissue. Close inspection shows numer-




FIG. 12. AN ENLARGED VIEW OF A PORTION OF THE

SPOT SHOWN IN FIG. 11
The pycnidia are here clearly shown.




FIG. 13. PHYLLOSTICTA. A PYCNIDIUM IN SECTION, SHOWING SUBCUTICULAR DE-
VELOPMENT AND SHAPE OF THE PYCNIDIUM
The cavity was entirely filled with spores.



310



BULLETIN No. 213




Fig. 14. Phyllosticta, Pycnidia
viewed from above



ous very minute
dark pustules
(pycnidia)
(Figs. 11 and
12). On petioles
and stems the
spots are oval
1 en gthwise of
the petiole, dis-
tinctly sunken,
and quite dry.
S om e times a
spot extends for
nearly the whole
length of a peti-
ole, occupying
p e r haps only
one side or part
of one side of it.
In addition to
collections at




Fisr.



15. Phyllosticta spores
from a leaf spot




Fig.



16. Phyllosticta
small type



spores,



Bourbonnaise, others have also been made at Urbana, Kankakee, and
Champaign. Specimens collected at Ladoga, Indiana, in 1916, by Dr.
P. J. Anderson and at Racine, Wisconsin, by M. W. Gardener, bear the
same fungus. When prevalent to the extent shown in the cover illus-
tration, and to that degree on many leaves, as is the case in some fields,
the drain on the vigor of the plants must be large.

THE FUNGUS

Microscopic examination invariably revealed the presence of a
pycnidial fungus of the Phoma or Phyllosticta type. Fig. 12, an en-




FIG. 17. PORTION OF PETRI DISH SHOWING PHYLLOSTICTA COLONIES
This plating was made direct from rhubarb leaves.



1919]



Two ILLINOIS RHUBARB DISEASES



311




FIG. 18. A SINGLE COLONY OF THE PHYLLOSTICTA ON CORN-MEAL AGAR




FIG. 19. PORTION OP PETRI DISH SHOWING SEVERAL COLONIES OF THE

PHYLLOSTICTA

larged view of a single spot, shows the numerous pycnidia. These
are approximately circular, tan colored, about 120 to 150 /x in diameter,
with an ostiole 20 to 30 p. wide (Fig. 14). Under appropriate con-
ditions the spores issue in cirrhi or long agglutinated coils. From
the leaf spot the spores are rather large for a Phyllosticta ( 10 to 19 p
by 4 to 5/x), and oblong (Fig. 15) and straight or very slightly curved.
Specimens from the petiole and from some cultures to be described
later gave a much smaller type of spore, as small as 4 to 5 //, by 1.5 to
5 /x (Fig. 16).



312 BULLETIN No. 213 [January,

Isolation

The fungus was isolated on corn-meal agar in all of the ways
noted on page 301, and with the same general results ; i.e., the fungus
grew readily and normally in culture (Fig. 17). Photographs of
plates and colonies are given in Figs. 17, 18, and 19.



On plates on corn-meal agar, the fungus grows rapidly and pro-
duces pycnidia quickly. When thickly sown, sporiferous pycnidia are
produced in thirty-six hours ; when more thinly sown, pycnidia are not
so quickly produced. The colonies eventually become quite dark in
the central region. The pycnidia arise uniformly by the simple meri-
stogenous method. There is in corn-meal agar a very remarkable change
in spore size from that exhibited by the same fungus when in its
natural habitat. On this medium the spores are very rarely as
long as 7 p. (Fig. 16). In very old cultures spores of somewhat larger
size are found, but none as large as on the original leaf spots. On
various autoclaved vegetable plugs the fungus grew well and formed
numerous pycnidia normally. As with the Colletotrichum, Tropceolum
stems were the most favorable medium. On cabbage the growth was
exceptionally white. On rice the whole culture became black and
was nearly like that of Colletotrichum (cf. Fig. 9).

The conidia germinated readily in hanging drops of water, nearly
every spore germinating. There were formed no appressoria such as
were invariably formed under similar conditions by the Colletotri-
chum.

TAXONOMY

Regarding this as a Phyllosticta and its spore measurements to
be the extremes noted above; i.e., 4 to 19 /x by 1.5 to 5 /x, there are
829 species 1 listed by Saccardo from which, on a basis of spore size,
it would be indistinguishable. Accepting the narrower limits of spore
measurement obtained from the large spored forms from foliar mate-
rial ; i.e., 10 to 19 /x by 4 to 5 /*, there are 48 species listed by Saccardo
from which it could not be distinguished by spore measurements.
One only of these, however, occurs on any of the Polygonaceae. Ten
other species of Phyllosticta are listed by Saccardo on Polygonaceae,
but none of these has spores as large as the large-spored form described
in this paper. All of these, with the exception of one, have maximum
lengths of 12 /x, while in most of them the maximum length is below
8 11. The one large-spored species, P. straminella Bres., described on
Rumex acestosa, is given as having spores 12 to 20 p. long and is suf-
ficiently close in morphology to the form under consideration to be
regarded as the same species.



lr riiese facts are taken from a tabulation of the species of Phyllosticta listed
by Saceardo, prepared for future publication by Mrs. E. Young True.



UNIVERSITY OF ILLINOIS-URBAN*






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