Night-flowering jasmine (Nyctanthes arbor-tristis) is also known as Parijat jasmine and "tree of sorrow" because the flowers fade throughout the day (Rani et al., 2012). The flowers are fragrant, with five to eight lobed white corollas with an orange-red center. They are formed in clusters of 2 to 7, with individual flowers opening at dusk and finishing at dawn (Singh et al., 2018). The fruit is a flat brown heart-shaped to round capsule 2 cm in diameter, with two sections each containing a single seed. The flowers can be used as a source of yellow dye for clothing. Such an important ornamental crop is infected by powdery mildew disease. It can be recognized by the fine white or dark spots on the leaves of the plant, which will quickly spread to the stems. It leads to flowers dropping and the aesthetic value being reduced. Finally, the yield is reduced by about 60%. On the basis of macroscopic and microscopic analysis, causal organism was reported as Oidium sp. (Erysiphe sp.). The mildew can be to some extent eliminated by rubbing the leaves together, however, this won`t destroy the issue totally. The best treatment is to spray the plant week by week with a fungicide spray.

Taxonomical classification

Domain: Eukarya

Kingdom: Fungi

Division: Ascomycota

Class: Ascomycetes

Order: Erysiphales

Family: Erysiphaceae

Genus: Oidium; (Telomorphic name: Erysiphe sp.)

Symptoms:

Initially, on leaf circular, powdery white spots appear subsequently covering the entire leaf by a dusty-white to grey coating and talcum powder-like growth. Powdery growth usually is found on the upper surface of the leaves. It also infects the lower surface of the leaf, young stem, bud, flower, and young fruit. The infected leaves become distorted, turn yellow with small patches of green, and finally fall off prematurely.

Etiology:

Oidium sp. was identified as the causative organism through macroscopic and microscopic examination (Fig. 3 and 4). Oidium sp. prefers to grow on plant surfaces as an epiphyte. Hyphae occur on the upper leaf surfaces during the growth season. Haustoria are specialized absorption cells that extend into the epidermal cells of the plant to obtain nourishment and create epiphytic mycelium on the leaf surface. On specialized hyphae called oidiophores, oidium grow individually or in chains (Fig. 5). The epiphytic hyphae or the leaf stomata produce oidiophores. Powdery mildew fungi generate sexual spores known as ascospores in a sac-like ascus contained in a fruiting body called a cleistothecium at the end of the growing season. The cleistothecium is spherical with no natural opening; asci with ascospores are discharged when a rupture occurs in the fruiting body`s wall. Hyphoidal appendages appear on the cleistothecia`s surface.

Epidemiology:

Powdery mildew grows quickly in situations that promote it, such as dense plant growth, warm temperatures (28-30 degrees Celsius), low light intensity, and high relative humidity (80 %). Oidia (asexual spores) production is best around 20°C and rapidly diminishes above and below that temperature (Ward and Manners, 1974). However, spore germination does not require free water on leaf surfaces, and disease development can occur in the presence or absence of dew. Dry weather will not necessarily inhibit the spread of this disease, unlike other fungal diseases. The disease is propagated by ascospores or oidia (secondary spores) and survives in infected plant waste in the form of cleistothecium (resting structures). During periods of heavy rain, disease development is slowed (Merchan and Kranz, 1986).