Leaf spot of sorghum

The initial symptoms of the disease appear on sorghum leaves as small chlorotic spots that gradually develop a brown or reddish-brown coloration.

A distinctive characteristic is the elongated, elliptical shape of the spots, which align parallel to the leaf veins, creating a ladder-like appearance.

Under conditions of high humidity, a grayish or olive-colored fungal growth, representing the conidial sporulation, becomes visible on both sides of the leaf spots.

As the disease progresses, these spots may coalesce, covering a significant area of the leaf blade, eventually leading to premature senescence and tissue necrosis.

Infection typically initiates on the lower leaves and gradually spreads upward along the stem throughout the growing season.

The disease is caused by the phytopathogenic fungus Passalora fusimaculans (formerly and commonly known as Cercospora fusimaculans).

This organism is classified among the imperfect fungi and produces conidia, which serve as the primary source for the rapid spread of the infection in fields.

The fungus can survive for extended periods in plant debris and stubble, remaining in the soil or on the field surface after the harvest.

The pathogen is disseminated primarily through wind, rain splashes, and physical contact between healthy tissue and infected crop residues.

It exhibits a high level of host specificity, infecting primarily species within the Sorghum genus and significantly impacting plant physiology.

The development of leaf spot is most favored by periods characterized by high atmospheric humidity, frequent rainfall, and moderate temperatures.

Optimal conditions for fungal spore germination and successful plant tissue infection occur within the temperature range of +20°C to +28°C.

High planting densities create an microclimate with stagnant air and elevated humidity, which is highly conducive to the proliferation of the pathogen.

Poor agricultural practices, such as minimal crop rotation and inadequate management of crop residues, significantly increase the risk of disease incidence.

Periods of persistent fog and heavy evening dew are critical factors that promote intensive sporulation and secondary cycles of infection.

The primary impact of the disease is the substantial reduction of the effective photosynthetic surface area of the leaves.

Severe infection disrupts metabolic processes, weakening the overall vigor of the plant and reducing its ability to withstand environmental stress.

The premature loss of foliage leads to a decrease in biomass accumulation, which directly impacts grain yield and the quality of forage crops.

Affected plants often produce inferior grain, characterized by lower test weights and reduced starch accumulation in the developing kernels.

In severe epidemic years, crop losses can reach 10 to 25 percent, making it a significant concern for sorghum production efficiency.

The fundamental cultural practice for managing this disease is the implementation of a strict crop rotation cycle, with sorghum returning to the same site every 3–4 years.

Effective management includes deep plowing and thorough incorporation of crop residues into the soil to promote decomposition and reduce the initial inoculum.

Selecting and planting resistant or tolerant hybrid varieties is considered the most sustainable and cost-effective approach to long-term disease management.

Maintaining optimal plant spacing helps prevent canopy microclimates from becoming overly humid, thereby limiting the potential for fungal development.

If necessary, the timely application of systemic fungicides, such as triazoles or strobilurins, can be used to suppress the disease and protect the photosynthetic canopy.