Covered kernel smut of sorghum

Covered kernel smut, caused by the fungus Sporisorium sorghi, is a significant disease affecting various types of sorghum, including grain, forage, and sweet sorghum. This basidiomycete fungus acts as a systemic pathogen, infecting the plant at the seedling stage and eventually colonizing the developing reproductive structures of the host.

The disease cycle begins primarily with teliospores that contaminate the surface of sorghum seeds or persist in the soil. Once the seed germinates, the fungus penetrates the developing sprout. Because the infection is systemic, it moves with the growing point of the plant and remains largely invisible until the plant begins to produce heads or panicles.

The primary symptom is the replacement of the grain kernels with smut sori, also known as smut galls. These structures are elongated, cone-shaped, and covered by a persistent, tough, grayish-brown membrane. Unlike loose smut, the membrane in covered smut remains intact for a longer period, protecting the mass of dark-brown teliospores inside until the threshing process.

Environmental conditions play a critical role in the development of the disease. The fungus thrives when soil temperatures are moderate, typically between 15°C and 25°C, during the germination of the seed. Cool and moist soil conditions can delay seedling emergence, providing the pathogen with a wider window of opportunity to infect the vulnerable seedling tissues.

The economic impact of covered kernel smut is characterized by significant grain yield losses. Furthermore, the presence of smut spores reduces the quality of the harvested grain, making it unsuitable for feed or industrial purposes due to contamination. To mitigate these risks, agricultural producers should implement the following control strategies:

• Apply effective systemic fungicides as a seed treatment before planting to eliminate surface-borne spores.
• Utilize resistant or tolerant sorghum hybrids developed by breeding programs.
• Ensure timely planting in warm soil to promote rapid seedling establishment, which limits the susceptibility period.
• Maintain clean harvesting equipment and storage facilities to prevent the cross-contamination of healthy seed lots.