In this article we will discuss about the green ear disease of bajra caused by fungi.
Green Ear Disease Introduction
Green ear disease of Bajra is a widespread disease that has been reported in India, Iran, Israel, China, Fiji, Japan, and other countries where the Bajra crop is grown. Butler (1907) was the first to report and study the disease in India, believing it to be sporadic and not causing significant crop damage.
Mitter and Tandon (1930) confirmed Butler’s observations by reporting the disease from Allahabad, Uttar Pradesh (1907). Since then, the disease on Bajra has been reported in all states where Bajra is grown as one of the ‘Kharif’ season crops.
Effect of Green Ear Disease
Initially, the disease was thought to be sporadic, causing little crop damage except in low-lying fields where the loss could be significant. However, Rai and Sinha (1965), Mathur and Dalela (1971), and Nene and Singh (1976) estimated a loss of up to 27-30%. During 1975, the disease became epidemic in Karnataka and Maharashtra, causing nearly total crop loss.
Symptoms of Green Ear Disease
The disease’s symptoms have been classified into two stages: the downy mildew stage, which is visible on the leaves, and the green ear stage, which affects the inflorescence (ears). Since it has been noted that the pathogen strain found in India generates more oospores than sporangia, the latter symptom is more noticeable than the former.
Figure 1: Green ear disease of Bajra: A. Partially transformed ear, B. Fully transformed ear, and C. Downy mildew stage on a leaf.
The affected plants’ growth is stunted, and they appear sickly and pale yellow in colour. This symptom can be seen in young plants as well. The leaves soon begin to show chlorosis in streaks on the upper surface, while the lower surface shows downy fungus growth.
After a while, chlorotic areas turn brown, and shredding of the leaves has been observed in advanced stages. Because the plant is unbranched, the disease stimulates nodal bud formation and the development of lateral shoots, which is an abnormal characteristic.
The leaves also display additional symptoms in connection with it. The typically green colour of the leaves might partially or completely shift to a yellowish or brown hue. Younger leaves show a whitening along the streaks, whilst older leaves show a brown colour change.
This occurs more frequently in older plants, particularly in the leaves from which the inflorescence develops. The twisting, folding, and shredding of leaves towards the tip causes the colour to change.
The upper leaves of older plants with green ears are mostly brown and split. A dense mass of white or twisted brown leaves develops from a large number of leaf buds. These buds’ axils produce green leafy human ear-like structures.
The leaves exhibit additional symptoms in conjunction with it. The normally green colour of the leaves may change entirely or partially to whitish or brown. The younger leaves are white along the streaks, while the older leaves are brown.
When the inflorescence transforms into green leafy ears, the disease becomes visible. This is because floral parts are transformed into twisted leafy structures, giving the ear the appearance of a green leafy mass. As a result, the disease is also known as ‘Green ear.’
Types of Green Ear Disease
(i) The entire ear (inflorescence) changes into a green leafy mass.
(ii) The lower part of the inflorescence is transformed into a green leafy mass, whereas the upper part bears seeds.
(iii) The development of the inflorescence is completely suppressed, and in its place, a small cluster of leafy structures forms.
The disease alters both the florets and the floral organs. The spikelet’s bristles become twisted and hypertrophied. On the same pedicel, the number of spikelets may change from one to two. The increase in the number of florets per spikelet is a common trait.
The reproductive organs, particularly the stamens and carpels, typically sterilise, resemble leaves, and are suppressed to simple peg-like structures. The stamens can occasionally just be brown, pointed bodies with no discernible filament or anther.
In severely attacked plants,’ the carpels rarely develop and is replaced by small, leafy shoots or horn like outgrowths. This may be attributed to hypertrophy and hyperplasia of the affected tissues.
Causal Organism & Disease Cycle
The causal organism is Sclerospora graminicola (Sacc.) Schroet. It is an obligate parasite.
Figure 2: Disease cycle of ‘Green ear’ disease.
The primary infection is caused by oospores or mycelium found in the seeds of diseased ears. When favorable conditions return, the oospores in the soil germinate and enter the young seedlings of the host through the underground parts, causing primary infection.
When mycelium-containing seeds are sown in fields, they germinate and grow, and the mycelium grows alongside them, causing primary infection. The infection spreads throughout the plant as it grows from seedling to mature plant.
Symptoms appear on leaves in the early stages as downy growth of the fungus on the lower surface of the leaves. After a while, the diseased plants develop the distinctive green ears.
Secondary infection occurs as a result of primary infection via sporangiophores and sporangia produced on the host. Wind, water, and insects disperse these sporangia, which land on susceptible parts of the host and germinate, producing zoospores.
By using germ tubes, these zoospores grow and spread infection. The likelihood of secondary infection is slim because the sporangia are reported to germinate only in the presence of sufficient dew on leaf surface.
Towards the end of the growing season of the crop, the pathogen produces oospores through sexual reproduction. The oospores are thick walled and serve as resting spores, keeping the pathogen alive during the unfavourable conditions.
During harvesting of the crop, the oospores along with plant debris are left in the soil of the fields where the oospores perennate waiting for the favorable conditions to return back so that these can serve as the source of primary inoculum.
Control of Green Ear Disease
Green ear disease is primarily soil borne and the following control measures have been suggested:
(i) Rotation of crop, with non-host types may check the disease.
(ii) Removal of diseased plants and their burning within a month of the disease detection checks the disease to a large extent.
(iii) Spraying with Dithane M-45 also helps in controlling the disease.
(iv) Seed treatment with 0.1 percent Agrosan GN and 0.4 percent thiram has been reported to control the disease to 50 percent.
(v) Use of disease resistant varieties like HB-15, PHB-10 and PHB-14 has been recommended.