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Bunt, or stinking
smut |
Causal organism - Tilletia tritici
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The spores on the seed surface germinate with the seed. Each produces a short fungal thread terminating in a cluster of elongated cells. These, after a process of conjugation, produce secondary spores. These infect the coleoptiles of the young seedlings before the emergence of the first true leaves. The mycelium grows up with the shoot eventually to infect the developing ear. Affected plants develop apparently normally until the ear emerges when it can be seen that grain sites have been replaced by bunt balls containing the masses of spores which will contaminate the grain at harvest.
Because each bunt ball contains millions of spores the capacity for contamination of healthy grain in the same field is enormous. Thus, if seed is continually saved and re-sown without treatment the disease can build up very rapidly. Dry spores can survive for several years. Harvesting or handling equipment contaminated by spores from an infected crop can thus serve to introduce the pathogen into seed lots harvested in the following season. Wind blown spores, particularly from late-harvested crops, can contaminate neighbouring fields and survive on bare soil ready to infect the next crop. In damp soil spores usually germinate and then, in the absence of the host plant, die. In dry seasons, however, they may (especially if they are protected within the glumes of shed ears) survive in the soil from the harvesting of one crop to the sowing of the next. Long-term survival of bunt spores in soil in Sweden was first reported in 1990 but this report conflicted with other results from many researchers who generally agree that the viability of bunt spores in soil is very short, particularly if the soil is moist. There are cases in France and Denmark where bunt spores have survived overwinter to affect subsequent crops in 'normal' soils i.e. not very dry soils. Similar cases have occurred in the UK. These cases confirm that overwintering of bunt spores (probably as intact bunt balls encased in glumes on intact ears) is possible.
