First report of Bipolaris leaf blight of coconut (Cocos nucifera) caused by Bipolaris incurvata in mainland India

A. Kamalakannan*¹, V. Valluvaparidasan², R. Rabindran², Salah Eddin Khabbaz² and D. Ladhalakshmi²

¹ Agricultural Research Station, Tamil Nadu Agricultural University, Bhavanisagar – 638 451, Erode District, Tamil Nadu, India
² Department of Plant Pathology, Tamil Nadu Agricultural University, Coimbatore-641 003, Tamil Nadu, India

Accepted for publication 29/09/05

Coconut (Cocos nucifera) is an important plantation crop in India. It is cultivated for oil and raw materials used in the coir industry. Between October and December 2004, a disease survey was conducted in Erode, Salem, Coimbatore and Dharmapuri districts in the state of Tamil Nadu. A new leaf blight disease was observed on 6-10 year old coconut palms (Fig. 1) in all areas surveyed. Symptoms first appeared as tiny, water-soaked flecks on the leaflets of the lower fronds. These flecks expanded and became circular or oval shaped, brown spots with a dark brown border, often surrounded by yellow halo. As the disease developed, these leaf spots coalesced (Fig. 2). In due course, the same symptoms were observed on the upper palm fronds.

Figure 1: Coconut palm infected with Bipolaris incurvata

A fungus was consistently isolated from diseased leaves onto potato dextrose agar (PDA) medium at 20 ± 2oC under near UV light. The mycelium was septate, branched and brown in colour. Straight or flexuous conidiophores mainly arose in small groups and were, 500 µm long, 7- 12 µm thick and often swollen to 16-20 µm at the base. The conidia were typically curved, fusiform, septate with 8-13 pseudo-septa, pale straw coloured with a size ranging from 100-150 x 19- 22 µm (Fig. 3). Based on the symptoms, mycelia and conidial characters, the fungus was identified as Bipolaris incurvata (Ellis, 1971).

Figure 2: Leaf spot and blight symptoms on Coconut leaves

To confirm pathogenicity, a spore suspension (3.5 x 10⁶ conidia per ml) was sprayed onto wounded leaf pieces in petri dishes lined with moist cotton. Detached, wounded leaf pieces sprayed with sterile distilled water served as control. The petri dishes were maintained at 20 ± 2°C under near UV light. Inoculated leaf pieces produced typical necrotic spots after seven days incubation. The fungus was consistently re-isolated from infected leaf pieces onto PDA medium, completing Koch’s postulates. In contrast, the control plants did not show any symptoms.

Figure 3: Conidia of Bipolaris incurvata

This pathogen has previously been reported on coconut from Indian Andaman Islands (Rao, 1988), USA (Florida), Jamaica (Farr et al., 1989), Australia, the Philippines and the Seychelles (Uchida & Aragaki, 1991). To the best of our knowledge, this is the first report of this disease on coconut in mainland India.

Ellis MB, 1971. Dematiaceous Hyphomycetes. Kew, England: Commonwealth Mycological Institute.

Farr DF, Bills GF, Chamuris GP, Rossman, AY, 1989. Fungi on Plants and Plant Products in the United States. St. Paul, Minnesota, USA: APS Press.

Rao SC, 1988. Leaf Spot disease of coconut incited by Bipolaris incurvata (Bernad) Alcorn. Current Science 57, 674-675.

Uchida JY, Aragaki M, 1991. Bipolaris and Exserohilum leaf spots. In: Chase AR, Broschat TK, eds. Diseases and Disorders of Ornamental Palms. St. Paul, Minnesota, USA: APS Press, 55.

The British Society for Plant Pathology