Pseudomonas syringae pv. mori: Mulberry Bacterial Blight and qPCR Detection

Pseudomonas syringae pv. mori belongs to the domain Bacteria, phylum Proteobacteria, class Gammaproteobacteria, order Pseudomonadales, family Pseudomonadaceae, and genus Pseudomonas.

It is classified as a pathovar of Pseudomonas syringae, distinguished by its host specificity toward mulberry (genus Morus, family Moraceae). This host-adapted classification reflects its co-evolution with mulberry plants.

The bacterium is a Gram-negative, short rod measuring approximately 0.5–1.0 × 1.5–3.0 μm, with one or more polar flagella enabling motility. On nutrient agar (NA) or King’s B (KB) medium, colonies appear circular, smooth, moist, and milky white after incubation at 28°C for 48 hours. Many strains produce fluorescent pigments under UV light.

It is strictly aerobic, with an optimal growth temperature of 25–28°C and a maximum tolerance around 34°C. Biochemically, it is typically oxidase-negative and catalase-positive. It can utilize sugars such as glucose, sucrose, and mannitol but does not hydrolyze starch. Indole production, methyl red, and Voges–Proskauer reactions are generally negative.

Infection begins when the bacterium enters plant tissues through stomata, hydathodes, or wounds. Once inside, it colonizes intercellular spaces and activates a complex virulence program.

A key mechanism is the Type III Secretion System (T3SS), which delivers effector proteins into host cells to suppress immune responses, alter cellular metabolism, and promote bacterial survival.

The pathogen also produces toxins (such as syringomycin-like compounds) that disrupt plant cell membranes, leading to leakage of cellular contents and providing nutrients for bacterial growth. Some strains may produce phytohormone-like compounds, further disturbing host physiology.

In response, mulberry plants activate innate immune defenses, including reactive oxygen species (ROS) production and defense gene expression, though these responses are often insufficient to halt disease progression.

Leaf symptoms: Initial lesions appear as water-soaked, dark green spots on young leaves, which expand into angular or irregular brown to black lesions often surrounded by yellow halos. Under humid conditions, bacterial exudates may appear and later dry into shiny films. Severe infections lead to leaf blight and defoliation.

Shoot symptoms: Young shoots develop water-soaked streaks that later become sunken, cracked, and form dark cankers. Severe infections can cause dieback of shoots.

Overall impact: Infected mulberry plants exhibit reduced vigor, leading to decreased leaf yield and quality, which negatively affects silkworm rearing.

Diagnosis combines field observation with laboratory methods. Traditional approaches include isolation, morphological and biochemical characterization, and pathogenicity testing. Molecular techniques such as PCR and real-time qPCR targeting specific genes (e.g., 16S–23S rRNA ITS or virulence genes) provide rapid and accurate detection.

The pathogen overwinters in infected branches, fallen leaves, plant debris, and soil. In spring, under favorable conditions, it becomes active and spreads to new tissues via rain splash, irrigation water, and agricultural activities.

Disease development is strongly influenced by environmental conditions. Warm (20–28°C), humid, and rainy weather promotes outbreaks. Poor drainage, dense planting, excessive nitrogen fertilization, and inadequate ventilation increase disease severity.

Significant differences in resistance exist among mulberry cultivars, providing opportunities for breeding and selection.

Management relies on an integrated strategy combining cultural, biological, and chemical approaches.

Agricultural practices: Use resistant varieties where available. Optimize planting density, pruning, and ventilation. Apply balanced fertilization with adequate phosphorus and potassium. Remove and destroy infected plant material during dormant seasons to reduce inoculum.

Biological control: Beneficial microorganisms such as Bacillus spp., Pseudomonas spp., and actinomycetes can suppress the pathogen through competition, antibiosis, or induction of host resistance.

Chemical control: Copper-based bactericides and antibiotics (e.g., kasugamycin, validamycin) may be used during early disease stages. Rotation of active ingredients is recommended to prevent resistance development and ensure safety in sericulture systems.