Loose smut of sorghum occurs in Africa, Southern Europe, USA, and Asian countries like China, Iran, and India. This disease is less common than the grain smut and closely resembles the latter in its appearance and sometimes mistaken for it.
In India, the disease is distributed in Andhra Pradesh, Maharashtra, Karnataka, Madhya Pradesh, and Tamil Nadu. The disease not only affects the grains but also renders reduced plant growth. Thus, both grain as well as fodder yield are reduced.
Symptoms of Loose Smut Disease:
The diseased plants are shorter than the healthy ones and possess thinner stalks and marked tillering. Thus, the affected plants can easily be detected before the ears come out. Also, the ears come out earlier than in normal plants and are more loose than the healthy ones. The glumes are hypertrophied.
Normally, all the spikelets of the head are affected. Smut sori may be borne on glumes and pedicels, besides the essential organs of the spikelets. Frequently, the lemma and plea as well as the ovary contain smut sori. The covering membrane of the sorus ruptures easily releasing the powdery mass of smut spores. The columella becomes exposed and persists after spores are released. The size of the sorus is variable and depends upon the varieties infected.
Causal Organism of Loose Smut Disease:
The mycelium is dikaryotic. The smut spores are round or shortly elliptical, dark brown, with echinulate wall, surrounding un-branched well-developed columella, and measure 5-10 µm in diameter. They germinate and give rise to 4-celled pro-mycelium (basidium) with laterally borne sporidia (basidiospores). The pro-mycelium may develop into un-branched or branched hyphae also without developing sporidia.
Sporidia are formed usually at lower temperatures while the direct germination of pro-mycelium is common at higher temperatures. At least, three physiologic races of this species are known. S. cruenta possesses two sex groups and can hybridize with S. sorghi and S. reiliana. However, the fungus can be easily cultured on agar media such as potato dextrose agar and Czapek’s agar.
Loose Smut Disease Cycle:
The pathogen is externally seed-borne. It may be soil-borne also to some extent in dry soils. The smut spores retain viability for four years in dry conditions. Primary infection of seedlings takes place at the time of seed germination and before emergence of seedlings.
Germ tubes developed by infection inoculum enter into the seedlings through the radicle, mesocotyl, or hypocotyl. Further development of the pathogen in the host is on the same pattern as in other systemic smuts. Spread within the crop by floral infection of air-borne smut spores (i.e. secondary infection) has been recorded.
Predisposing Factors:
Smut spores best germinate between 18° to 32°C temperature, while sporidia germination is favoured by temperatures on the lower side. Low temperature and low soil moisture as well as deep sowing favour infection.
Management of Loose Smut Disease:
(i) Crop rotation and field sanitation are highly recommended in dry soil conditions where soil survival of smut spores is possible.
(ii) Other measures used to manage grain smut disease of sorghum are almost equally effective in controlling loose smut of sorghum.
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