Abstract
Incidence and distribution of seedborne Bipolaris
sorokiniana throughout the wheat growing areas of Bangladesh have
been studied. Altogether 118 seed samples of two widely
cultivated wheat cultivars - Kanchan and Sonalika from different
wheat growing districts were collected, tested and screened for
different levels of infection by B. sorokiniana. There were
highly significant differences in the number of seed samples at
different infection levels. The maximum number of seed sample was
recorded in the range 3-21 per cent infection. Significant
correlations between seedborne infection and subsequent disease
development in plants were obtained. Germination of the seeds
significantly decreased with the increase of infection level and
as much as 71.5% and 68.0% reduction was recorded in the two
cvs. respectively, at the maximum infection level of 90%. Seed to
plant to seed transmission of the pathogen and its effect on
disease development and yield was studied both in vitro and in
vivo. Seed rot and/or germination failure, pre- and postemergence blight, foot rot, coleoptile infection, stunting growth
and poor vigour, black point or embryo blight diseases were
recorded as the effect of seed to plant transmission of the
pathogen. Seed rot was positively correlated with the level of
seedborne infection. There was a highly significant trend of
total disease development with the increase of seedborne
infection level for both the cultivars. The nature and behaviour
of the seed to plant to seed transmission of B. sorokiniana along
with its effect on disease development have been studied under
controlled conditions. The pathogen overwintered as resting
mycelium mainly in pericarp. During germination of the seeds the
pathogen got its transmission from seed to the growing plant both
intra- and inter-cellularly through the coleoptile which was
exclusively the first and primary phase of all the subsequent
disease development in plants. The infection resulted progressive
destruction of the host cells and tissues which were lignified
and killed and secondary invasion occurred mainly through the
vascular vessels. Before penetration into the adult stages of the
plant the pathogen usually grew and proliferated in the space
between coleoptile and first leaf sheath usually till anthesis.
Significantly increased infection severity was resulted with the
increase in age of the seedlings under controlled condition in
vitro. Regressions of infection severity on different growth
characters- number of leaves per plant, shoot length, number of
root per plant and root length showed positive relationship with
the seedling age. As much as 63.85% seedlings were found affected
with almost stagnancy of their growth within 21-28 days. After
anthesis the pathogen was found to be activated and transmitted
gradually upwards infecting leafsheath, leaf, node and internode
resulting leaf blight disease. The pathogen has been successfully
recovered by different techniques from different parts of the
infected plants including rachis, lemma, palea and kernel
confirming the plant to seed transmission of the pathogen. The
disease severity increased significantly with the increase of
seedborne infection level and varied depending upon the location,
growing season and cultivar, ultimately affecting the yield which
was also gradually decreased with the increase of seedborne
infection level in both the cultivars. The highest yield
reduction of 57.6% and 64.5% was recorded for the cvs. Kanchan
and Sonalika respectively. Yield performance of the cv. Kanchan
was better in almost all locations and years as compared to cv.
Sonalika. Threshold levels of seedborne infection fixing at 14%
for the cv. Kanchan and 5% for the cv. Sonalika have been
interim
recommended as the seed standards for the production of quality
seeds as well as proper management of the disease(s) caused by
the pathogen.