Phyllosticta leaf spot of sorghum

Taxonomic classification and nature of the pathogen: Phyllosticta leaf spot is caused by the fungus Phyllosticta sorghina (also recognized as Phoma sorghina). This pathogen is a member of the Fungi kingdom, phylum Ascomycota. It acts as an obligate or facultative parasite, primarily attacking the leaf tissues of sorghum crops, leading to necrotic spots and reduction in photosynthetic capacity.

Diseases and host plants: The pathogen causes the characteristic leaf spot disease of sorghum. It primarily affects grain sorghum, sweet sorghum, and forage varieties. Beyond sorghum, the fungus can survive on various wild grass species, which serve as alternative hosts for the pathogen during the off-season.

Biology and life cycle: Phyllosticta sorghina overwinters on crop residues and in the soil as pycnidia. When favorable environmental conditions arrive, pycnidia produce conidia that are disseminated via rain splashes and wind currents. The fungus infects the host through stomata or wounds, initiating the localized necrosis on leaves.

Conditions for development and impact: High humidity, frequent rainfall, and moderate temperatures (ranging between 20°C and 25°C) significantly promote the spread and severity of the disease. The primary economic impact involves reduced leaf area, which limits carbon assimilation, causes premature leaf senescence, and results in lower grain yield and poor forage quality.

Management and control strategies: Controlling this pathogen requires a multi-faceted approach focusing on reducing the initial inoculum and limiting environmental favorability:

• Implementing crop rotation to break the cycle of infection.
• Deep plowing and residue management to bury infected plant debris.
• Selecting resistant or tolerant sorghum hybrids.
• Applying foliar fungicides if disease incidence thresholds are reached.
• Managing weed populations around the field to eliminate alternative host reservoirs.