© 2009 Plant Management Network.
A New Leaf Spot Disease of Calotropis gigantea Caused by Alternaria alternata in Rajasthan, India
Satish K. Sain and H. N. Gour, Department of Plant Pathology, Rajasthan College of Agriculture, Maharana Pratap University of Agriculture & Technology, Udaipur-313001, India; P. Sharma and P. N. Chowdhry, Division of Plant Pathology, Indian Agricultural Research Institute, New Delhi-110012 India
Corresponding author: Satish K. Sain. firstname.lastname@example.org
Sain, S. K., Gour, H. N., Sharma P., and Chowdhry, P. N. 2009. A new leaf spot disease of Calotropis gigantea caused by Alternaria alternata in Rajasthan, India. Online. Plant Health Progress doi:10.1094/PHP-2009-0331-01-BR.
Madar [Calotropis gigantea (L.) R. Br. ex. Ait.] is a medicinally important wild shrub native to India. It is used to treat a number of illnesses including dysentery, ipecacuanha, intermittent fevers, cold, cough, and asthma. The seed floss is used for furniture and bedding stuffing and the bark is stripped to make fishing nets and twine. In January and February 2005, we observed a leaf spot epidemic of madar growing on wasteland sites near the Sikar district of Rajasthan, India. Disease incidence was greater than 80% and shrubs suffered extensive defoliation. Symptoms on infected leaves were small, circular spots with large yellow to dark brown haloes on both sides of the lower and middle leaves (Fig. 1). The spots gradually enlarged in size and later became irregular in shape or remained circular with concentric rings or zones. During the later stage of infection, these spots coalesced to cause necrosis of the leaves causing withering, extensive drying, and ultimately death of leaves.
Small pieces of diseased leaf tissue were surface sterilized in 0.2% NaOCl and placed on potato dextrose agar (PDA) with a pH of 7. Grayish black colonies of Alternaria consistently grew from the tissue after incubation at 22 ± 3°C with a 12-h photoperiod for 3 to 4 days.
Colonies of Alternaria on PDA had off-white centers with fluffy margins (Fig. 2). Conidiophores arising singly or in small groups were 80.8 to 173.2 µm in length and 3.2 to 6.3 µm thick, simple or branched, straight or flexuous, some times geniculate, pale to mid-olivaceous or golden brown, and smooth. The fungus produced dark brown conidia with a short, 2 to 5-µm thick, pale beak. (Fig. 3). Conidia were 20-63 × 9-18 µm with 3 to 7 transverse septa and produced 5 to 10 germ tubes. Conidia on plants were pale to golden brown, obclavate, obpyriform, ovoid, or ellipsoidal with cylindrical beak not more than one third of the length of the conidium, and borne singly or in short chains. The conidia had 5 to 8 transverse septa and a few longitudinal septa (Fig. 3). On the basis of the morphological characteristics of the conidiophores and conidia, the pathogen was identified as of Alternaria alternata. (Fr.) Keisseler. The culture was deposited with the National Center of Fungal Taxonomy (NCFT), New Delhi, who confirmed the identity of the isolate (Accession No. NCFT 2002).
Koch’s Postulates were completed by spraying 75-day-old healthy madar plants with an aqueous spore suspension of 1 × 106 conidia per ml using an atomizer in the late evening. Inoculated plants were placed in an environmental chamber at 23°C with 8 h of daily light and 75% relative humidity for 48 h. Thereafter, inoculated and non inoculated plants were moved to a 25°C green house bench and watered twice per day. Leaf spots initially appeared on the upper surface of older leaves after 7 days and by 12 to 15 days, leaf spot symptoms similar to those described previously developed on every inoculated plant. No symptoms developed on plants sprayed with distilled water. A. alternata was consistently re-isolated from the inoculated plants.
A leaf spot of Madar (Calotropis procera) caused by A. alternata has previously been reported from the Punjab state of India (1) but this is the first record of the disease from the Sikar district of the Rajasthan state of India.
1. Saini, S. S., Kumari, S., and Kaur, J. 1989. Fungi of Punjab: III, New host records of Alternaria. Indian Phytopathol. 42:599-600.