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Background and Objectives:
- To determine the duration of survival of Fusarium graminearum on
kernels exposed on the soil surface or buried at various depths under controlled and field
conditions, and
- To investigate grass species as possible alternative hosts for
Fusarium
species and as a reservoir for F. graminearum, the predominant Fusarium
species causing FHB.
Procedure and Project Activities:
PROJECT 1
Field Study
The study investigated the survival of F. graminearum under field conditions on
fusarium damaged kernels (FDK) when buried at different depths and in various soil types.
A severely infected sample of Roblin wheat (93% infected with F. graminearum) was
used for the experiment. Fusarium damaged kernels were selected from the sample and placed
in small nylon mesh bags. In October 1998, bags were placed on the soil surface or buried
at 5 or 10 cm at Glenlea Research Farm, MB in a four replicate randomized complete block
design (1 bag, 50 FDK per replicate). Bags were harvested at 6 month intervals for 24
months. Additional bags were placed at Winnipeg, Morden and Portage la Prairie, MB in
October 1999, and harvested on a monthly basis from May through September 2000. For each
treatment, 200 kernels were examined. Soil samples were collected and percent sand, silt,
and clay contents were determined using the hygrometer method.
Controlled Experiment
Under controlled experimental conditions, the survival of F. graminearum on FDK
that were buried at different depths and exposed to various temperatures was investigated,
using infected seed from two spring wheat cultivars, Roblin and AC Domain. In November
1998, bags of FDK were either left on the surface or buried at 5 cm in sterile or
non-sterile soil. The trays were placed in either: i) freezer at -10°C,
ii) coldroom at +2°C, or iii) greenhouse at +20°C. Four bags per treatment were harvested at 6 and 12 months and
examined for the presence of F. graminearum. For each treatment, 100 kernels were
examined.
PROJECT 2
Wild grasses were sampled at three locations in southern Manitoba: Living Prairie
Museum, Winnipeg (no FHB), Tall Grass Prairie Preserve, Tolstoi (low FHB), and Glenlea
Research Farm (high FHB). Grasses were collected in June, July and August (1999 and 2000)
and identified to species. Florets were surface-sterilized and incubated on potato
dextrose agar (PDA) at 20°C under continuous cool white light
for five days, after which Fusarium species growing from the florets were isolated
and identified.
Results and Discussion:
PROJECT 1
At Glenlea (1998 test), after 24 months there was little decrease in the survival of G.
zeae on FDK from the soil surface or buried at 5 and 10 cm; survival ranged from 85 to
99%. At 24 months (October 2000), FDK left on the soil surface were difficult to retrieve
due to the breakdown of the nylon bags. The remaining seeds were still intact, but few
(20/200) were recovered. Of these, 17 (85%) yielded G. zeae.
The soil at Morden was characterized as a sandy loam, at Portage a silty loam, and at
Winnipeg a silty clay loam. At Morden, Portage la Prairie, and Winnipeg (1999 test),
retrieval of FDK was high following the winter and monthly sampling (a total of 11
months). Fungal survival ranged from 97 to 100%. The kernels buried at 5 and 10 cm
from all three locations had a significantly higher rate of decomposition than the FDK
left on the soil surface (P=0.05). After 331 days, the biomass of FDK located on
the surface at all locations had decreased by approximately 50%.
Effects of Sterile vs. Non-sterile Soil and Temperature on G. zeae
Survival
on FDK under Controlled Conditions
Under controlled conditions, temperature and depth of burial did not influence survival of
G. zeae on kernels sampled from 2°C and -10°C; after 24 months survival ranged
from 98 to 100%. No statistical analysis was performed on the data from 2°C and -10°C
because no decrease in survival was observed. Survival of G. zeae on FDK located on
the surface at 20°C was 92% and 96% in sterile and non-sterile soil, respectively,
whereas survival on FDK buried at 5 cm significantly decreased after 24 months, with
values of 83% and 76%, in sterile and non-sterile soil, respectively.
Other fungal species isolated from 1% to 24% of the kernels, under both field and
controlled conditions, included F. culmorum, F. equiseti (Corda) Sacc.,
Epicoccum nigrum, Alternaria spp., and Penicillium spp. Link.
Perithecial Development
Perithecial production was observed on kernels from all locations and treatments. Under
field conditions at Glenlea, perithecia were first noted in April 1999 after 6 months
exposure on FDK, both at the soil surface and buried at 5 and 10 cm. However, no
ascospores were detected. The kernels buried at Winnipeg, Morden, and Portage la Prairie
in October 1999 did not develop perithecia until June 2000 after 9 months exposure.
Kernels at all burial depths developed perithecia, but ascospores developed only in
perithecia on the kernels left at the soil surface. Under controlled conditions, kernels
exposed to 20°C and 2°C either on the soil surface or buried at 5 cm developed
perithecia and only those incubated at 20°C formed ascospores. Few perithecia formed on
buried kernels that were incubated at -10°C and none formed on kernels at the soil
surface at this temperature.
PROJECT 2
No symptoms were observed on any of the grass species collected. Some species had other
pathogens present, including Claviceps purpurea (ergot), Stagonospora nodorum,
Septoria tritici, Pyrenophora tritici-repentis, and Cochliobolus sativus. These
last four are common leaf spot pathogens, especially of wheat. Fusarium graminearum
was isolated from 9% of the grasses examined (149/1766). Other Fusarium species
isolated included F. sporotrichioides (2.3%), F. equiseti (1.2%), F.
oxysporum (1.1%), F. avenaceum (0.9%), F. culmorum (0.8%), F. poae
(0.7%). Of these, only F. avenaceum and F. culmorum are associated with FHB.
Fusarium graminearum is the predominant species causing FHB in southern Manitoba,
accounting for more than 95% of all isolations from kernels collected in annual
surveys in recent years. F. graminearum occurred on 15 grass species, seven of
which have not been previously reported.
At Glenlea, the highest numbers of isolations were from Agropyron trachycaulum, Bromus
inermis, and Calamagrostis canadensis (Figure 1). Many of these grass species
are commonly found adjacent to fields. Fusarium graminearum was isolated from Agropyron
trachycaulum, Bromus inermis, Bromus ciliatus, Calamagrostis
canadensis, Phleum pratense, and Poa pratensis (Figure 2). The grass
species with the highest incidence of Fusarium species are also commonly found in
pastures as forage crops, which raises questions of toxin levels produced by Fusarium
species. At the Living Prairie Museum, F. sporotrichioides was the only species
isolated. Fusarium graminearum was the dominant species isolated from Glenlea and
Tolstoi, 62 and 59%, respectively. The other species ranged from 13 to 3%.
Conclusions:
PROJECT 1
In Manitoba, survival of G. zeae on seed was not diminished after 24 months and
was not influenced by burial depth, suggesting that rotations of at least two years are
necessary to avoid infection of new crops by seed-borne G. zeae under conditions of
zero-till and conventional tillage. Temperature also influenced the survival of G. zeae.
At high temperatures, survival was found to decrease after 24 months. Storing FDK at low
temperatures tended to extend the viability of G. zeae perithecia, and ascospore
development occurred mainly on FDK located on the surface of the soil providing an
inoculum source that could contribute to disease outbreaks. On buried FDK, perithecia
production was low, but fungal survival was not compromised.
Placement of FDK
influenced the rate of decomposition. Once residues decompose, duration of survival may be
reduced, but under Manitoba conditions re-isolation of G. zeae from kernels was
still possible after 24 months after burial.
PROJECT 2
Wild grass species can harbour many species of Fusarium including F.
graminearum, the predominant species causing FHB. Grass hosts are symptomless carriers
of Fusarium graminearum as well as other species of Fusarium. Wild grass
species may help bridge the gap between hosts and increase the inoculum potential.
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