Symptoms
Broad bean mosaic disease is caused by different strains of the bean yellow mosaic virus (BYMV), and the symptoms vary depending on the strain. The disease can be classified into two main types: mosaic type and yellow type. In the mosaic type, infected plants are typically stunted, with curled and thickened leaves that show a mottled pattern of green and yellow. Mildly infected plants may not show significant dwarfing but might have yellowing or curling at the top. Severely affected plants are significantly shorter than healthy ones, fail to flower, and appear weak. In the yellow type, plants are also stunted, with thin, yellow leaves and erect stems. They generally do not wilt, and older leaves tend to fall off later in the season.
Pathogen
The causal agent of this disease is the Bean Yellow Mosaic Virus (BYMV), also known as the Bean Yellow Macular Mosaic Virus. It is a viral pathogen with linear-shaped particles. The virus is inactivated at temperatures between 55°C and 65°C, and its dilution limit ranges from 1,000 to 10,000 times. In vitro, it remains viable for 1 to 3 days. It causes mosaic symptoms on petunia and amaranth, while it produces necrotic spots on common tobacco, heart tobacco, and mandala.
Transmission Routes and Conditions
The virus overwinters on legume crops, especially in warm regions, or on winter vegetables and greenhouse legumes during colder months. The primary vector is the aphid *Aphis medicaginis*, and the disease often has a clear epidemiological center once it appears. Broad beans grown in fields or greenhouses are particularly susceptible. Early sowing, pod formation under dry soil conditions, and prolonged dry weather contribute to higher disease incidence.
Control Methods
(1) There are differences in resistance among bean varieties. It is recommended to select and plant disease-resistant varieties, and to use seeds from disease-free fields that are also healthy and full-sized.
(2) Choose planting sites carefully, keeping them away from areas where peas or other legumes are grown, as well as from greenhouses or sheds.
(3) Sow at the appropriate time, avoiding early planting. Remove diseased plants promptly to prevent the formation of disease centers and further spread.
(4) Apply 50% triazophos wettable powder diluted 2,500 to 3,000 times.
(5) When faba bean plants reach the stage of "under the hemp leaf, middle node, and flowering," topping can help reduce disease severity.
(6) If necessary, spray 0.5% mushroom proteoglycan water at a 300-fold dilution or 20% virus A wettable powder at 500 times, or 15% virus Bick wettable powder at 500–700 times.
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