Abstract: Not available
Alternaria leaf petiole and stem blight is an important disease of sweet potato (Ipomoeae batatas) in East and Central Africa. In surveys conducted from 2001 to 2003, disease incidences ranged from 1 to 49% in Uganda (Osiru et al., 2007). Symptoms typical of Alternaria leaf petiole and stem blight disease were observed on sweet potato. Symptoms initially manifested as small, dark gray to black oval lesions with lighter lesion centers on stems and petioles. Under humid conditions, lesions enlarged on stems and petioles resulting in petiole and stem girdling (Fig. 1). Some leaves exhibited chlorotic and dry symptoms, while brown circular lesions with concentric rings were observed on older diseased leaves. In severe cases, defoliation occurred and soil beneath diseased vines was carpeted with blackened leaf debris (Fig. 2).
To confirm the pathogenicity of Alternaria sp. on sweetpotato, the pathogen was isolated from vines and leaves showing characteristic symptoms of Alternaria disease and cultured on Sweet Potato Decoction Media (SPDM) and Potato Dextrose Agar (PDA). Pathogenicity tests (inoculation and re-isolation) conducted on leaves of two sweet potato cultivars, New Kawogo (CIP 441743) and NASPOT 1 (NIS/91/52), resulted in the typical symptoms above. All isolates showed similar conidia morphology, shape, size and number of septae (Fig. 3). Conidia were solitary, pale brown to brown elongate-obclavate, with two directional septae. The muriform, transverse septae were 5-8 and longitudinal sepate were 2-4. The average dimensions of 40 conidia obtained from colony grown on PDA were 46 μm (34-160 μm range) x 19 μm (15-42 μm range). These data were similar to those reported by Van Bruggen (1984) and were in contrast to the beakless conidia observed by Lopes and Boiteux (1994).
| Fig. 1(a,b): | Black lesions girdling sweetpotato stem and petioles |
| Fig. 2: | Characteristic debris under plants infected by Alternaria sp. |
| Fig. 3: | Conidia of A. bataticola isolated from cultivar NASPOT 1 |
The representative isolates were identified as Alternaria bataticola and deposited at the CABI Plant Health Clinic.
There have been frequent observations of disease caused by Alternaria sp. on sweet potato cultivars in farmers’ fields in many districts of Uganda and in research plots particularly in central and South-Western Uganda. The disease reports from farmers’ fields and research observations (Osiru et al., 2007) have indicated yield losses of over 70% depending on the cultivar and cropping season. Due to the fact that sweet potato is a very significant crop in Uganda, cultivated by approximately 75% of households in the country, pathogen identification is crucial for effective disease management in sweet potato production in this region. This communication confirms previous record of sweet potato leaf petiole and stem blight disease caused by A. bataticola in Uganda (Osiru et al., 2007).
ACKNOWLEDGMENTS
We greatly acknowledge the work of CABI-Biosciences scientists in isolations, identification and confirmation of pathogen species as well as in manuscript revision.