Pseudomonas syringae pv. sesami: Sesame Bacterial Angular Leaf Spot and qPCR Detection

Pseudomonas syringae pv. sesami belongs to the species Pseudomonas syringae, a diverse group of Gram-negative plant pathogenic bacteria. This pathovar is highly specialized for infecting sesame plants.

The bacterium is rod-shaped and motile, possessing 2–5 polar flagella. It is aerobic and grows optimally around 30°C, with a maximum tolerance near 35°C. Colonies on media such as King’s B (KB) or SPA appear smooth, moist, and creamy white.

Like other members of the species, it can survive both epiphytically on plant surfaces and as a pathogen within host tissues, enabling persistence across growing seasons.

Seed-borne transmission is the primary source of infection. The pathogen can survive on contaminated sesame seeds for up to 11 months, making seeds the most important initial inoculum source. Infected plant residues also contribute to overwintering.

In the field, bacterial spread depends on water. Rain splash and irrigation distribute bacteria from infected to healthy plants. Entry occurs through stomata or small wounds on leaf surfaces.

Warm and humid conditions favor disease development, especially during the rainy season. High nitrogen fertilization leading to dense canopy growth further increases humidity and disease severity. Recent reports indicate the expanding global distribution of this pathogen, highlighting its emerging importance.

The pathogenicity of P. syringae pv. sesami depends on its Type III Secretion System (T3SS), which delivers effector proteins into plant cells to suppress host immunity. Recent studies suggest that epigenetic regulation, such as DNA methylation, may influence virulence gene expression and bacterial colonization.

Leaf symptoms: The most characteristic feature. Initial lesions appear as small water-soaked spots that expand into angular, dark brown to black lesions constrained by leaf veins. Lesions typically measure 2–8 mm in diameter. Under humid conditions, bacterial exudate may appear as milky droplets, later drying into shiny films. Severe infections lead to leaf yellowing, drying, and premature abscission.

Stems and petioles: Dark, elongated streaks or lesions develop, which may affect structural integrity and growth.

Capsules (pods): Infected capsules show brown lesions, and seeds inside may become shriveled and discolored, reducing oil content and quality.

The disease can affect sesame plants at all growth stages, with severe seedling infections causing basal stem blackening and plant death.

Field diagnosis is based on angular leaf lesions with dark coloration and possible bacterial exudate. Differentiation from fungal leaf spots is important, as fungal lesions are usually more circular and produce visible fungal growth rather than bacterial ooze.

Microscopic examination of infected tissue in water may reveal bacterial streaming, a rapid indicator of bacterial infection. Isolation on suitable media allows observation of colony morphology.

Definitive identification relies on molecular techniques. PCR targeting conserved genes such as 16S rRNA and housekeeping genes (e.g., gyrB, rpoD) enables accurate identification. Probe-based real-time qPCR provides highly sensitive and specific detection, supporting early diagnosis and disease monitoring.

Effective management follows an integrated approach emphasizing prevention. The use of disease-free seeds and seed disinfection is critical. Treatments such as hot water soaking or chemical disinfectants can significantly reduce seed-borne inoculum.

Field management practices include crop rotation with non-host species, proper plant spacing, raised beds for drainage, and balanced fertilization to avoid excessive nitrogen. Removal of infected leaves and plant debris helps reduce pathogen reservoirs.

Chemical control may be applied at early stages or during high-risk conditions. Copper-based bactericides and antibiotics can be used, but should be rotated and carefully managed to prevent resistance.

Emerging research highlights plant-derived compounds such as erucamide that can inhibit bacterial virulence systems like T3SS, offering promising directions for environmentally friendly disease control strategies.