Black Spot Disease
Seedlings grown from infected seeds often have round, dark brown to black sunken spots on leaves and stems. The growth of the seedlings is compromised and they might die prematurely or present stunted growth. During secondary infections, leaf veins and petioles develop angular brick-red to black lesions, first on the underside of leaves, later also on the upper side. Circular, light brown to rust-colored lesions, surrounded by a black margin, appear on pods and stems. In severely infected pods, these lesions can shrivel and deform slightly, adopting a sunken canker aspect. Infected seeds are often discolored and develop brown to black cankers. Common bean plants are very susceptible to this disease.
Black spot disease is caused by the fungus Colletotrichum lindemuthianum. It is mainly seed-borne, but also survives on crop residues and in alternative hosts. When the environmental conditions are favorable, it releases its spores and is spread in the field by wind and rain. Cool to moderate temperatures (13-21°C), periods of high humidity, dew, wet foliage or frequent rainfall favor the life cycle of the fungus and the progression of the disease. Since the fungus is disseminated in the presence of water, it can also spread due to mechanical injuries during field work carried out while the foliage is wet. The fungus can invade the pod and infect the cotyledons or seed coat.
Extracts of neem oil applied every 7 to 10 days throughout the warmest period of the growing season limit the growth of the fungus. Biological agents can also help to control the infection. Biocontrol agents such as the fungus Trichoderma harzianum and the bacterium Pseudomonas fluorescens for example, limit the growth of Colletotrichum lindemuthianum if used as a seed treatment. Immerse the seeds in hot water (50°C) for 10 minutes to kill the fungi.
Always consider an integrated approach with preventive measures along with biological treatments available. Foliar applications of seed fungicides may reduce disease severity in the field, but the are rarely economical. Apply fungicides containing mancozeb, chlorothalonil, flutriafol, penconazole, or copper-based products when the foliage is dry.