Globally, green gram or mung bean (Vigna radiata) are prone to the common and destructive fungus disease rust. It is caused by the fungus Uromyces vignae and can result in substantial yield declines in the growth of green grams. Small, yellow-orange pustules on the lower leaf surface are the initial symptoms of rust disease; as the condition worsens, they enlarge and change brown. Premature leaf drop is another disease that lowers the plant’s ability to photosynthesize, resulting in stunted growth and decreased yield.
Green gram rust disease management can be done through strategies that combine cultural, chemical, and biological techniques. Planting resistant cultivars, crop rotation, and ensuring proper plant nutrition and spacing are examples of cultural techniques. Fungicides such as azoxystrobin, trifloxystrobin, and propiconazole are used in chemical control.
The use of fungicides should be limited, though, as it could result in the emergence of pathogen strains that are immune to them. Utilizing helpful microbes like Trichoderma harzianum, which can inhibit the growth and development of the rust pathogen, is a key component of biological control. Also, good sanitation practices like clearing and destroying infected plant residue can help prevent rust disease from spreading.
Rust Management in Green Gram
The Causal Organisms of Rust Disease
Rust disease caused by Uromyces phaseoli typica (Syn: U. appendiculatus) is a long-cycle rust that affects several plants, including green gram. It is an autoecious pathogen meaning all spore stages occur on the same host. The disease is characterized by yellowish-brown, unicellular, globose or ellipsoid uredospores with echinulations and chestnut-brown unicellular teliospores pedicellate with warty papillae at the top. The disease also produces yellow pycnidia on the upper surface of leaves and orange cupulate aecia on the lower surface of leaves. Aeciospores are elliptical and unicellular.
The Disease Cycle of Rust Disease
The rust disease cycle starts with the pathogen’s survival as teliospores in the soil and as uredospores in crop debris. Primary infection occurs when sporidia develop from teliospores, while the secondary spread is via wind-borne uredospores. The fungus also survives on other legume hosts.
During the season, the spores produced in the pustules of green gram infect gramma grasses (Bouteloua spp.) around the fields, causing elongated brownish or black spots on their leaves. The spores produced on these grasses infect cotton plants at the beginning of the next season, completing the cycle.
Causes/Conditions Favorable for Rust Disease in the Field
Specific environmental conditions favor rust disease in green gram. The disease-causing spores penetrate plant cells directly, especially in high humidity, moderate to warm temperatures and leaf wetness. The disease thrives in cloudy and humid weather conditions, with temperatures between 21-26°C. In addition, nights with heavy dews provide the ideal environment for the disease to develop and spread.