Abstract:

The effect of delayed harvest on the occurrence and incidence of seed-borne Fusarium
spp. and their effects on seed quality was investigated using four maize cultivars
(Pioneer 3551, 3591, 3709 and 3475) over two seasons (1989/90, 1990/91) at Massey
University, Palmerston North. As harvest was delayed from April to July, the
percentage of cobs showing Fusarium mould increased. Cultivar 3551 tended to
develop Fusarium cob mould later in the season (June) than the other three cultivars.
In both seasons the percentage of seeds of all four cultivars infected with
Fusarium spp. increased as harvest was delayed. However, there was a difference
between the two seasons; in 1989/90 the mean percentage of seeds carrying
Fusarium spp. was 26%, 39%, 70% and 82% for April, May, June and July harvests
respectively, while the corresponding levels for 1990/91 were 1%, 9%, 31% and
40% respectively. Between season differences were ascribed to climatic differences,
the former season being wetter and warmer than the latter. There were only minor
differences among cultivars for the percentage of seeds carrying Fusarium spp. F.
graminearum was the species most consistently detected in all cultivars in both
seasons, being recorded from 16%, 31%, 53% and 72% of seeds from the 1989/90
April to July harvests respectively, and from 0%, 6%, 25% and 30% of seeds from
the same harvest times in 1990/91. F. subglurinans, F. poae and other Fusarium spp.
were also detected, but their incidence was generally low.
Seed-bome Fusarium did not significantly reduce seed germination or vigour.
In both seasons germination was between 86-99% for all cultivars. However, any
dead seeds bore evidence of F. graminearum mycelial growth. Mycotoxins were
recorded in seeds from all harvests in both seasons and mycotoxin levels increased
as harvest was delayed. However, there were differences between seasons, as mean
levels of Zearalenone, aZearalenol, Nivalenol and Deoxynivalenol ranged from 0.06 -
1.42 mg/kg seed in 1989/90, but from 0.0 - 0.54 mg/kg seed in 1990/91. In all
cultivars and at most harvests in both years, levels of aZearalenol and of Nivalenol
increased earlier than those of Zearalenone and Deoxynivalenol. Mycotoxin
differences among cultivars and the precise nature of the relationship between specific
Fusarium species and mycotoxin development urgently requires further study, because
of the potential for human and animal health problems.
Fusarium spp. from seed-culture colony were initially identified
macroscopically on Malt Agar (MA), with pure cultures later being verified by the
International Mycological Institute (UK). Subsequently, cultures were studied or
Potato Dextrose Agar (PDA), Malt Extract Agar (MEA) and on Carnation Leaf Agar
(CLA), with the final identity of seed-culture colonies being verified on CLA.
Colony texture and colour (including agar pigmentation) were initially used
to separate Fusarium species detected on MA from infected seeds after harvest into
a series of groups, ie ’red and fluffy’, ’red centre’, ’red and lobed’, ’cream and
fluffy’, and ’cream and lobed’ for F. graminearum. F. crookwellense was also
separated as a ’red centre’ type of colony while F. culmorum was separated as ’cream
and flat’, F. subglutinans ’purple and strands’ type, and F. poae as
’purple/white/cream and powdery’ type. While it was possible to differentiate the
five types of F. graminearum on MA, it was not possible to distinguish F.
graminearum ’red centre’ type from F. crookwellense, although F. culmorum was
relatively easy to differentiate from F. graminearum and F. crookwellense. Use of
PDA or MEA pure cultures to differentiate F. graminearum from F. crookwellense
or F. culmorum was not successful because the colony morphology of these three
species was similar. However, F. subglutinans and F. poae were readily identified
macroscopically on MA and MEA.
F. graminearum seed-culture colonies which did not sporulate on MA or MEA
in most cases readily formed perithecia of Gibberella zeae on CLA (in the presence
of 40W NUV light) regardless of whether the cultures were initiated by single
germinated spores or by mass transferred inoculum. Those colonies which did
sporulate on MA or MEA formed abundant sporodochia on CLA but not perithecia.
CLA was also used to identify F. graminearum (G. zeae) from maize seeds or
seedlings by direct plating of these structures after surface disinfection, Full
descriptions of the Fusarium colonies on the various media used are presented.
Fusarium survival in seed during storage depended upon seed moisture content
(SMC) and storage temperature. F. graminearum was eliminated from seed at 14%
SMC stored at 30°C and 25°C after 3 or 6 months storage, respectively, but survived
at low levels (1-5%), together with F. subglutinans (1-7%), F. poae (1-2%) at these
temperatures and 10% SMC. F. subglutinans and F. poae in seeds at 14% SMC did
not survive after 9 months storage at 30°C. In seed stored at 5°C, Fusarium spp.
infection levels did not decline after 12 months of storage at both 10 and 14% SMC.
These results suggest a possible control strategy for producing Fusarium free seed,
providing seed moisture content is not greater than 10%. At a storage temperature
of 30°C, the post-storage germination of seed at 14% SMC had dropped to under
10% within 3 months, but seed at 10% SMC maintained its germination (88-97%)
throughout the storage trial. After 12 months seed storage at 5°C (sealed storage) or
25°C (open storage), mycotoxin levels were similar to pre-storage levels.
The requirements of Koch’s postulates were fulfilled in demonstrating that
seed-borne F. graminearum was transmitted from maize seeds to seedlings under
aseptic conditions in a glasshouse maintained at a temperature of 14°C to 17°C. The
mean transmission rate (48%) was similar to the original seed-borne inoculum which
suggests that under favourable environmental conditions, the pathogen will be
effectively transferred from the seed to seedlings. F. graminearum had little effect
on seedling emergence or survival, but was associated with a high percentage of
seedlings with scutellum-mesocotyl/scutellum-main root lesioning. In the field, F.
graminearum was consistently isolated from seedlings, but seed transmission could
not be confirmed because of the presence of soil-borne inoculum, ie the pathogen was
isolated from up to 37% of seedlings from a seed lot which carried only 1% seedborne
inoculum.
F. subglutinans was also proved to be seed transmitted under the same
glasshouse conditions as described for F. graminearum. The significance of surface-
borne inoculum of this pathogen was demonstrated in that the mean transmission rate
for non-surface disinfected seed lots was 81%, whereas it was only 7% for surface
disinfected seed lots. F. subglutinans was associated mainly with ’above sand level’
seedling infection (coleoptile-node infection, leaf/shoot blight, shoot wilt and seedling
stunting). However, F. subglutinans was rarely detected in seedlings from the field,
possibly because of the antagonistic effects of mycopathogenic fungi such as =
Gleocladium roseum.
These results are discussed, particularly in relation to the significance of F. I
graminearum and F. subglutinans as seed-borne pathogens of maize, and the i
difficulties inherent in the identification of Fusarium spp. following seed health |
testing. It is likely that these seed-borne Fusarium spp. are more important because !
of their association with mycotoxins, than with any effects they have as an inoculum
source for diseases of maize.