Diseases Of Sorghum Plant – Agricultural study material

DISEASES OF SORGHUM

1. CHARCOAL ROT-Macrophomina phaseolina

Symptoms

The fungus invades through the roots and the lower stem of the infected plants become soft and weak resulting in lodging and poor grain filling. When infected stem is split open, vascular fibres could be seen disintegrated and black sclerotia can be seen heavily coated in the tissues and hence the name charcoal rot.

Mode of spread

The pathogen spreads through transfer of infested soil by agricultural implements and irrigation water. 

Survival 

The pathogen survives as sclerotia in soil and plant debris

Epidemiology

Drought stress makes the plant weak and susceptible. High soil temperature with rain or irrigation during grain filling stage encourages infection.

Management

• Collection and destruction of infected plant debris and improving the drainage facility.
• Growing resistant genotypes like SPV 248, 249, 428, 488.
• Use of seed treatment chemicals like Thiram, Captan or Carbendazim @ 4 gm/Kg of seed. 
• Use of antagonistic organisms like T. viride, P. fluorescens as seed treatment and soil application helps to reduce the inoculums.

2. DOWNY MILDEW OR LEAF SHREADING – Perenosclerospora sorghi

Symptoms

Whitish soft downy growth can be seen on the lower surface of the leaves with corresponding yellowing on the upper surface of the leaves. Finally chlorotic streaks develop and turn brown as the leaf tissues die. After the oospore formation in the vascular bundle, leaf blade tear in to stripes with the midrib remaining intact producing the characteristic “leaf shredding” symptom.

Mode of spread

Oospores form the primary inoculum and the air borne sporangia forms the secondary inoculum. 

Survival 

The pathogen survives as oospores in soil for 3-4 years and survives as mycelium in seeds.

Epidemiology

Cool weather, high Relative Humidity, light drizzling encourages the disease development. 

Management

• Crop rotation with pulses and oil seeds.
• Deep ploughing and rogueing out the affected plants
• Seed treatment with metalaxyl at 4 g/kg of seed
• Foliar spraying with metalaxyl @ 500g/ha or mancozeb or ziram or zineb @ 1kg/ha
• Grow resistant varieties like CO 25, CO 26

3.RUST – Puccinia purpurea

Symptoms

Rust is a severe disease of sorghum capable of infecting the leaves at all stages of the crop growth when the environment is favorable. Rust pustules appear mostly on the lower surface of the leaves. The reddish brown sori are minute, round or elliptical and 1-2 mm in diameter. Later black telia develop in linear patterns mostly on the lower surface of the leaves resulting in premature drying of leaves.

Mode of spread and Survival 

Uredospores are spread by air. Teliospores survive in soil and infected debris (uredial and telial stages survive on sorghum. Pycnial and aecial stages survive on the alternate host Oxalis corniculata) 

Epidemiology

Generally rainy weather and low temperatures favour the disease development.

Management

• Removal of alternate host
• Dusting with sulphur @ 25 kg/ha and Spray with mancozeb / zineb @ 1.25 kg/ha
• Grow resistant varieties like IS 3443, 3547, 18758, and SPV-247

4. GRAIN SMUT / KERNAL SMUT / COVERED SMUT – Sporisorium sorghi

Symptoms

The disease appears only at the time of grain formation. The individual grains are replaced by smut sori which are oval / conical shaped and cream to light brown in colour. These are covered by cream to light brown skin (Peridium) which remains unbroken upto thrashing. Depending on intensity 20-90% florets are damaged.

Mode of spread and Survival 

The pathogen is externally seed borne and the smut spores survive up to 13 years.

Epidemiology

Low temperature, dry soil moisture and high humidity encourages the disease

Management

• Use disease free seeds
• Crop rotation
• Seed treatment with sulphur/ thiram @ 4g/kg of seed
• Grow resistant varieties like CSH 5, 9, SDM-9, SPV-102, 115, 138 and 245

5. LOOSE SMUT – Sporisorium cruenta

Symptoms

The infected plants produce earheads earlier (Up to two weeks) than healthy one and often shows increased tillering. All the spikelets of the infected ears are malformed and hypertrophied.

Mode of spread and Survival

The fungus is externally seed borne and soil borne. Smut spores survive up to 4 years under dry conditions.

Epidemiology

Low temperature, dry soil moisture and high humidity encourages the disease.

Management

• Crop rotation and field sanitation
• Seed treatment with sulphur/ thiram @ 4g/kg.

6. LONG SMUT – Tolyposporium ehrenbergii

Symptoms

Usually a few smut sori appear and scattered on earhead. The sorus is covered by a whitish to dull yellow, fairly thick membrane (Peridium) and is much longer than those of the other two smuts. The sori are cylindrical, elongated and creamy brown coloured. 

Mode of spread and Survival  

The pathogen spreads through air borne smut spores/sporidia.                                                                                                                                       

The pathogen survives in the soil for considerable period.

Epidemiology

Low temperature and high humidity helps in the disease development

Management

• Rogueing, Crop rotation and field sanitation

7. HEAD SMUT – Sporisorium reiliana

Symptoms

The disease appears only at the time of flowering. The head is partially or fully replaced by a large sorus when still in the boot leaf. The galls are first covered by a whitish grey membrane in the boot leaf, later turn to brown/ black coloured masses.

Mode of spread and Survival

The pathogen is soil borne and survives in soil for two years.

Epidemiology

Dry cool climate and low temperature favours the disease

Management

• Collect the smutted ear heads in cloth bags and dip in boiling water [50⁰ C]
• Crop rotation
• Grow resistant varieties

8. ERGOT/SUGARY DISEASE – Sphacelia sorghi

Symptoms

The disease occurs only at the time of flowering. The ovary is converted into a mass of fungal tissue. Small droplets of light honey-coloured dew like substances exude from the infected spikelets. Finally, the infected ovary turns in to small dark brown ‘’sclorotia’’. 

Mode of spread and Survival

The pathogen spreads through air borne conidia and insects. The honey dew attracts the insects and ants which help in spreading the disease. The pathogen survives as sclerotial bodies in soil. 

Epidemiology

    High rainfall, cloudy weather during flowering stage encourages the disease development.

Management

• Adjust the sowing date so that the flowering does not coincide with the rains
• Immerse the seeds in 10 % salt solution and remove the floating sclerotia
• Spray with mancozeb or zineb or zirum @ 1 kg/ha

DIFFERENCE BETWEEN SORGHUM SMUTS

9 .LEAF BLIGHT – Helminthosporium turcicum

Symptoms

In the initial stage, small, narrow, elongated spots appear on the leaf. On older plants, the typical symptoms are long elliptical necrotic lesions, straw coloured in the center with dark margins. Under humid conditions conidiophores and conidia can be observed on the lesions.

Mode of spread and Survival   

The pathogen spreads through infested seed and the secondary spread is through the air borne conidia. The pathogen survives in the infected plant debris.

Epidemiology

Cool moist weather, high humidity and rainfall encourage the disease development.  

Management

• Use disease free seeds
• Seed treatment with captan or thiram @ 4g/kg of seed
• Spray mancozeb 1.25 Kg/ha or captafol at 1 kg/ha
• Grow moderately resistant varieties like CO 21, 22, 23, CSH – 6, CSV – 6 

10. RECTANGULAR LEAF SPOT – Cercospora sorghi

Symptoms

Small rectangular leaf spots of dark brown colour, bounded by veins with irregular borders appear on both the surface of the leaves. 

Mode of spread and Survival

The pathogen spreads through air borne conidia and survives in the infected plant debris and collateral hosts.

Epidemiology

Cool moist weather, high humidity (above 90 %) and rainfall favour the disease development. 

Management

• Spray ziram 0.2% or dusting sulphur @ 25Kg/ha

11. ANTHRACNOSE and RED ROT – Colletotrichum graminicola

Symptoms

The pathogen causes both leaf spot and stalk rot diseases. The disease appears as small red coloured spots on both surfaces of the leaf. Finally elliptical to spindle shaped spots with whitish centre and brown margin, embedded with dark dots at the centre [acervuli] appear. The red rot phase appear in the stalk or inflorescence. 

Mode of spread and Survival

The primary infection is through infested seeds and secondary spread is through air borne conidia. The pathogen survives in the infected plant debris and collateral hosts.

Epidemiology

Continuous rain, a temperature range of 28-30 ⁰ C and high humidity favours the disease development.

Management

• Seed treatment with captan or thiram @ 4g/kg
• Spray zineb 0.2% or Mancozeb 1.25 kg/ha controls the disease.

Chief characters of three of the major leaf spot diseases of sorghum

12. WITCH WEED – Striga asiatica and Striga densiflora

Symptoms

Striga is a small plant with bright green leaves, grows upto a height of 15-30 cm. It’s a partial root parasite on sorghum, cumbu and sugar cane. The root exudate of sorghum stimulates the seeds of the parasite to germinate. It always occurs in clusters of 10-20/ host plant. Each fruit contains minute seeds, which survive in the soil for several years. The parasite attaches to the host by haustoria and grows below the soil surface and produce underground stem and roots.

Mode of spread and survival   

The seeds are spread by wind, water and farm tools/machinery contaminated with the infested soil. The seeds of the parasite are found viable for many years in the soil. 

Management

• Regular weeding and inter cultural operations during early stage of parasite growth
• Spraying with sodium salt of 2,4-D 80% at 450g/500 lit of water kills the parasite. 
• Grow resistant varieties like CO-20