SYMPTOMS OF PLANT DISEASES

Any change external or internal in the host plant which serves to recognize the disease is called symptoms. The most important type of symptoms is as follows.

I. Necrosis: It includes death of the infected tissues. The different necrotic symptoms are as follows.

1. Wilts: In wilts, the host plant loses water and gets dehydrated.
2. Damping off: The tissues at the base of the stem in young seedling collapse and degenerate leading to their death in this region.
3. Rot: In rot, the affected tissues are completely disorganized leading to their death in this region.
4. Streak or Stripes: In stripes, the affected tissue appears in the form of elongated and narrow lesions.
5. Canker: canker is the sunken necrotic lesions developed in cortical tissue of stem, leaf and fruit.
6. Spot: Spot is produced due to death of localized region in the root, stem or leaf.
7. Blight: Blight is the burnt appearance due to sudden death of a plant or its organ.
8. Die-back: It is charaterised by dying of plant organs, especially stem or branches, from tio backwards.

II. Hyperplasia (Hypertrophy): It includes increase in cell division or increase in cell size leadind to outgrowth. The different hyperplasia symptoms are as follows;

Galls: Galls are globose, elongated or irregular malformation or outgrowth developed on the affected part of the host plant. Small galls are called warts or tubercules, while larger one are called knots.
Pocket or Bladder: Bladders are formed when the affected fruit is considerably enlarged, distorted and often hollowed to form pockets.
Curl: Curl is formed due to hypertrophy in localized area of the leaf.
Witches broom: It is closely grouped clusters of fine slender braches generally arranged parallel to one another.
Floral abnormalities: The floral parts of some host plants are enlarged and become fleshy and leaf like due to the infection of certain fungi.

III. Hypolysia (Dwarfing or atrophy): This is the reduction in the size of plant parts. The different dwarfing is as follows:
Chlorosis: Poor development of chlorophyll leads to chlorosis. It is caused due to low temperature, mineral deficiencies or certain infections.
Variegation: It is appearance of white or yellow spot in the green chlorophyllous region of the infected leaves.
Stunting: It is caused due to stunting growth leading to dwarfing.
Mosaic: It is development of light green or yellow patches with dark green pathches.
Rossetting: It is caused due to shortening of the internodes of shoot. It leads to crowning of the foliage in cluster assuming in rosette form.
Vein-clearing: It is caused due to yellowing of tissue near the vein.

Plant Pathology

The branch of science which deals with the study of nature, development and control of plant diseases is known as Plant pathology or Phytopathology.
De Bary is considered as "father of modern plant pathology".

Important terms related with plant pathology

Disease: Disease may be defined as morphological and physiological disturbance in the part caused by some external agencies.
Pathogen: An agent that generates a disease is called pathogen.
Host: Host is an organism attacked by a pathogen.
Infection: Infection is the establishment of pathogen in the host tissues.
Incubation: Incubation is the interval between infection and the appearance of a disease.
Susceptible: Host which are easily attacked by some pathogen.
Resistant: Resistant are the plants which are not attacked by Some pathogens.


Classification of Plant Diseases

I. Based on severity of infection and geographical distribution

1. Endemic diseases: A disease when more or less constantly present from year to year in a moderate to severe form is called endemic disease.
e.g. Wart disease of potato.
2. Epidemic (Epiphytic) diseases: When the disease is widespread and severe. It is known as epidemic disease.
e.g. Rust diseases of cereal crops.
3. Sporadic diseases: These are endemic disease which occurs at very irregular interval, locations and relatively in few instances.
e.g. Blotch disease of cucumber.
4. Pendemic diseases: Diseases which occure all over the world and cause mass mortality.
e.g. Late blight of potato.

II. Based on extent of infection on host

1. Localised disease: Such diseases are confined to a particular region.
e.g. only a part or organ is infected.
2. Systemic diseases: Such diseases spread on entire plant and associated with nearly every stage of its life cycle.

III. Based on nature of causal organism

1. Infectious or parasitic diseases: Such diseases are incited by living agent which are parasitic on the host. According to their causal organisms, these diseases can be mycoplasmal, viral bacterial, fungal etc.
e.g. Little leaf of brinjal (Mycoplasmal)
Leaf role of potato (Viral)
2. Non-infectious or non-parasitic diseases: Such diseases are incited by non living agent and thus non infectious. Unfavourable environment condition, deficiency of nutrients etc. are the causal factors of these diseases.
e.g. Tip burn of rice (due to high soil moisture content), Read leaf of mango (mineral deficiency)

Classification of Living Organisms

Two Kingdom System of Classification

Linnaeus (1758) gave this system of classification, according to which all living organisms are grouped into two large Kingdoms.
Plantae- (includes all plants)
Animalia- (includes all animals)
Plants can be differentiated from animals on the basis of their nutrition, movements, types of growth, body structure and cellular structure. But there are few exceptions to all these characteristics and because of these exceptions this system has been criticized.

Four Kingdom System of Classification
This system of classification was given by Copeland (1956). He has divided all organisms into four kingdoms.
Kingdom Mycota or Monera- They are group of organisms having incipient nucleus.
e.g. Bacteria and Blue green algae.
Kingdom Protoctista- They are the organisms having true nucleus.
e.g. Protozoa, red and brown algae and fungi.
Kingdom Plantae- These are multicelluler plant having cell wall and chloroplast. e.g. Algae, Bryophyta, Pteridophyta, Gymnosperms and Angiosperms.
Kingdom Animalia- They are multicelluler animals showing eukaryotic organization.
e.g. All eukaryotic animals.


Five Kingdom System of Classification
R. H. Whittaker (1969) gave this system of classification. He is divided all organisms into five kingdoms.
1. Kingdom-Monera- It includes all the prokaryotic organisms lacking nuclear membrane, mitochondria and plastids.
e.g. Blue-green algae and bacteria.
2. Kingdom-Protista- It includes unicellular, aquatic eukaryotes having nucleus, plastids, mitochondrion, endoplasmic reticulum and golgi bodies.
e.g. Amoeba, Euglena etc.
3. Kingdom-Plantae- It includes all the coloured, multicelluler, photosynthetic, terrestrial and acquatic plants.
e.g. Algae, Bryophyta, Pteridophyta, Gymnosperms and Angiosperms.
4. Kingdom-Fungi- It includes multicellular heterotrophic organisms lacking plastids and photosynthetic activity.
5. Kingdom-Animalia- It includes all multicelluler animals showing eukaryotic organization. They lack plastids and photosynthetic pigments and shoe holozoic mode of nutrition.
e.g. All eukaryotic animals.

SUMMARY OF LOSS BY PLANT PARASITIC NEMATODES

The table indicated that the loss of crop production by plant parasitic nematode on a world wide and on a country basis.

LIFE CYCLE

The life cycle completed in 25 days at 27 °C, but it takes longer time at lower or higher temperatures. When the egg hatch, the infected second stage juveniles may migrate from within galls to adjacent parts of the root and cause new infections in the same root, or they may emerge from root and infects other roots of the same plants or roots of the other plants. The greatest numbers of root-knot nematode are usually in the root zone from 5-25 cm below the surface. The schematic diagram of the root-knot nematode life cycle shows the clear life span of the nematode.

The Pathogens: Meloidogyne spp.

The male and female root-knot nematodes are easily distinguishable morphologically.
The males are wormlike and about 1.2-1.5 mm long × 30-36 μm in diameter. The female are pear shaped and about 0.4-1.3 mm long × 0.27-0.75 mm wide.

Each female lays approximately 500 eggs in a gelatinous substance produced by the female. The first and second stage juveniles are wormlike and develop inside each egg. The Second stage juvenile emerges from the egg into the soil. This is the only infected stage of the nematode. If it reaches into the susceptible host, the juvenile enters into the root, becomes sedentary, and grows thick like sauasage.

SYMPTOMS

The above ground symptoms are reduced growth, fewer, small, pale green, or yellowish leaves that latter tends to wilt. Affected plants unusually linger the growing season and are seldom killed prematurely. The underground symptoms shows infected roots swell at the point of invasion and developed into typical root-knot galls that are two to several times as larger in diameter as the healthy root. Several infections takes place along the same root, and developing galls gives root rough, clubbed appearance. However, infected roots remain smaller; show various stages of necrosis, and developing rooting, particularly late in season.

ROOT-KNOT NEMATODES: MELOIDOGYNE

Root-knot nematodes occur throughout the world. It attack more than 2000 species of plants, including almost all cultivated plants, and reduced the world crop production about 5 %. Losses in individual fields, however, may be much higher.
Root-knot nematodes damage plants by devitalizing root tips and causing formation of swellings of the roots. These effects not only deprive plants nutrients but also disfigure and reduced the market value of the crops.

PGPR and its Use

Some bacteria are associated with roots of crop plants and exert beneficial effects on their hosts and are denoted to as plant growth-promoting rhizobacteria (PGPR) (Kloepper et al., 1980). PGPR may promote plant growth directly through improving uptake of minerals and water or the production of growth-stimulating compounds, but in many cases improved growth can be attributed to the suppression of deleterious microorganisms that are harmful to the plant (Schippers et al., 1987; Glick et al., 1999). PGPR can, thus, promote plant growth by suppressing diseases caused by various soil-borne pathogens (Van Loon and Glick, 2004).

Genera of PGPR include Azotobacter, Azospirillum, Pseudomonas, Acetobacter, Burkholderia, Bacillus, Paenibacillus, and some are members of the Enterobacteriaceae. Direct use of microorganisms to promote plant growth and to control plant pests continues to be an area of rapidly expanding research. Rhizosphere colonization is one of the first steps in the pathogenesis of soil borne microorganisms. It is also crucial for the microbial inoculants used as biofertilizers, biocontrol agents, phytostimulators, and bioremediators. Pseudomonas spp. are often used as model root-colonizing bacteria (Lugtenberg et al., 2001). The beneficial effects of these rhizobacteria have been variously attributed to their ability to produce various compounds including phytohormones, organic acids, siderophores, fixation of atmospheric nitrogen, phosphate solubilization, antibiotics and some other unidentified mechanisms (Glick, 1995). Motile rhizobacteria may colonize the rhizosphere more profusely than the non-motile organisms resulting in better rhizosphere activity and nutrient transformation. They also eliminate deleterious rhizobacteria from the rhizosphere by niche exclusion thereby better plant growth (Weller, 1988). Induced systemic resistance has been reported to be one of the mechanisms by which PGPR control plant diseases through the manipulation of the host plant’s physical and biochemical properties.

How Mycorrhizal Fungi Benefit Plants

Better uptake of nutrients
With the help of mycorrhizal fungi, a plant can take up many times more nutrients, particularly phosphorous, than would be possible in the absence of the fungi. When a dependent plant lacks mycorrhizae, growers often have to load the soil with high levels of soluble nutrients. This heavy feeding is expensive, and further damages the health of the soil and water.

Soil improvement
Mycorrhizae enhance the soil by improving the structure of soil. This helps to increase water holding capacity, and traps nutrients that otherwise could be leached by rains.

Faster rehabilitation of degraded sites
Because they enhance the plant's ability to take up nutrients and water, mycorrhizal fungi can help plants compensate for low nutrient availability, poor soil structure, low water holding capacity often prevalent on harsh sites.

Healthier plants, less disease and fewer pests
Most experts in integrated pest management say that plant health is the most important aspect of pest management--healthy plants have much fewer pest problems. Better nutrition and water uptake through mycorrhizae helps plants stay healthy.

Biocontrol of certain pathogenic organisms
By infecting the root system of a plant, mycorrhizae can interfere with pathogenic organisms, effectively protecting the host plant from diseases.

Tolerance for problem soils
Mycorrhizal fungi may also help regulate the uptake of soil toxins, allowing plants to better tolerate salty or problem soil conditions.

Mechanism of disease suppression by mycorrhizal fungi

Parasitisim of pathogen by mycorrhizal fungi
Ø Mycorrhizal chlamydospores have been reported to occupy seeds and dead insects in soil and have limited saprophytic capabilities. It seems likely that mycorrhizal fungi colonize only stressed or weakened nematode eggs.
Ø The nematode parasitism by mycorrhizal fungi is opportunistic and depends on the carbon nutrition from autotrophic symbionts, rather than being representative of a true host-parasite relationship.

Change in root morphology
Ø Root offers structural supports to plants, functions in of water and mineral nutrients, are the site of production of growth regulators, are a sits of starch storage and provide a nutrient supply for a wide range of microorganisms.
Ø Change in root morphology ultimately affects their responses to other organisms.
Ø Mycorrhizal plants also have a strong vesicular system which imparts greater mechanical strength to diminish the effects of pathogens.

Histopathological changes
Ø Mycorrhizal fungi had fewer giant cells and smaller syncytia which confer resistance against the nematode in the host plants.
Ø Nematodes developed in mycorrhizal plants were smaller in size and took longer time to develop into adults.

Physiological and Biochemical changes
Ø The physiological changes caused by mycorrhizal fungi in the host plant generally reduced the severity diseases.
Ø Phenolic compounds formed after mycorrhizal colonization has been thought to play important role in the defense resistance.
Ø Inoculation of mycorrhizal fungi also increased the level of phytoalexin in plants, which play a major role in the host defense system against the pathogens.
Ø Inoculation of mycorrhizal fungi increased the level of amino acids (phenyalanine and serine) in the host plant and it is assumed that these amino acids having inhibitory effect against the root pathogens.

Change in host nutrition
Ø Improved the phosphorus nutrition.
Ø Mycorrhizal fungi induced decreased in root exudation, which has beeen the cause of reduction of soil borne diseases.
Ø Mycorrhizal fungi can also affect changes induced by environmental stress in root growth, root exudation, nutrient absorption and host physiology.
Ø Mycorrhizal fungi in P-deficient plant affected membrane permeability and exudation pattern in a way similar to that caused by P-fertilization in non-mycorrhizal plants.

What are Mycorrhizae?

The word mycorrhiza was first used by German researcher Albert Bernard Frank (1885). It originates from the Greek word ‘mycos’ meaning ‘fungus’ and ‘rhiza’ meaning ‘root’. It forms a symbiotic association in a similar fashion to the root-nodule bacteria in legumes. The fungus takes carbohydrates from the plant and in turn supplies the plant with nutrients, hormones, ect.
Types of mycorrhizal associations

1. Ectomycorrhizae
2. Endomycorrhizae
3. Ericoid mycorrhizae
4. Orchidaceous mycorrhizas

Ectomycorrhizae
Ø Fungus forms a sheath around the root, with hyphae emanating through the soil, greatly increasing the surface area
Ø Fungus penetrate between the cells of the cortex to facilitate nutrient exchange
Ø Fungus is almost a Basidiomycota although a few are Ascomycota species.

Endomycorrhizae
Ø Also known as Vesicular arbuscular-mycorrhizae
Ø Fungus does not forma a sheath around the root fungus, penetrate into the cortical cell but does not penetrate the cell membrane
Ø Fungus is the member of Glomeromycota
Ø More common than Ectomycorrhizae

Ericoid mycorrhizae
Ø The fungus grows loosely over the lateral "hair" roots of the host plant and the septate hyphae penetrate the single layer of cortical cells, often at several points, and fill them with intracellular hyphal coils. The apex of the "hair" root is not colonised and the stele is never penetrated.

Orchidaceous mycorrhizae
Ø The fungus grow into the plant cell, invaginating the cell membrane and forming hyphal coils within the cell. These coils are active for only a few days, after which they lose turgor and degenerate and the nutrient contents are absorbed by devloping orchid.
Ø The mycorrhizal fungi in orchids are Basidiomycota, and in particular species of Rhizoctonia.
Ø In mature orchids, mycorrhizae also have roles in nutrient uptake and translocation.
Ø The ericoid type is the most important and is found in such genera as Calluna, Erica, Rhododendron and Vaccinium.


Effects of Mycorrhizae on plants

ð Increase in nutrient uptake
ð Selective uptake of certain elements
ð Increase in drought resistance
ð Increase in survival after out planting
ð Increase in growth rate
ð Protection from root pathogens

Benefits
Ø Increased plant nutrient supply by extending the volume of soil accessible to plants.
Ø Increased plant nutrient supply by acquiring nutrient forms that would not normally be available to plants.
Ø Some ECM and ericoid fungi have the capacity to breakdown phenolic compounds in soils which can interfere with nutrient uptake.
Ø Root colonisation by ECM and VAM fungi can provide protection from parasitic fungi and nematodes.
Ø Non-nutritional benefits to plants due to changes in water relations, phytohormone levels, carbon assimilation, etc. have been reported, but are difficult to interpret.
Ø Mycorrhizal benefits can include greater yield, nutrient accumulation, and/or reproductive success. Mycorrhizas can cause growth form changes to root architecture, vascular tissue, etc.
Ø Suppression of competing non-host plants, by mycorrhizal fungi has been observed.
Ø Significant amounts of carbon transfer through ECM fungus mycelia connecting different plant species have been measured. This could reduce competition between plants and contribute to the stability and diversity of ecosystems.
Ø Networks of hyphae supported by dominant trees may help seedlings become established or contribute to the growth of shaded understorey plants.
Ø Nutrient transfer from dead to living plants can occur.

Arbuscular Mycorrhizal (AM) Fungi

AM fungi occur over a wide range of agro climatic conditions and are geographically ubiquitous. They form symbiotic relationships with roots of about 90% land plants in natural and agricultural ecosystems (Brundrett, 2002). The AM association has been observed in 200 families of plants representing 1,000 genera and about 3, 00,000 plant species (Bagyaraj, 1991) and it is as normal for the roots of plants to be mycorrhizal as it is for the leaves to photosynthesize (Mosse, 1986). These fungi are included in the phylum Zygomycota, order Glomales (Redecker et al., 2000) but recently they have been placed into the phylum ‘Glomeromycota’ (Schussler et al., 2001). The Glomeromycota is divided into 4 orders, 8 families, 10 genera and 150 species. The common genera are Acaulospora, Gigaspora, Glomus and Scutellospora (Schussler, 2005). They are characterized by the presence of extra radical mycelium branched haustoria like structure with in the cortical cells termed as arbuscules and are the main site of nutrient transfer between two symbiotic partners (Hock and Verma, 1995; Smith and Read, 1997).

AM fungi colonize plant roots and ramify into surrounding soil extending the root depletion zone and the root system. They supply water and mineral nutrients from the soil to the plant while AM is benefited from carbon compounds provided by the host plant (Smith and Read, 1997; Siddiqui et al., 1999). AM fungi are associated with improved growth of host plant species due to increased nutrient uptake, production of growth promoting substances, tolerance to draught, salinity and synergistic interactions with other beneficial microorganisms (Sreenivasa and Bagyaraj, 1989).
The soil conditions prevalent in sustainable agriculture are likely to be more favorable to AM fungi than those under conventional agriculture (Bethlenfalvay and Schuepp, 1994; Smith and Read, 1997). Any agricultural operation that disturbs the natural ecosystem will have repercussions on the mycorrhizal system (Mosse, 1986). The preceding crops affect the growth and yield of subsequent crops (Karlen et al., 1994). The inclusion of non-mycorrhizal crops within rotations has been shown to decrease both AM fungal colonization and yield of following crops (Douds and Galvez, 1997; Arihawa and Karasawa, 2000). In addition to crop sequence, varietals selection, cultivation and fallowing have all been shown to affect mycorrhizal activity (Ocampo et al., 1980; Hetrick et al., 1996; McGonigle and Miller, 2000). However, impact of soluble fertilizers on colonization and function of AM fungi is contradictory. The application of soluble phosphorus decrease root colonization (Abbott and Robson, 1984) with occasional reports of increases (Gryndler et al., 1990). Similarly, contradictory results have also been reported with nitrogen fertilizer (Baltruschat and Dehne, 1988; Gryndler et al., 1990; Liu et al., 2000).