Phoma leaf spot of sorghum

Phoma leaf spot, caused by the fungus Phoma insidiosa, is a common foliar disease affecting sorghum crops globally. The pathogen acts as an opportunistic fungus that colonizes leaves, sheaths, and sometimes stalks, leading to significant yield losses if left unmanaged in conducive environmental conditions.

The host range of Phoma insidiosa primarily includes various sorghum species (Sorghum bicolor). The fungus is a necrotrophic pathogen, meaning it kills plant tissue to obtain nutrients. It survives in crop debris and contaminated soil, acting as a primary source of inoculum for the subsequent growing season in susceptible cultivars.

Symptoms typically begin as small, irregular, or circular spots on lower leaves. These lesions often show a distinct brown border with a lighter tan center. As the infection matures, numerous small, black, spherical structures known as pycnidia develop within the dead tissue of the lesion, which serve as sites for asexual spore production.

Environmental conditions play a critical role in the development of Phoma leaf spot. High humidity, frequent rainfall, and moderate temperatures ranging from 20°C to 25°C are ideal for the release and germination of conidia. Splash dispersal from rain or irrigation systems spreads the spores to upper leaves, facilitating the epidemic development of the disease throughout the field.

The economic impact of this disease is significant, as severe infections result in premature senescence of leaves, which reduces the plant's photosynthetic capacity and ultimately leads to shriveled grains and reduced biomass. Integrated pest management (IPM) is essential for control, focusing on using disease-free seeds, rotating crops to non-host species, burying infected crop residues to accelerate decomposition, and applying fungicides when weather conditions favor disease progression.