plant disease


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downy mildew,fungal wilt, bacterial blight of rice and cucumber mosaic.


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“a series of harmful physiological processes caused by continuous irritation of the plant by a primary agent “ – by JAMES BLUENT (1952). “a harmful deviation from the normal functioning of physiological processes”- by JEFFERY MILLER (1956). “a continuous impairment of metabolism” – by SAMBRCK JOSEPH . “a physiological disorder or structural abnormality that is harmful to the plant or to any of its part or products that reduces the economic value”- by STATMAN & HARRAR (1967). “a series of visible and invisible responses of plant cells and tissues to a pathogenic microorganism or environmental factors that result in adverse changes in the form ,function , or integrity of the plant and may lead to partial impairment or death of the plant or its parts”- by AGRIOS (1997).

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CAUSES OF PLANT DISEASE 1. Living (animate) 2. Non- Living (inanimate) CLASSIFICATION OF PLANT DISEASE 1. on the basis of extent to which diseases are associated with plant : a) Localised diseases b) Systemic diseases 2. on the basis of mode of primary infection : a) Soil-borne diseases b) Air – borne diseases c) Seed- borne diseases 3. on the basis of kind of symptoms : a) Rust b) Smut c) Mildews d) Root- dots e) Blights f) leaf spots g) Wilts h) Cankers i) fruit – rots and etc…

Disease Caused By Fungi:

Disease Caused By Fungi 1)DOWNY MILDEWS Downy mildew refers to any of several types of oomycete microbes that are obligate parasites of plants. Downy mildews exclusively belong to Peronosporaceae . Development and severity depend greatly on the presence of a film of water on the plant tissue and on high humidity in air during cool or warm periods. Bordeaux – 1 st Fungicide (1885) It may destroy 40-90% of young plant heavy losses of crop yield .

Downy Mildew Of Grapes ( Vitis vinifera):

Downy Mildew Of Grapes ( Vitis vinifera ) Symptomatology: # small ,pale yellow ,irregular spot on upper surface of leaf. # downy growth of the sporangiosphore of the fungus appear on the underside of spots. White downy spore masses on the lower surface of the leaf. #Later, infected leaf areas are killed and turn brown ; spots often enlarge, unite or combine to form large dead areas and finally result to defoliation.

#berries become leathery ,distorted ,thickened , wrinkled and may die . :

#berries become leathery ,distorted ,thickened , wrinkled and may die . #Downy mildew rapidly kills highly susceptible young bunches , but berries are immune well before pea-size.

Etiology ::

Etiology : Caused by Plasmopara viticola.

Control: :

Control: *sanitation *use of Resistant Varieties ( Several North American species show resistance to downy mildew (e.g. V. labrusca and V. rotundifolia ) ) . *Fungicidal Sprays – Bordeaux, Zineb & Mineb(0.2%), Captan (0.2-0.5%),Metalaxyl with Copper Oxychloride(0.3%). * Automatic weather stations (AWS) monitor the vine canopy for leaf wetness, rainfall, relative humidity and temperature, critical to downy mildew.

Downy Mildew Of Bajra:

Downy Mildew Of Bajra Symptomatology: # the infected plants are dwarfed because of shortening of internodes & tillering is excessive. # the foliage become pale & chlorotic & on the underside covered by sporangia. Leaves become distorted and wrinkled . #the whole inflorescence reduced and converted into leafy bearded structure without bearing any grains.


Etiology: Causal organism of Downy mildew of Bajra Sclerospora graminicola Control: # Seed Treatment : hot water treatment or treatment with Cresan & Agrosan GN. # Resistant Varieties . # crop rotation & avoidance of water logging will reduce the primary inoculums.

2) Fungal Wilt.:

2) Fungal Wilt. *A wilt disease is any number of disease that affect the vascular system of plants. Vascular wilts are widespread, very destructive , spectacular and frightening plant disease . *which characterized by rapid wilting, browning and dying of leaves and succulent shoots of plant followed by death of the whole plant. *the infected plant loose turgidity, become flaccid and light green to yellow , droop , finally wilt  brown  die. *xylem vessel may be clogged with mycelium ,spores or polysaccharides produced by fungus. Clogging is further increased by gels and gums formed by accumulation and oxidation of breakdown products of plant cells attacked by fungal enzymes. *fungus in leaves cause reduced chlorophyll synthesis along the vein (vein clearing) and reduced photosynthesis, disrupt the permeability of the leaf cell membranes and their ability to control water loss through transpiration , and thereby result in leaf epinasty ,wilting , interveinal necrosis , browning and finally death.

Fusarium Wilt ::

Fusarium Wilt : *Fusarium wilt is a common vascular wilt fungal disease , exhibiting symptoms similar to Verticillium wilt . The pathogen that causes Fusarium wilt is Fusarium oxysporum . *F. oxysporum is a common soil saprophyte that infects a wide host range of plant species around the world. Development of the disease is favored by high temperatures and warm moist soils. *The fungal pathogen Fusarium oxysporum affects a wide variety of hosts of any age. Tomato, tobacco, legumes, cucurbits, sweet potatoes and banana are a few of the most susceptible plants, but it will also infect other herbaceous plants. * Fusarium oxysporum generally produces symptoms such as wilting, chlorosis, necrosis, premature leaf drop, browning of the vascular system, stunting, and damping-off. The most important of these is vascular wilt.  On older plants, symptoms are more distinct between the blossoming and fruit maturation stages.

Fusarium Wilt Of Tomato ::

Fusarium Wilt Of Tomato : Symptomatology : # slight vein clearing on outer leaflets and drooping of leaf petioles . # leaves Epinasty caused by drooping of Petioles. # Later the lower leaves wilt, turn yellow and die and the entire plant may be killed, often before the plant reaches maturity. Wilting of leaves , marginal necrosis , brown streaks, defoliation. # yellowing of the foliage, beginning with the lower leaves and working upward. Yellowing often begins on one side of the vine. Infected leaves later show downward curling, followed by browning and drying.

# In many cases a single shoot wilts before the rest of the plant shows symptoms or one side of the plant is affected first .Main stem is cut, dark, chocolate-brown streaks may be seen running lengthwise through the stem. :

# In many cases a single shoot wilts before the rest of the plant shows symptoms or one side of the plant is affected first .Main stem is cut, dark, chocolate-brown streaks may be seen running lengthwise through the stem. # This discoloration often extends upward for some distance and is especially evident at the point where the petioles joins the stem. Vascular browning can be seen in infected stems and large leaf petioles.

# root systems are stunted. The degree of stunting depends upon time of root infection and in some cases adventitious roots are formed.:

# root systems are stunted. The degree of stunting depends upon time of root infection and in some cases adventitious roots are formed.


Etiology: Fusarium oxysporum f. sp. lycopersici The pathogen is soil borne and persists for many years in the soil without a host. Most infections originate from the fungus associated with infected tomato debris.

Control ::

Control : # Use of Resistant Varieties ( Amelia , BHN-444, Carolina gold, Empire, Florida 47, Floralina , Mountain pride , etc….) # Soil sterilization . # hot water treatment to seeds. # Solar heating .

3) Bacterial Blight of Rice ::

3) Bacterial Blight of Rice : Damage is due to partial or complete blighting of the leaves or due to complete wilting of the affected tillers leading to unfilled grains. This disease known since 1881 in Japan , become quite severe in India since 15 years. In leaf blight disease, the spots enlarge in size, rapidly necrotic, and through combine of several small spots, may produce large dead areas of various shapes. Bacterium induce either wilting of plant or leaf blight .Wilting syndrome known as ‘Kresek’ occurs sporadically in the field causing serious damage. Bacterial blight commonly occur within 3-4 weeks after transplantation of crop. Bacterial blight of rice causes annual losses as high as 60%.

Symptomatology ::

Symptomatology : # Linear yellow to straw colored stripes with wavy margin developed usually on both the edges of the leaf. The stripes usually develop from the tip towards the base  dry rapidly  wither. # Spots formed on leaf sheath combine to form straw brown large lesions. # Bacterial ooze, pale amber in color, is found on the affected portions. On drying, these drops make crust and make the leaf surface rough. # As disease advances, the lesions cover the entire blade, turn white & later become grayish due to the growth of saprophytic fungi.

# wilt affected tillers leading to unfilled grains.:

# wilt affected tillers leading to unfilled grains.


Etiology: Xanthomonas campestris pathovar oryzae Gram –ve rod , 0.5 – 1.0 micron ,non – sporing with a single polar flagella, aerobic and TDP – 53 degree Celsius . On N.A , colonies are tiny , circular & bright yellow. The pathogen may survive on crop debris in soil. The bacterium is carried over through infected seeds. Rainy weather , strong wind and temperature of 22-26 C favor the rapid spread of disease in the field.


Control: # Seed Treatment – soaking of seeds for 12 hrs in 0.025% aq. sol. of Agrimycin + 0.05% wettable Ceresan. # Five foliar sprays of Agrimycin + copper oxychloride at 12 days intervals. # Resistant Varities – N 22 (highly resistant), IR 20 , IR 22, Ratna (mildly resistant) IR 8 , Jaya & IR 24 (susceptible). #Destroy plant debris. #Use certified seed . #Disinfect cutting knife tools, etc. with sodium hypochlorite , (1:5000) HgCl2 .

4) Cucumber Mosaic ::

4) Cucumber Mosaic : Cucumber mosaic virus (CMV) is a plant pathogenic virus from the family Bromoviridae and genus Cucumovirus. This virus has a worldwide distribution and a very wide host range. It can be transmitted from plant to plant both mechanically by sap and by aphids in a stylet-borne fashion . It can also be transmitted in seeds and by the parasitic weeds, Cuscuta sp. This virus cause mosaics, stunting of plant and leaf and fruit malformations.


Symptomatology: # This virus causing Mottling or discoloration and distortion of leaves , flower , and fruits. # Infected plant reduced in size may die . # leaves become mottled, spotted , distorted & wrinkled.

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# Stem internodes & petiole being shorter  plant appear dwarf . # Fruit produced on the plant after the infection shows pale green to yellow and white areas intermingled with dark green , raised areas; the latter often form rough , wart like projections and cause distortion of fruit.


Etiology: Cucumber Mosaic is caused by Cucumber mosaic virus. ( +ssRNA) , spherical virus with icosahedral symmetry , 29nm in diameter , genome consists 3 RNAs (ss). RNA 1 – 3.2 Kb & RNA 2 – 3.1 Kb (codes for protein for rna replication). RNA3 – 2.2 Kb (codes for protein help in cell- to – cell movement of virus). RNA4 1.0Kb(minor) codes for coat protein.

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CMV is mainly transmitted by aphids . It can also be spread mechanically by humans. The virus is easily spread through cultivating and even touching healthy plants after touching infected plants and can also be transmitted by seed. This virus, like many, replicates in the cytoplasm. Movement through the plasmodesmata occurs for cell-to-cell transfers but the phloem is utilized for long distance movement within the plant.


Control: # Resistant Varieties - Bush Champion F1, marketmore. *“trap crop ” ( In this method farmers plant resistant varieties around the perimeter of their fields and place the susceptible plants in the middle. Their intention is that the aphids carrying disease will first land on the resistant varieties and by the time they have eaten their way to the susceptible varieties they will no longer be carrying the virus .) # spray mineral oil or insecticide to control aphids. # antiviral spray – Virol -H, Bruceine – D, Anti – VSD. # use virus free seed and transplants.

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REFERENCES : * George . N. Agrios (2000), Plant pathology , 4th Ed academic Press. *Mehrotra & Aggarwal (2001), Plant pathology , Ed . PHI , India. *Dr. P.D Sharma (2004), Plant Pathology , Published by Rajson Printer , New Delhi. *www.

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