Oval leaf spot of sorghum

The initial symptoms of the disease appear on the leaves as small, circular or oval spots that gradually enlarge over time.

The spots are often tan or straw-colored, typically surrounded by a distinct dark brown or purple margin, which serves as a diagnostic feature.

Under conditions of high humidity, the centers of older spots may develop fungal structures, appearing as tiny gray or whitish dots on the leaf surface.

As the infection progresses, individual spots can coalesce, covering large portions of the leaf blade and leading to premature necrosis of the tissue.

Infection usually starts on the lower leaves and progresses upwards, significantly reducing the green surface area of the sorghum plant.

The disease is caused by the fungus Ramulispora sorghicola, which belongs to the group of imperfect fungi.

The pathogen overwinters in crop residues, infected stems, and seeds, serving as the primary inoculum source for the following growing season.

Primary infection is initiated by conidia, which are dispersed to healthy plants by wind, splashing rain, or insect activity.

The biological cycle of the fungus is closely synchronized with the sorghum growing season, with multiple infection cycles possible under favorable conditions.

The pathogen is known to affect various types of sorghum, including grain sorghum, sweet sorghum, and sudangrass, making it a widespread concern.

Development and spread of the disease are favored by high relative humidity and moderately warm temperature conditions.

Frequent rainfall, persistent dew, and high atmospheric moisture provide the necessary conditions for spore germination and mycelial colonization of plant tissues.

High-density planting that limits air circulation promotes moisture retention within the canopy, thereby accelerating the spread of the disease.

The fungus thrives at temperatures ranging from 20–25°C, although it remains capable of limited activity in a wider range of thermal conditions.

Poor field sanitation, including the presence of crop debris from previous harvests, significantly increases the likelihood of severe outbreaks.

The main impact of oval leaf spot is the reduction of functional leaf area, which severely limits the plant's photosynthetic capacity.

Severe infection leads to water stress, early leaf death, and impaired grain filling, resulting in substantial losses in overall crop yield.

In addition to grain yield losses, the nutritional quality of the forage or silage produced from infected plants can be significantly degraded.

Under conducive environmental conditions, yield losses ranging from 15% to 25% have been reported in heavily infested sorghum fields.

Plants weakened by the disease exhibit increased susceptibility to secondary pathogens and pests, complicating overall field management.

Adopting a proper crop rotation strategy is essential to break the survival cycle of the pathogen in the soil and on crop residues.

Thorough tillage practices that bury plant debris deeply into the soil help accelerate decomposition and reduce the initial inoculum pressure.

Selecting and planting resistant or tolerant sorghum cultivars is considered the most effective and sustainable management practice.

Seed treatments with systemic fungicides are recommended to protect seedlings during the early stages of development and prevent early infection.

Foliar fungicide applications during the vegetative stage may be necessary in high-pressure years, following local regulations and guidelines.