Black & Green Gram
The infection can occur at any growth stage and is visible on leaves, stems, petioles and pods. If the infection occurs after seed germination or if the seeds are infected, seedlings will show minute rust specks that slowly enlarge to form eye-spots and finally get blighted. On older plants, initial symptoms appear as small and irregular dark brown to black water-soaked spots, more commonly on the lower surface of leaves or petioles. Over time, the spots grow into sunken lesions with darker centers and yellow, orange or bright red margins, and also appear on the upper surface of leaves. Pods have rust-colored lesions and might shrivel and dry up. In case of heavy infection, affected parts might wilt and wither off. The development of cankers on stems and petioles is usually followed by defoliation.
The fungus Colletotrichum lindemuthianum survives in the soil and on infected seeds and plant debris. It also overwinters in alternative hosts. The spores are transferred to the growing seedlings via rain, dew or field work when foliage is wet. It is thus important to restrict the activity in the field (workers, treatments...etc) when the foliage is wet from rain or dew. Cool to moderate temperatures (13-21°C) and periods of frequent rainfall are also favorable to the fungus and its transmission, resulting in increased occurrence and severity of the infection.
Biological agents might help to control the infection. The fungus Trichoderma harzianum and the bacteria Pseudomonas fluorescens used as a seed treatment compete with Colletotrichum lindemuthianum. Fungicide sprays include products based on copper oxychloride 3g/l at 15 days interval.
Always consider an integrated approach with preventive measures together with biological treatments if available. Chemical treatment of the disease may not be economically viable if weather conditions are not favorable. Seeds can be soaked and treated with appropriate fungicides, for example thiram 80% WP @ 2 g/l or captan 75WP @ 2.5 g/l of water. Fungicide sprays include products based on folpet, mancozeb, thiophanate methyl (0.1%) or copper oxychloride 3g/l at 15 days interval.