| The most interesting finding from the distribution of the infected florets
in the intact spikelets and spikes is that the infected florets and spikelets
are clustered around the spikelets of primary infection in each row and
direction and are not distributed randomly as reported earlier. It was also
found that the extent of damage to the seed from Karnal bunt reduced progressively
as the distance of the infected florets increased from the primary infection
sites. The spikelets at the primary infection site had mostly 100% of the
florets infected while the infected spikelets away from the primary infection
sites had some healthy grains. This indicates that the Karnal bunt disease
spreads systematically to the adjacent florets and spikelets in each
row in both directions of the spike from the primary infection site. Under
the favourable environmental conditions the disease would spread to more
spikelets and cause greater damage to infected grains. In 1981 when the
environmental factors were more favourable, not only the spikes had higher
number of primary infection sites but also had greater spread of the disease
to the adjacent spikelets than in 1982. There are 1-4 florets in a spikelet. It takes about 7 to 10 days for the completion of anthesis within a spikelet as it first starts in the floret nearest to the rachis and then in the 2nd, 3rd and 4th florets in each spikelet. The fact that 100% of the florets in the spikelets at primary infection sites and some adjacent spikelets are infected irrespective of 7 to 10 days difference in their anthesis, the probability of infection of individual florets in a given spikelet, with all infected florets, by separate sporidia at the time of anthesis is very very small as compared to the observed frequency, confirming that the spread of disease after primary infection is highIy systematic. The infected florets (grains) away from the primary infection site at different stages of grain development, such as dough had only a small black spot at the embryonic end indicating that the infection had just started. This shows that the secondary infection can probably take place at any stage of caryopsis development after anthesis. From the observations reported here it can be concluded that Karnal bunt spreads very systematically to different florets and spikelets from the primary infection site (floret) and the secondary infection can take place at any stage of carropsis development after anthesis up to dough stage. References BEDI, K.S., SIKKA, H.R. & MUNDKUR, B.B. 1949. Transmission of wheat bunt due to Neovossia indica (Mitra) Mund. Ind. Phytopath 2: 20-26. |
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