Occurrence of leaf rust of Cannain Singapore caused by Puccinia thaliae

Canna is the only genus in the family Cannaceae. This tropical to sub-tropical native of the New World is widely grown in Singapore. The rhizomes of the plant are a rich source of starch (Tanaka, 2004), and it is also an attractive and popular ornamental flowering plant in Singapore. Many different cultivars of C. indica and C. glauca are used in landscaping of parks and gardens throughout the island.

Although Canna is not demanding to grow and is usually disease-free, rust disease is often evident on the leaves and can be severe in plants throughout Singapore. Diseased leaves are covered with small, discrete orange lesions that erupt, releasing numerous spores. Severely infected leaves have coalescing lesions that are chlorotic and become shrivelled. This leads to loss of vigour and decline of the entire plant. Diseased leaves were collected from plants growing in the Nanyang Technological University (NTU) campus, west of Singapore and from the Singapore Botanical Gardens, in central Singapore. Uredinia appeared as yellow-orange hypophyllous lesions, 0.5-2 mm in diameter (Fig. 1), and were subepidermal and erumpent (Figs. 2 & 3). Urediniospores were ovate (Fig. 4), measuring 36.63 ± 1.63 µm x 26.88 ± 0.88 µm (including wall thickness), and were yellow-orange with oily refractile globules. Walls of urediniospores were thick and echinulate (covered with coarse spines) (Fig. 5). Spermogonia, aecia and teliospores were not found in any of the 20 infected leaves examined. The fungus was identified as Puccinia thaliae based on morphology and symptoms on the host. Herbarium specimens of diseased leaves (S226) were deposited in the Natural Sciences and Science Education Department of NTU, Singapore.

Koch’s postulates were satisfied by inoculating uredinia on to excised Canna leaves. The leaves were kept in moist chambers under natural light at 25°C. Uninoculated excised Canna leaves were kept in moist chambers under the same conditions as controls. Inoculated leaves developed symptoms similar to those observed in the original infection 17 days after inoculation. Controls remained healthy. Although rust on Canna has been observed in Singapore since the early 1970s, this is the first published record of its presence in Singapore. The rust, caused by P. thaliae, has been reported previously from Hawaii, India and South Africa (Gardner & Martinez, 1985; Jeeva et al., 2004; van Jaarsveld et al., 2006).