The optimum temperature for growth is 28–32°C, the minimum temperature for growth is 9°C, and the maximum temperature for growth is 37°C (Fig. 2). 

Asexual Structures

Sporangiophores:

Sporangiophores are sympodial.

Sporangia:

Sporangia were described as semipapillate by Katsura (1976), but Ho et al. (1984) and Ho (1986) restudied the species and found sporangia to be nonpapillate and noncaducous and to produce new sporangia by internal proliferation (Ho et al., 1995). Sporangia are ovoid and obpyriform, with rounded bases, and can have various shapes. Sporangia are 23–46 × 33–75 µm (average 33.6 × 48.8 µm) (Fig. 3B–H).

Chlamydospores:

Katsura (1976) described chlamydospores as spherical, often intercalary, and 16.3–52 µm in diameter (average 31.2 µm). However, Ho et al. (1984) did not find chlamydospores for P. melonis (Fig. 1).

Hyphae:

Hyphal swellings are not reported by Katsura (1976).

Sexual Structures

P. melonis was originally considered homothallic by Katsura (1976) but heterothallic by Ho et al. (1984).

Antheridia:

Antheridia are amphigynous and often spherical.

Oogonia:

Oogonia are 27.5–35.0 µm in diameter.

Oospores:

Oospores are 25–32.5 µm in diameter (average 29.3 µm) and aplerotic (Fig. 3I and J).

Host Range and Distribution

P. melonis is the cause of late blight or foot rot of cucumber (Cucumis sativus). The pathogen also affects watermelon and pumpkin.

Symptoms

P. melonis causes infection in leaves, stems, and fruits of cucumber plants. The fungus initiates infection in the collar region, which turns greenish brown and becomes soft. The stem then shrinks, and the plant collapses. Lesions on the leaves turn darker green than the rest of the leaf and progress toward the pedicel. Fruit infection is initiated at the tip and leads to dark green, water-soaked lesions. Eventually a soft rot of the fruit develops. If moist conditions prevail, the fruit surface exhibits fungal mycelium and powdery masses of sporangia.

References