STEM AND LEAF SPOT INFECTIONS CAUSED
BY SEPTORIA MVSIVA AND S. POPVLICOLA
ON POPLAR SEEDLINGS
by
H. ZALASKY
Northern Forest Research Centre
5320 - 122 Street
Edmonton, Alberta
T6H 3S5
Phytoprotection 59 (1)
43-50 (1978)
ABSTRACT
Septoria musiva and S. populicola were isolated, respectively, from
overwintered leaves of hybrid poplar and balsam poplar, Populus balsa­
mifera
in
Manitoba
as
perithecia
of
Mycosphaerella
populorum
and
M. populicola. Isolates were sporulated on barley meal in flasks and inoc­
ula were prepared by suspending conidia in water. Seedlings of P. balsa­
mifera grown from 90 trees in 18 localities in six source areas in Manitoba
were inoculated in the grenhouse. S. Musiva caused the highest percentage
of canker and leaf spot of balsam poplar seedlings from all areas. Seed­
lings
from two-thirds
of the
localities
did
not
develop
S.
populicola
cankers, and from two localities no leaf spots. S. musiva also caused canker
and leaf spot on one seed source sample (15 trees) of eastern cottonwood,
P. deltoides, a differential host of S. popuiicola, which caused occasional
leaf spot but no canker. Infected plants responded with a canker that is
delimited laterally by phloem fibers. Twelve selections of seedlings were
resistant to Septoria canker caused by S. popu/icola and one to S. musiva.
Although S. mllsiva generally had a high inoculum potential, the fungus
did not produce as many spores as S. populicola in infected balsam poplar
seedlings. Monitoring leaf infections and the change of fungus activity
from colonization to sporulation may help to time control measures under
nursery operations.
Septoria musiva et S. popuiicola furent isoles respectivement de feuil­
les de Iitieres de Peupliers hybrides et de Peupliers baumiers (Populus
balsamifera), apres hivernation, au Manitoba. II s'agissait de peritheces
de Mycosphaerel/a populorum et M. populicola. Les isolats furent sporules
dans des flacons de gruau d'orge et les inoculums furent prepares en de­
posant des conidies en suspension dans l'eau. On inocula en serre les semis
de P. balsamifera provenant de 90 arbres en 18 endroits de 6 regions­
sources du Manitoba. S. mush'a causa Ie plus fort pourcentage de chan­
cres et de tache des feuilles sur les semis du Peuplier baumier de toutes
44
les regions. Les semis provenant des deux tiers des endroits ne develop­
perent pas de chancres S. populicola et ceux de deux endroits ne subirent
pas de tache des feuilles. S. mllsiva provoqua aussi Ie chancre et la tache
des feuilles sur une provenance (15 arbres)
deltoides).
Sur cet hote distinct, S.
de Peupliers delto"ides (P.
populicola
causa occasionnellement
la tache des feuilles mais aucun chancre. Les plants infectes subirent un
chancre lateralement delimite par les fibres du phloeme. Douze choix de
semis resisterent au chancre Septoria cause par S. populicola et un autre
resista a S. musiva. Bien que generalement S. mllsiva put etre facilement
inocule, Ie champignon ne produisit pas autant de spores que S. popillicoia
chez les semis de Peuplier baumier infectes. La surveillance etroite des
infections des feuilles et de la modification des activites du champignon
delaissant la colonisation pour la sporulation pourraient aider les pepinieris­
tes a trouver Ie temps propice a l'application des mesures de repression,
soit Ie temps d'arrosage.
INTRODUCTION
Cankers discovered on whips of hybrid poplar in stooling beds at a forest
tree nursery in Saskatchewan were shown to be caused by Septoria sp. (Ross
1930) . Later, two indigenous species, Septoria musiva Pk. and S. populicoia Pk.,
were tested for differential host reactions (Thompson 1941) . Other tests have
demonstrated that S. musiva causes canker in current saplings of several hybrid
poplar (Bier 1939) , exotic poplars (Bier 1939, Thompson 1941) , and a native
poplar, Populus deltoides Marsh. (Filer et al. 1971) . S. musiva was not found
in the rhytidome of poplar either as fruiting bodies or by isolation (Waterman
1954) . Stem infections appeared to occur before the formation of the rhytidome,
which forms by annual renewal of the periderm (Esau 1969) .
This paper reports on stem and leaf spot infections by S. musiva and S.
populicola on balsam poplar, P. balsamifera L., and eastern cottonwood, P.
deltoides Marsh., seedlings from Manitoba sources maintained under greenhouse
conditions and describes host reaction between and within host species. The
information will be useful to nurserymen and plant breeders because the more
resistant seedlings can be selected for breeding stock and for outplanting (Skinner
1966) .
MATERIALS AND METHODS
Seeds of open-pollinated balsam poplar and eastern cottonwood were col­
lected in Manitoba during June-July, 1968. Each seed sample collected from an
individual tree was dried on screens and cleaned to remove fluff. Ninety samples
of balsam poplar were collected from six source areas: Red River valley,
Whitemouth River valley, Whiteshell-Lac du Bonnet area, Turtle Mountain Pro­
vincial Park, Spruce Woods Provincial Park, and Riding Mountain National
45
Park. Three localities were selected in each source area, with five trees sampled
at each locality. In each locality the selected trees were mostly isolated or semi­
isolated by aspen or conifers and were separated a minimum distance of 0,3 km.
Fifteen seed samples of eastern cottonwod were collected from trees separated a
minimum distance of 0,5 km and located along the Red River within 45 km of
Winnipeg.
Balsam poplar was propagated from two seed source areas at a time and
maintained in the greenhouse at Winnipeg, Manitoba. All seed germinated except
that from one locality in Riding Mountain National Park (see Table 1). A 2-year
period was required to test balsam poplar seedlings from all seed sources. The
number of balsam poplar seedlings from an individual locality used in subsequent
tests varied from 160 to 1158. Cottonwood seedlings from the Red River valley
were tested last and similarly maintained in the greenhouse. The seedlings were
grown to a maximum of the tenth-leaf stage. Total growing time allowed before
inoculation was 12 weeks.
Hadashville, Manitoba was selected as the source area of inocula after
evaluating field surveys of summer leaf infections in 1967 and abundance of
perithecia on overwintered leaves in Manitoba and Saskatchewan. Perithecia of
Mycosphaerella populorum G.E. Thomp. and M. populicola G.E. Thomp. were
obtained in June 1967 respectively from overwintered leaves of hybrid poplar
in a nursery (Roller and Thibault 1966) and from a regeneration balsam poplar
stand. The inocula of S. musiva and S. populicola were started from ascospores
in 1968.
Perithecia were placed in a hanging drop of sterilized distilled water inside
the top petri plate to allow ascospores to disperse evenly over the water agar
below. Each germinating ascospore was transferred aseptically to nutrient potato
dextrose agar slants within 2 or 3 days. The fastest-growing culture was homo­
genized and transferred to autocIaved mealy barley contained in flasks. Septoria
conidia were then produced in each culture at room temperature (20°C). Flask
cultures of S. musiva and S. populicola were stored in a deep freeze to be used
when required for each group of seedlings.
Seedlings of each source tree were divided into three groups, one each for
inoculation with S. musiva and S. populicola and one for the control. Inocula
were prepared by suspending Septoria conidia in water. Seedlings were atomized
with conidial suspension containing 6 X 104 Septoria spores/ml and then cov­
ered with plastic bags for 48 h to ensure a moist environment for infection.
Noninoculated controls were similarly covered in plastic bags. Seedlings were
maintained in the greenhouse. Infections were dated by daily examination for
canker and leaf spot. Six weeks after inoculation, pathogenicity of the two fungi
was compared on the basis of leaf and stem infection and sporulation of pycnidia.
Spore counts were made using a haemacytometer. It was not convenient to
measure the percentage of leaf infection except by estimation.
fIGURE
47
RESULTS
In balsam poplar seedlings, S. musiva and S. populicoia infections were
confined to small areas of the leaf and stem tissues; more infections occurred in
the leaf tissues. Cankers (Figs. 1 and 2) and leaf spots generally developed 21
days after inoculation. Most S. musiva canker and leaf spot percentages were
higher than those of S. populicoia. Cankers caused by the two fungi were indis­
tinguishable. The controls remained free of leaf and stem infections.
The percentage of cankers in balsam poplar seedlings of different source
areas varied from 0 to 3 1,6% for S. musiva and 0 to 14,4% for S. populico/a
(Table 1). Balsam poplar seedlings of source trees from Libau, Thalberg, and
Hadashville had the highest percentage of S. musiva cankers: 3 1,6, 14,4, and 13,7%
respectively. Percentages of S. musiva cankers in seedlings from all other source
localities were lower than 10%, and that in Bird's Hill seedling from five source trees
was zero. Balsam poplar seedlings of source trees from Libau, Thalberg, and
Brokenhead River had the highest percentage of S. populicola cankers: 14,4,
13,7, and 9,8% respectively. Only a few seedlings from three other localities
were cankered (Table 1).
Balsam poplar seedlings from each source area were estimated to have
26-75 % S. musiva leaf spot (Table 1), but seedlings from only five localities
were estimated to have 26-75% S. populicola leaf spot. Seedlings of most other
localities were estimated to have 1-25 % S. populicola leaf spot, except in two
stands at Spruce Woods Provincial Park with 0% leaf spot.
Damage by S. musiva canker and S. populicola canker was greatest in Red
River valley, Whitemouth River valley, and Whiteshell-Lac du Bonnet seedlings.
Damage from S. musiva leaf and stem infections was usually higher in seedlings
from all source areas than from S. populicola leaf and stem infections (Table 1).
The seedlings from the mesic source localities, i.e., the rich loam soil of the Red
River valley, Whitemouth River valley, and Whiteshell-Lac du Bonnet area, had
the highest numbers of S. musiva cankers. The only exception was seedlings from
Bird's Hill, where source trees are on elevated sandy ground with a high water
table. Seedlings from the higher elevation source trees of Spruce Wods Provincial
Park, Riding Mountain National Park, and Turtle Mountain Provincial Park
and from the Woodridge sandy locality of Whitemouth River valley had no S.
populicola cankers and generally fewer S. musiva cankers. Most seedlings from
all areas were affected uniformly with S. musiva leaf spot; the only noticeable
discrepancies were slight differences by tree sources for Brokenhead River, Hadash­
ville, and Rennie.
Pycnidia and spore production in leaf spots of S. musiva and S. populicota
varied among source areas of seedlings from southeastern Manitoba bordered
by Thalberg, Bird's Hill, and Woodridge. S. musiva produced three times the
number of pycnidia and spores than S. populico/a in Red River valley infected
seedlings. S. populicola, however, produced five times as many spores in White-
TABLE 1
-
S. mllsiva AND S. populicola CANKER AND LEAF SPOT INFECTION OF BALSAM POPLAR SEEDLINGS, P. balsamifera, FROM VARIOUS
SOURCE AREAS AND LOCALITIES IN SOUTHERN M ANITOBA 6 WEEKS AFTER INOCULATION.
Seed source
areas and
localities
Septaria mllsiva
No. of
seedlings
Estimated
% leaf spot
% canker
Septaria populicola
No. of
seedlings
Estimated
% leaf spot
% canker
Red River Valley
Bird's Hill
Libau
Thalberg
97
26-75
0,0
117
26-75
0,9
101
26-75
31,6
104
26-75
14,4
90
26-75
14,4
96
26-75
13,7
Whitemouth River
Brokenhead River at # 1 Highway
96
26-75
6,1
71
26-75
9,8
Hadashville
131
26-75
13,7
153
1-25
Woodridge
117
26-75
5,9
138
26-75
3,2
0 ,0
East Braintree
108
26-75
2,7
110
26-75
0,0
Rennie
287
26-75
8,3
232
1-25
0,0
Seven Sisters
273
26-75
5,4
263
1-25
1,9
Whiteshell - Lac du Bonnet
Spruce Woods Provo Park
Stand 1 north of Hwy.
297
26-75
3,0
384
o
0,0
Stand 2 south of river
332
26-75
1,8
349
1-25
0,0
Stand 3 north of river
602
26-75
1,6
556
o
0,0
Audy Lake
188
26-75
4,2
122
1-25
0,0
West Park gate
276
26-75
3,9
231
1-25
0,0
Riding Mountain Natl. Park
North Park gate
Turtle Mountain Provo Park
Stand 1 (Centre)
107
26-75
8,4
113
1-25
0,0
Stand 2 (East)
197
26-75
7,1
310
1-25
0,0
Stand 3 (N.E.)
83
26-75
7,2
77
1-25
0,0
'" No seeds germinated
49
mouth River valley infected seedlings and seven times as many spores in the
Whiteshell-Lac du Bonnet infected seedlings than were produced by S. musiva.
Production of pycnidia and spores fell sharply as the number and size of leaf
spots decreased in seedlings from source areas of higher elevations in forest
preserves and parks elsewhere in the province.
S. musiva leaf and stem infections in 67 eastern cottonwod seedlings were
rated 1-25% and 28,3%, respectively, and S. populicola leaf and stem infestions
in 23 seedlings were rated 1-25% and 0% respectively. The 20 controls were
free of infection.
The cankers were elongated, while the leaf spots were relatively oval and
angular; bark and growth rings nearest the cankers and the non-infected portions
of the leaf were of normal thickness except for a slight collapse of tissues.
Septoria killed the tissues, colonized, and fructified without further colonization
of healthy tissues surrounding the canker and leaf spot. In bark, the mycelium
colnized the cortex and phloem along the vertical axis and was delimited laterally
by phloem fibers and the xylem. In leaves, the mycelium was delimited by the
vascular bundles of leaf veins and midrib. Pycnidia formed in groups or in a
targetlike pattern within the central white area of the canker and leaf spot.
DISCUSSION
The relative ease of demonstrating Septoria canker in balsam poplar and
eastern cottonwood seedlings may be attributed to the thin bark. In seedlings,
the bark undergoes a period of transition as it looses the epidermis and produces
its first periderm and phloem fibers (Esau 1969). These changes take place
over a period of several weeks. The Septoria canker starts as necrosis of the
cells at the initial site of infection at the epidermis and developing periderm.
Necrosis reaches the phloem as the fungus invades from cell to cell of the cortex.
After the phloem is reached by the fungus, the direction of spread of necrosis
and the fungus change to vertical orientation along the phloem. The xylem is
not colonized during the time the fungus forms pycnidia. In poplar leaves
Septoria mycelium is delimited by the leaf veins and midrib and ceases growth
as pycnidia mature, as was found in soybean by MacNeil and Zalasky (1957).
The type of Septoria canker obtained with sprayed conidial inoculations
was similar to that reported by Filer et al. (1971) when stems were not wounded.
There was no wide variation in host reaction to stem infections and canker
development. The variations reported by Bier (1939) and Waterman (1954)
are probably due to wounding, which was shown by Brown (1937) to stimulate
the formation of irregular swellings in poplar. Variations in host reaction to leaf
and stem infections, however, are useful in defining inter- and intrahost differ­
entiation for different species of pathogens. The evaluation of leaf and stem
infections has to be done on the basis of area, locality, and tree seed sources. The
variations may be clearly discernible in seedlings on the basis of area and
50
locality of tree sources as in S. populicola infections, or just on the basis of
tree source, as in S. musiva infections. The pathogens could conceivably be
contained by mass selecting resistant seedlings, selecting differential host clones,
and certification of seedlings designated as commercial stock.
Supplementary control measures of Septoria leaf spot in commercial stock
may also be desirable if the leaf spot enhances chances of infection in the bark.
Control can be achieved by the application of fungicides (Carlson 1972, 1974).
In warm weather, 10 days is generally required for the production of conidia in
the black fruiting bodies located centrally in white areas of leaf spots. Fungicidal
spray should be applied when leaf spots first color from yellow to brown but
before white spots appear centrally. In cool weather, conidial production is
delayed beyond the 10-day period as the fungus reacts to climate and host. With
proper monitoring, this allows more time to select correct weather conditions for
an effective spray operation.
REFERENCES
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Sec. C: 195-204.
BROWN, A.B. 1937. Activity of the vascular cambium in relation to wounding in the balsam
poplar, Populus balsamifera L. Call. I. Res. 15, Sec. C: 7-31.
CARLSON, L.W. 1972. Fungicidal control of poplar leaf spots in Alberta and Saskatchewan.
Can. Plant Dis. SUIT. 52: 99-101.
CARLSON, L.W. 1974. Fungicidal control of poplar leaf spots. Can. Plant Dis. Surv. 54: 81-85.
ESAU, K. 1969. The phloem. Gebriider Borntraeger, Berlin.
FILER, T.H., F.T. MCCRAKEN,
C.A. MOHN, and W.K. RANDALL. 1971. Septoria canker on
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MACNEIL,
B.H.
and
H. ZALASKY.
1957.
Histological
study
of
host-parasite
relationship
between Septoria glycines Hemmi and soybean leaves and pods. Can. I. Bot. 35: 501-505.
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K.J.
and D.H. THIBAULT.
1966.
Preliminary test of
some
one-year-old
poplar
cultivars in Manitoba. Can. Dep. For., For. Res. Lab., Winnipeg, Manit. Intern. Rep.
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SKINNER, F.L. 1966. Horticultural horizons. Mallit. Dep. Agrie. No. 48004.
THOMPSON, G.E. 1941. Leaf spot disease of poplars caused by Septoria musiva and S.
populieola. Phytopathology 31: 241-254.
WATERMAN, A.M. 1954. Septoria canker of poplar in the United States. U.S. Dep. Agrie.
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(Accepted for publication November 17, 1977)