Green gram, or mung bean, is susceptible to a fungus disease called dry root rot. Caused by the fungi Rhizoctonia solani, which can live in soil for years without dying off, this disease is a major agricultural problem. Green gram producers face a serious issue with dry root rot, which can lead to huge losses in crop production. Green gram shows signs of dry root rot, such as wilting, leaf yellowing, and eventual death.
There may be dark or brown lesions on the stems of diseased plants, and the roots themselves dry out and become brittle. Destruction of plants is possible in extreme situations. Extreme heat and humidity promote the rapid spread of dry root rot, which is especially problematic in densely planted areas. Infected seeds or plant residue material are common sources for spreading the disease to new areas.
Green gram needs biological and chemical control methods to prevent dry root rot. The possibility of infection can be lowered by rotating crops, not sowing in proper spacing and seed variety areas, and employing disease-free seeds. In addition, fungicides can be used to manage the disease, though their effectiveness may be moderate.
Dry Root Rot Management in Green Gram
The Causal Organisms of Dry Root Rot Disease
The causal organism of dry root rot disease is the fungus Rhizoctonia bataticola, also known as Macrophomina phaseolina in its pycnidial stage. The fungus produces dark brown, septate mycelium with constrictions at hyphal branches. It also produces abundant minute, dark, round sclerotia, the primary means of survival and pathogen spread. The fungus also forms dark brown, globose oscillated pycnidia on the host tissues. These pycnidia contain thin-walled, hyaline, single-celled elliptical pycnidiospores responsible for disseminating the pathogen.
The Disease Cycle of Dry Root Rot Disease
The disease cycle of dry root rot in green gram begins with the survival of the fungus Rhizoctonia bataticola (or Macrophomina phaseolina in its pycnidial stage) in infected plant residue, a facultative parasite in the soil. In the next stage, the sclerotia germinate in the presence of moisture and infect the host plant’s roots.
The fungus then colonizes the plant’s vascular system, leading to the characteristic symptoms of dry root rot. The secondary spread of the disease occurs through air-borne pycnidiospores, produced by the fungus in dark brown, globose ostiolated pycnidia on the host tissues. These pycnidiospores are responsible for disseminating the pathogen to healthy plants and causing new infections.
Causes/Conditions Favorable for Dry Root Rot Disease in the Field
- The soil-borne fungi Macrophomina phaseolina causes green gram dry root rot.
- The disease thrives in hot, dry conditions with daytime temps around 30°C.
- The disease thrives in dry periods followed by irrigation, because the fungus can attack plant roots.
- Late blooming and podding plants suddenly wilt and dry from the disease.
- Seed-borne and soil-borne, the fungi can live in sclerotia for years. Temperature and moisture stress can intensify the disease in hot damp areas.
Symptoms of Dry Root Rot Disease
- Yellowing and drying of leaves.
- Straw-colored leaves and stem.
- Brown discoloration on lower leaves and stems, brittle taproot, scattered drying of plants.
- Dark brown lesions on the stem at ground level.
- Shredding symptom on bark, black minute sclerotia on the rotten stem, and root tissues.
- Small spots with pale brown centers and reddish brown margins on branches and pods, severe leaf spotting and defoliation du