Diseases of Vegetable, Ornamental and Spice Crops

Exercise 4: DISEASES CAUSED BY POWDERY MILDEW FUNGI

a) Powdery mildew of pea/ fenugreek / coriander (Causal organism Erysiphe pisi DC., Syn: E. polygoni DC.)

Kingdom : Fungi
Phyllum : Ascomycota
Class : Ascomycetes
Order : Erysiphales
Family : Erysiphaceae
Genus : Erysiphe
Species : pisi/ polygoni

Symptoms: Small white patches appear on upper surface of leaf, petioles, tendrils, stems and pods. These patches coalesce together and cover entire plant surface. The plants look as if sprayed with wheat flour from distance. The affected pods also show white floury patches consisting of white powdery mass which first turn light brown and finally dark brown in colour and become necrotic. After infection, the pods remain small in size, shrivel and pod number and pod weight are also reduced.

Mycelium: Ectophytic, branched, septate, hyaline sending haustoria into the epidermal cells to draw nutrients.

Asexual reproduction: Conidiophore short, septate, arise vertically on superficial hyphae. Conidia are ovate, barrel-shaped, hyaline, and unicellular in chains (Fig 1).

Sexual reproduction: Cleistothecia appear as dark coloured, round structures, sharp, minute bodies scattered in the mycelial web, bears unbranched myceloid appendages with free ends (Fig. 1(a)). Asci, ovate, nearly sessile and originate from a single point, 3-8 ascospores, which are elliptical, hyaline and unicellular.

Fig 1 (a) Microscopic details of Erysiphe (b) Conidiophores of Erysiphe

b) Powdery mildew of cucurbits (Causal organism Erysiphe cichoracearum DC. and Sphaerotheca fuligenia (Schlecht) Poll.)

Kingdom : Eumycota
Phyllum : Ascomycota
Class : Ascomycetes
Order : Erysiphales
Family : Erysiphaceae
Genus : Erysiphe, Sphaerotheca
Species : cichoracearum, fuligenia

Symptoms: Symptoms of this disease appear on all above ground parts of the plants. Most common symptoms of the disease caused by S. fuliginea and E. cichoracearum are the appearance of small white, circular ectophytic spots on the host surface, which gradually enlarge to coalesce with each other covering the entire leaf surface. When the spots are in the advance stages of growth, similar spots start appearing on the stem, petioles etc. The powdery spots generally appear first on the lower surface of the leaves, which gradually spread to the upper surface as well. Fruit infection is rare.

Mycelium: Ectophytic, branched, septate, floury white sending haustoria into the epidermal cells to draw nutrients.

Asexual reproduction: Conidiophores short, septate, arise vertically on superficial hyphae. Conidia are barrel to cylindrical, hyaline, and unicellular in chains.

Sexual reproduction: Cleistothecia appear as dark coloured, round structures, sharp, minute bodies scattered in the mycelial web, bears unbranched myceloid appendages with free ends (Fig. 2). Asci, ovate, nearly sessile and originate from a single point and their number ranging from 10-25, each asci contains 2-3 ascospores, which are elliptical, hyaline and unicellular.
In case of Sphaerotheca fuligenia there is one ascus in each cleistothecium and each ascus contains eight ascospores.

Fig.2: Microscopic details of Sphaerotheca fuliginea

c) Powdery mildew of bell pepper/ chillies/ tomato (Causal organism Leveillula taurica (lev.) Arnaud (anamorph = Oidiopsis sicula Scalia Syn. Oidiopsis taurica (Lev.) Arnaud)

Kingdom : Fungi
Phylum : Ascomycota
Class : Ascomycetes
Order : Erysiphales
Family : Erysiphaceae
Genus : Leveillula
Species : taurica

Symptoms: The symptoms first appear as white to light grey-coloured spots on the older leaves which progresses to upper leaves, stems and buds. On the under surface of the leaves, white to grey coloured spots appear while their corresponding upper surface exhibits yellow lesions with brown necrotic centres. Premature senescence of the leaves results in early defoliation.

Mycelium: Endophytic.

Asexual reproduction: Conidiophores are long and multi-branched. Conidia are dimorphic (pyriform and cylindrical). Conidia are borne singly or in short chains.

Sexual reproduction: Cleistothecia are formed within the stroma but only rarely. Appendages are hyphae like, short, indistinctly branched and colourless to olivaceous brown. Usually there are 20-35 asci within each cleistothecium. Each ascus contains two ascospores. Ascospores are large, cylindrical to pear shaped and slightly curved.


d) Powdery mildew of rose (Causal organism Sphaerotheca pannosa Wallr. ex Fr.) authority

Kingdom : Fungi
Phylum : Ascomycota
Class : Ascomycetes
Order : Erysiphales
Family : Erysiphaceae
Genus : Sphaerotheca
Species : pannosa

Symptoms: Small, discrete lesions appear on the stems, leaves or sometimes on the flowers. Warm and humid weather with cool nights favours the development of the disease. Production of numerous conidia gives the affected plants a white powdery appearance. Newly unfolded leaves are most susceptible, becoming more resistant with age. The young growing tips are completely covered by the mildew.

Asexual reproduction: Conidiophores are long and multi-branched. Conidia are dimorphic (pyriform and cylindrical). Conidia are borne singly or in short chains.

Sexual reproduction: Cleistothecia are formed within the stroma but only rarely. Appendages are hyphae like, short, indistinctly branched and colourless to olivaceous brown. Usually there are 20-35 asci within each cleistothecium. Each ascus contains two ascospores. Ascospores are large, cylindrical to pear shaped and slightly curved (Fig. 3).

Fig. 3: Microscopic details of Sphaerotheca pannosa