Pseudoperonospora cubensis on Sechium edule in India

Sechium edule is grown in the hills of Meghalaya, Manipur, Mizoram, Nagaland, Sikkim and West Bengal in India (Rai et al. 2002). Edible immature fruits, tender shoots and large starchy roots of S. edule are eaten by tribal communities in north-east India. Roots are often used as a substitute for potato (Yadav et al. 2005). Sechium edule is also grown in Mexico, Brazil, Taiwan, China and Cuba. Fruits are used for preparing pasta, sauces and juice. Medicinal uses, including leaf infusions forz dissolving kidney stones, and for treatment of arteriosclerosis and hypertension, have also been documented (Saade 1994).

Downy mildew symptoms were observed on the leaves of S. edule during September 2008 in Barapani, Meghalaya (north-east India). Symptoms included yellow spots on the upper leaf surface. Heavy sporulation was observed on the lower surface of the leaves. Later on leaves turned necrotic. Voucher specimens have been deposited in Herbarium Cryptogamme India Orientalis, New Delhi, India (HCIO No. 48787) and in Institute Herbarium of ICAR Research Complex for NEH Region, Meghalaya, India (ICARHNEH-56).

Fungal structures were harvested by dislodging them from infected tissue onto a strip of clear tape (Correll et al. 1987). Sporangiophores emerging from stomata were of 184–(220)–288 × 5–(6.4)–8 μm (n = 50). Overall branching pattern of sporangiophores was monopodial. Ultimate branchlets measured 5–(7)–12 × 1–(1.9)–3 μm (n = 50). Brown sporangia (25–(28.1)–33 × 18–(18.8)–20 μm) were ovoid to ellipsoidal in shape with l/b ratio 1.3–(1.6)–1.7 (n = 50). Prominent hyaline papilla were also present on sporangia. The sporangia were narrowing toward the apical and distal ends (Fig. 1). Haustoria appeared to be clavate-branched. Based on these observations the organism was identified as Pseudoperonospora cubensis (Choi et al. 2005; Ko et al. 2008; Voglmayr et al. 2008). Monophyly of the genus Pseudoperonospora has also been confirmed through extensive molecular analysis (Riethmüller et al. 2002). Pathogenicity was confirmed by spraying a sporangial suspension on healthy twigs of S. edule; non-inoculated twigs served as control. Inoculated leaves developed symptoms after 12–16 days whereas control plants remained healthy.

Fig. 1.  Sporangiophores and sporangia of Pseudoperonospora cubensis on Sechium edule (HCIO No. 48787). Bar = 10 μm.

This pathogen has been reported on S. edule from Brazil, Cuba, Panama, China and Taiwan (Farr et al. 2005; Ko et al. 2008). Many species have been reported to be the host of P. cubensis viz. species of the genera Benincasa, Lagenaria, Luffa, Momordica, Trichosanthes, Cucumis, Cucurbita and Citrullus (Crous et al. 2004). To our knowledge, this is the first report of P. cubensis on S. edule from India. Downy mildew is of concern to tribal populations of north-east India since this crop is grown extensively for human and animal consumption.