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 categorized into two main types: mosaic type and yellow type. In the mosaic type, infected plants show stunted growth, with curled and thickened leaves that display mottled patterns of yellow and green. Mildly affected plants may not show significant dwarfing, but their top leaves may turn yellow or curl. Severely infected plants are significantly shorter than healthy ones, often fail to flower, and exhibit weak growth. Yellow-type plants are also stunted, with thin, yellow leaves, upright stems, and generally no wilting. As the disease progresses, older leaves tend to fall off easily.
Pathogen
The pathogen responsible for this disease is the Bean Yellow Mosaic Virus (BYMV), also known as Bean Yellow Macular Mosaic Virus. It is a plant virus with linear-shaped particles. The virus is inactivated at temperatures between 55°C and 65°C, and its dilution limit ranges from 1000 to 10,000 times. In vitro, the virus can survive for 1 to 3 days. It causes mosaic symptoms on petunia and amaranth plants, while it produces necrotic spots on common tobacco, heart tobacco, and marigold.
Transmission Routes and Conditions
The southern strain of the virus overwinters on legume crops or becomes more active during the summer season. In winter, it mainly survives on winter vegetables or greenhouse legumes. The primary vector is the Aphis medicaginis aphid, and the disease often has a clear epidemiological focus once it appears. Broad bean fields or greenhouses with poor management are highly susceptible. Early sowing, pod formation under dry soil conditions, and prolonged dry weather can all contribute to severe outbreaks.
Control Methods
(1) There are differences in disease resistance among varieties. Therefore, it is important to select resistant varieties and use seeds from disease-free fields. Ensure that seeds are healthy, clean, and free from contamination.
(2) Choose planting locations carefully, avoiding areas near vegetable pea fields, sheds, or greenhouses where the virus might persist.
(3) Sow at the appropriate time and avoid early planting. Prompt removal of diseased plants is crucial to prevent the formation of disease centers and further spread.
(4) Apply 50% triazophos wettable powder diluted 2500–3000 times as soon as symptoms appear.
(5) During the "hemp leaf, middle node, and flowering" stage of faba bean growth, topping can help reduce disease incidence.
(6) If necessary, spray 0.5% mushroom proteoglycan water at a 300-fold dilution, or 20% Virazole wettable powder at 500 times, or 15% Virbex wettable powder at 500–700 times. These treatments can help manage the disease effectively when applied at the right time.
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