PAT 219. Applied Plant Pathology (2+1)

Objectives

            The subject covers the basic aspects of plant disease management through physical, chemical and biological methods devising for effective management techniques to reduce the losses.

Unit – 1 Principles of Plant disease management

                        History of disease management – definition for Plant diseases – Causes of Plant diseases – Classification of Plant diseases – Principles of Plant disease management – Prophylaxis and Immunization – Prophylaxis: Avoidance, Exclusion, Eradication and direct protection.

History of Plant disease management

1000 B.C. – Homer, the Greek poet made a mention of sulphur with pest controlling properties.

1761 – Sculthess first suggested the use of copper sulphate on wheat seed against stinking smut.  

1807 - Copper sulphate was recommended for wheat seed treatment against bunt by Prevost who for the first time established the fungitoxic value of the compound.

1821- Robertson of England stated that Sulphur is effective against powdery mildew of peach.

1882 - The modern era of chemical control of plant diseases started with the discovery of Bordeaux mixture during the epidemic of grape downy mildew in France during 1875. The mixture of copper sulphate and slaked lime had been sprinkled on the grapevines to deter the pilferage.

1887 - Mason of France introduced Burgundy mixture using copper sulphate and sodium carbonate in place of lime.

1887 – Jensen developed hot water treatment for control of wheat smut.

1889 – Weed of the USA for the first time used fungicides in combination with insecticides.

 1891 – N.A.Cobb devised first scale to measure plant disease intensity.

1910- The first description of a germination test for evaluating fungicides was done by Donald Reddick and E.Wallace.   

1929 –Cross protection or Pre-immunization technique was developed by H.H.Mckinney

1934 – W.H.Tisdale and I. Williams reported fungitoxicity of dithiocarbamates.

1940 – Burlingham and reddish proposed the “Zone of inhibition” technique for fungicidal bioassays.

1952 – Kittleson introduced captan as a fungicide. (Kittleson’s killer)

1966 - Systemic fungicides introduced by Van Schleming and Marshal Kulka. (oxathiins, Carboxin)

1968 – Delp and Klopping reported systemic fungicidal properties of benomyl.

1978 – Fosetyl –Al, a metal based systemic fungicide effective against phycomycetes with pronounced basipetal translocation was developed under code number LS-74-783.

1981 – Fungicide Resistance Action Committee (FRAC) was founded.

1996 – First Strobilurins fungicide launched, which was isolated from wood rotting mushroom fungi (Strobilurus tenacellus; Basidiomycetes fungus). 

1931 - Sanford and Broadfort were the first to use the terms Biological control and suppressive effect in Plant pathology. 

1932 – The first suggestion for the use of Trichoderma spp. as biocontrol agent was given by Weindling.

1963 – The first commercially available bio control agent was Peniophora gigantean for the control of Heterobasidium annosum.

1914 –(03.02.1914) In India, Destructive insect and Pests Act (DIPA/ DIP ACT) was passed prohibiting or restricting the import of plant and plant materials, insects, fungi to India form foreign countries.

1964 – Thirumalachar reported the development of Aureofungin, a new antifungal antibiotic for use in plant disease control.

1975 – S.Nagarajan and H.Singh formulated a set of synoptic conditions called the “Indian Stem Rust Rules” for predicting possible outbreaks of wheat stem rust.

1980 – S.Nagarajan and L.M.Joshi defined the route of long distance spread of black stem rust of wheat between the source and the target as “Puccinia path” of India and also suggested the possibility of avoiding epidemics by deployment of resistance genes along this path.

J.E.Vanderplank – Father of Plant Disease Epidemiology

Paul Neergaard – Father of Seed Pathology

S.D.Garrett – Father of Biological Control

Jacov Katan – Father of Soil Solarization

T.S.Thind – Father of Indian Fungicides

Edmond Thenard – Father of Soil fumigation (CS₂)

Haberlandt - Father of Plant tissue culture

N.A.Cobb – Father of Nematology (American)

Important textbooks/ reference books related to this course

1971 – Y.L.Nene and P.N.Thapliyal wrote the first book on fungicides in India – “Fungicides in Plant Disease Control”. (now in 3^rd edition)

1984 – S.C.Vyas wrote a book entitled “Systemic Fungicides”.

1980 – V.K.Agarwal derived a method known as “NaOH Seed Soak Method” for detection of Karnal and paddy bunt infection in wheat and rice seed samples.  He also wrote a book entitled “Principles of Seed Pathology”.  (now in 2^nd edition with J.B.Sinclair)

1974 – The first book devoted wholly to the subject of biocontrol of plant pathogens was published by K.F.Baker and R.J.Cook entitled “Biological control of Plant Pathogens”.

1963 – Publication of classical book “Plant Diseases: Epidemics and control” by J.E.Vanderplank

History of Plant Nematology

Needham – Discovery of wheat seed gall nematode Anguina tritici

Atkinson – 1^st report of root knot nematode and Fusarium  complex in vascular wilt in cotton

Hinger - 1^st report of root knot nematode and Bacterial  wilt in tomato

Hewitt, Raski & Goheen – Transmission of grapevine fan leaf virus by Xiphinema index 

Butler – reported Ufra disease on rice is caused by Ditylenchus angustus

Dastus – reported white tip disease on rice is caused by Aphelenchoides besseyi

 

Definition for Plant diseases

·         Disease is a malfunctioning process that is caused by continuous irritation, which results in some suffering producing symptoms. (APS & British Mycological Society (BMS))

·         A plant is said to be „diseased‟ when there is a harmful deviation from normal functioning of physiological process (Federation of British Plant Pathologists).

·         Any malfunctioning of host cells and tissues that result from continuous irritation by a pathogenic agent or environmental factor and leads to development of symptoms (G.N.Agrios).

 

Causes of Plant Diseases

            Biotic agents (Animate agents/ Parasitic agents)

1. Prokaryotes

True bacteria or bacteria E.g.. Rice BLB

Rickettsia-like bacteria (RLB / FVB) E.g. Citrus greening, Pierce's disease of grape

Mollicutes or wall-less prokaryotes

i. Mycoplasma-like organism (MLO)/ Phytoplasma:  e.g. Tomato bunchy top, Sesame phyllody

ii. Spiroplasma e.g. Corn stunt, Citrus stubborn

2. Eukaryotes

Fungi   E.g. Late blight of potato

Fungi like Protozoa E.g. Club root of cabbage

Algae E.g.  Red rust of guava

Plants - Parasitic flowering plants or phanerogamic parasites - Broomrape of tobacco.

Animals- Nematodes Ex. Rice Ufra disease – Ditylenchus angustus.

            Mesobiotic agents

Ex.  Viruses and Viroids. They are infectious agents. They can be crystallized and are considered as non-living. But their multiplication in the living plants ensures that they are living (Obligate parasites / Biotroph). Hence they are called as mesobiotic agents.

Viruses E.g.  BYVMV

Viroids E.g.  PSTVd

            Abiotic agents or Non-parasites / Inanimate agents / non-infectious / physiological disorders                      Black heart of potato – O₂ deficiency

Sunscald of fruits – High temperature

Blossom end rot of tomato – Ca deficiency

             

Classification of plant diseases

            Based on type of infection

a. Localized diseases: These diseases are limited to a definite area of an organ or part(s) of a plant. E.g. Leaf spot

b. Systemic diseases: In these diseases the pathogen spreads from a single infection point so as to infect all or most of the host tissues. E.g. Viral diseases

            Based on type of perpetuation and spread

a. Soil-borne diseases: The causal agents perpetuate and spread through soil. Ex. Damping-off, root rot

b. Seed-borne diseases: Seed or seed materials help in the perpetuation and spread of this disease.

E.g. Loose smut of wheat (Ustilago nuda tritici internally seed borne)

      Blast of rice (Pyricularia oryzae externally seed-borne).

c. Air-borne diseases: Anemochory (Most of the foliar diseases)

d. Water – borne diseases (Hydrochory): Soli borne diseases

            Based on extent of occurrence and geographic distribution

a.       Endemic diseases (enphytotic disease): When a disease is more or less constantly occurring year after year in a moderate to severe form in a country or locality then it is called as an endemic disease.

E.g. wart disease of potato (Synchytrium endobioticum is endemic in Darjeeling)

citrus canker is endemic in Asia

Sorghum rust is endemic in India.

b.       Epidemic or epiphytotic disease: An epidemic or epiphytotic refers to sudden outbreak of a disease periodically over a widespread area in a devastatingly severe form causing extensive losses or complete destruction.

E.g. wheat powdery mildew, rice Lakshmi / false smut disease

c.       Sporadic diseases: Sporadic diseases are those, which occur at irregular intervals over limited areas or locations. E.g. Fusarium wilt of cotton (Fusarium oxysporum f.sp. vasinfectum)

d.       Pandemic diseases: A disease is said to be pandemic when it is prevalent throughout the country, continent or world involving mass mortality. E.g. Late blight of potato and wheat stem rust.

            Based on the multiplication of inoculum

a.       Simple interest disease (monocyclic / Saturation curve):  the disease increase is just like simple interest in money. Here inoculum comes from a reservoir and hence amount of inoculum for a given season's crop is fixed. So there is no repetition of the disease cycle within the crop season. Hence the disease spread will be slow. e.g. Soil inhabiting pathogens like Pythium sp., Rhizoctonia sp. and Sclerotium sp.

b.       Compound interest disease (polycyclic / Sigmoid curve/ ‘S’ shaped progress curve): Inoculum is multiplied several times (every 7 to 15 days for wheat rust) during crop growth in a season. So the disease spread will be fast. e.g. Wheat stem rust, rice blast, powdery mildew diseases of different crops.

c.       Polyetic disease (Bimodal curve): Several years are required for completing their life cycle of this pathogen. E.g. Dutch elm disease, Citrus Tristeza, Pear decline

      Based on the crops affected

a.       Sugarcane red rot

b.       Wheat Tundu disease

c.       Club root of crucifers

      Based on the organs affected

a.       fruit rot

b.       Sheath blight

c.       Stem rot

            Based on Host Pathogen dominance system

a.       Pathogen dominant diseases: The pathogen is dominant over the host, but the relationship is transitory because the resistance of the host is less initially than it becomes eventually. E.g. Macrophomina, Pythium

b.       Host dominant diseases: The host is dominant and the pathogen is successfully only, when factors favour the pathogen over the host. E.g. Fusarium, Helminthosporium

 

Principles of Plant disease management

Methods for plant diseases management were first classified by Whetzel (1929) into Exclusion, Eradication, Protection and Immunization. Further advances in plant pathology leading to development of newer methods, two more principles - avoidance and therapy were created (NAS, 1968)

a.       Exclusion

b.       Eradication

c.       Protection

d.       Immunization

e.       Avoidance

f.        Therapy

The six principles that characterize the modern concept of plant disease management should be viewed from three stand points

(a) Reduction in the initial inoculums or the rate of disease development.

(b) Management of the pathogen population, the cure or induce defense of the suscept or modify the environment as it influences disease

(c) Interruption of dispersal, survival or the course of disease development.

Exclusion

            It means preventing the inoculums from entering or establishing in a field or area where it does not exist.

            Seed treatment

1.       Seed treatments with Brine Solution (20 % Nacl) for manage the BLB, Ergot and Tundu disease.

2.       Hot Water Treatment with Sugar cane setts @ 50⁰C  for 2-2and half hours for manage the grassy shoot and ratoon stunting disease.

3.       Hot Air Treatment with Sugar cane setts @ 54⁰C  for 8 hours for manage the grassy shoot disease (HAT).

4.       Aerated Steam Therapy with Sugar cane setts @ 50⁰C  for one hour for manage the grassy shoot (AST).

5.       Moist Hot Air Treatment with Sugar cane setts @ 54⁰C  for 2 hours for manage the grassy shoot (MHAT.

            Inspection and certification

Due to the inspection, necessary precautions are taken to remove the diseased plants, badly affected fields and roguing. By this method, inter and intra regional spread of seed borne diseases can be prevented,

            Quarantine

Ø  Quarantine the word which is derived from Italian language and means forty days retention period. Plant quarantine is defined as “a legal restriction on the movement of agricultural commodities for the purpose of exclusion, prevention or delaying the spread of the plant pests and diseases in uninfected areas”.

Ø  Plant quarantine laws were first enacted in France (1660), followed by Denmark (1903) and USA (1912) (Eradication of Barberry bush)

Ø  In India, plant quarantine rules and regulations were issued under Destructive Insects and Pests Act (DIPA or DIP Act) in 1914 (03.02.1914).

1. Domestic quarantine: Rules and regulations issued prohibiting the movement of insects and diseases and their hosts from one state to another state in India is called domestic quarantine. Domestic quarantine in India exists for two pests (Rooted scale and Sanjose scale) and three diseases (Bunchy top of banana, banana mosaic and wart of potato).

Bunchy top of banana: It is present in Kerala, Assam, Bihar, West Begal and Orissa. Transport of any part of Musa species excluding the fruit is prohibited from these states to other states in India.

Banana mosaic: It is present in Maharashtra and Gujarat. Transport of any part of Musa species excluding the fruit is prohibited from these states to other states in India.

Wart of potato: It is endemic in Darjeeling area of West Bengal, therefore seed tubers are not to be imported from West Bengal to other states.

2. Foreign quarantine: Rules and regulations issued prohibiting the import of plants, plant materials, insects and fungi into India from foreign countries by air, sea and land.  Foreign quarantine rules may be general or specific. General rules aim at prevention of introduction of pests and diseases into a country, where as the specific rules aim at specific diseases and insect pests. The plant materials are to be imported only through the prescribed ports of entry.

Airports: Bombay (Santacruz), Calcutta (Dum Dum), Madras (Meenambakam), New delhi (Palam, Safdarjung) and Tiruchurapalli.

Sea ports: Bombay, Calcutta, Vishakapatnam, Trivandrum, Madras, Tuticorin, Cochin and Dhanushkoti.

Land frontiers: Hussainiwala (Ferozpur district of Punjab), Kharla (Amritsar district of Punjab) and Sukhiapokri (Darjeeling district of West Bengal)

3. Embargo: It is government prohibition against the shipment of certain products to a particular country for economic or political reasons.

Crop	Countries from where import is prohibited
Coffee	Sri Lanka, Africa, South America
Sugarcane	Fiji, Australia, Philippines
Sunflower	Argentina, Peru

Phytosanitary certificate (PSC): It is an official certificate from the country of origin, which should accompany the consignment without which the material may be refused from entry.

PEQ- Post Entry Quarantine

ISPM – International Standards for Phytosanitary Measures

PRA – Pest Risk Analysis is done to protect the country’s agriculture from damages that can be caused by harmful quarantine pests which can be brought in along with imported commodities.

There are 4 stages of PRA are

1.       Pest Risk initiation

2.       Risk assessment

3.       Risk management

4.       Risk documentation

PFS Order – Plants Fruits and Seeds Order (1984)

FVS Order – Flowers and Vegetable seeds order

PSC – Phytosanitary certificate

IPPC – International Plant Protection Convention (1951)

APHIS – Animal, Plant Health Inspection service, USA

CBD – Convention on Biological Diversity (Rio Conversion, 1992) (Quarantine of invasive species)

Directorate of Plant Protection, Quarantine & Storage situated at Faridabad, Haryana

National Plant Quarantine station is situated at New Delhi

SPS – Sanitary and Phytosanitary agreement of WTO

Plant Quarantine (Regulation of Import into India) order 2003 under DIP Act

TRIPS – Trading Related Intellectual Property Rights

Sl. No.	Disease	Causal Organism	Year	Introduced into	From
1.	Late Blight of Potato	Phytophthora infestans	1883	India	Europe
2.	Coffee rust	Hemileia vastatrix (Wardia vastatrix)	1879	India	Srilanka
3.	Powdery mildew of crucifers	Erysiphe cichoracearum	1910	India	Srilanka
4.	Wheat flag smut	Tuburcinia agropyri	1906	India	Australia
5.	Downy mildew of grapes	Plasmopara viticola	1910	India	Europe
6.	Black rot of crucifers	Xanthomonas campestris	1929	India	Holland
7.	Powdery mildew of rubber	Oidium heveae	1939	India	Malaysia
8.	BLB of rice	Xanthomonas oryzae	1964	India	Philippines
9.	Blast of rice	Magnaporthe grisea	1918	India	South East Aisa
10.	Downy mildew of maize	Peronosclerospora sorghi	1912	India	Java
11.	Ergot of Bajra	Claviceps fusiformis	1957	India	Africa
12.	Black shank of tobacco	Phytophthora nicotianae	1938	India	Holland
13.	Panama wilt of Banana	Fusarium cubense	1920	India	Panama Canal
14.	Bunchy top of Banana	BBTV	1940	India	Srilanka
15.	Wart of Potato	Synchytrium endobioticum	1953	India	Netherland
16.	Fire blight of apple	Erwinia amylovora	1940	India	England
17.	Golden cyst nematode	Globodera rostochinensis	1961	India	Europe

            Eradication of insect vectors

Many crop diseases spread through insect vectors which transfer the inoculum from one region to another depending on their flight range. For prevention of entry of a new pathogen exclusion of insect vectors is also essential.

 

Eradication

            The process of reducing, inactivating, eliminating or destroying inoculums at the source, either from a region or from an individual plant in which it is already established is termed as eradication.

One of the most  extensive eradication operations carried out so far was to get rid of the citrus canker in the USA during 1927- 35 (Excellent cultural control)

            Biological control (Sanford and Broadfoot)

The reduction of inoculum density or disease producing activities of a pathogen or parasite in its active or dormant state by one or more microorganisms, accomplished naturally or through manipulation of the environment, hoist or antagonists or by mass introduction of one or more antagonists. (Baker and Cook, 1974)

The mechanisms of Biological control are Competition, Parasitism, Antibiosis & Lysis. 

Example of Antagonistic organisms:

Trichoderma viride – Fungal Bio Control Agent

Pseudomonas fluorescens – Bacterial Bio Control Agent

Ampelomyces quisqualis 10 – BCA for Powdery Mildew

1.       Suppressive soil: Soils that suppress certain diseases due to presence in soil of micro organisms antagonistic                                                                                                                                                                                          

to the pathogen. E.g. Pythium, Phytophthora, Gaeumannomyces diseases controlled by Suppressive soil. Numerous kinds of antagonistic microorganisms have been found to increase in suppressive soils; most commonly, however, pathogen and disease suppression has been shown to be caused by fungi, such as Trichoderma, Penicillium, and Sporidesmium, or by bacteria of the genera Pseudomonas, Bacillus, and Streptomyces.

2.       Mycorrhizae (VAM or AM): Fungus roots (Frank) to control soil borne diseases. E.g. Glomus, Gigaspora

3.       Cross protection or Pre-immunization: To introduce the mild strain of the virus in the healthy plant, so that to avoid the severe strain of the same pathogen. E.g. Citrus Tristeza Virus mild strain

4.       Hypovirulence: Reduced virulence of pathogen strain as a result of presence of transmissible double stranded RNA. E.g. Hypovirulent strain of Endothia parasitica , Hypovirulent strain of Rhizoctonia solani.

            Crop rotation

Continuous cultivation of the same crop in the same field helps in the perpetuation of the pathogen in the soil. Soils which are saturated by the pathogen are often referred as sick soils. To reduce the incidence and severity of many soil borne diseases, crop rotation is adopted.

It is also possible that different crops release such biochemical substances in their root exudates that either kill the pathogen directly or encourage development of antagonistic microorganisms. 

Uses of crop rotation are 1. Pathogen lives in soil for very short time, 2. Having short host range, 3. Annual diseases E.g. Panama wilt of banana (long crop rotation), wheat soil borne mosaic (6 yrs) and club root of cabbage (7 yrs), etc.

            Removal or destruction of diseased plant organs

1.       Sanitation

Collection and destruction of plant debris from soil will help in the management of soil borne facultative saprophytes as most of these survive in plant debris. (to reduce primary inoculum). Burying the debris deep in the soil turning ploughs also reduces the inoculum.

Late blight of Potato – Tubers and leaves

Mahali disease of arecanut – Infected and Fallen nuts

2.       Rouging

Removal of diseased plants or their affected organs from field, which prevent the dissemination of plant pathogens. E.g. Loose smut of wheat and barley, whip smut of sugarcane, red rot of sugarcaneof bajra, yellow vein mosaic of bhendi, khatte disease of cardamom, etc.

3.       Eradication of alternate and collateral hosts

E.g: Barberry eradication programme in France and USA reduced the severity of black stem rust of wheat

E.g. Eradication of Thalictrum species in USA to manage leaf rust of wheat caused by Puccinia recondita.

Eradication of collateral hosts, such as Panicum repens, Digitaria marginata will help in the management of rice blast disease (Pyricularia oryzae)

4.       Heat and chemical treatment of disease plants

  Luthra solar heat treatment : Its developed against wheat loose smut disease (Ustilago nuda tritici).

o   Wheat seeds are soaked in cold water for 4 hrs (Morning 8-12 noon)

o   Seeds are dried under sunlight conditions for the next 4 hrs (12-4 pm)

o   Seeds are treated with Carboxin @ 4-6g/kg of seeds

Banana suckers are treated or dipped with Carbendazim solution @ 1.0 % for 5 min. against Panama wilt disease.

                        Soil treatments

1.       Mixed cropping: Root rot of cotton (Phymatotrichum omnivorum) is reduced when cotton is grown along with sorghum. Intercropping sorghum in cluster bean reduces the incidence of root rot and wilt (Rhizoctonia solani).

2.       Summer ploughing: Ploughing the soil during summer months expose soil to hot weather which will eradicate heat sensitive soil borne pathogens.

3.       Soil amendments: Application of organic amendments like saw dust, straw, oil cake, etc., will effectively manage the diseases caused by Pythium, Phytophthora, Verticillium, Macrophomina, Phymatotrichum and Aphanomyces. Beneficial micro-organisms increases in soil and helps in suppression of pathogenic microbes.

E.g: Application of lime (2500 Kg/ha) reduces the club root of cabbage by increasing soil pH to 8.5

       Application of Sulphur (900 Kg/ha) to soil brings the soil pH to 5.2 and reduces the incidence of common scab of potato (Streptomyces scabies).

4.       Changing time of sowing: Pathogens are able to infect susceptible plants under certain environmental conditions. Alternation in date of sowing can help avoidance of favourable conditions for the pathogens. Ex: Rice blast can be managed by changing planting season from June to September/October.

5.       Seed rate and plant density: Close spacing raises atmospheric humidity and favours sporulation by many pathogenic fungi. A spacing of 8’X8’ instead of 7’X7’ reduces sigatoka disease of banana due to better ventilation and reduced humidity. High density planting in chillies leads to high incidence of damping off in nurseries.

6.       Irrigation and drainage: The amount, frequency and method of irrigation may affect the dissemination of certain plant pathogens. Many pathogens, including, Pseudomonas solanacearum, X. campestris pv. oryzae and Colletotrichum falcatum are readily disseminated through irrigation water. High soil moisture favours root knot and other nematodes and the root rots caused by species of Sclerotium, Rhizoctonia, Pythium, Phytophthora, Phymatotrichum, etc.

7.       Fallowing : Wilt of Banana and green ear

8.       Flooding : Panama wilt of Banana

9.       Heavy application of FYM – Wilt of cotton

            Manures and fertilizers

Excessive nitrogen application aggravates diseases like stem rot, bacterial leaf blight and blast of rice. Nitrate form of nitrogen increases many diseases, whereas, phosphorous and potash application increases the resistance of the host.

 

Protection

The protection of infection courts against the inoculums of many fast spreading infectious pathogen, brought by wind from neighboring fields or any other distant place of survival.

            Protection from Anemochory diseases

Application of fungicides by dusting and spraying to be controlled for rust, leaf spot, powdery mildew and downy mildew. The covering of fungicides upon host surface prevents germination of spores and subsequent penetration.

            Protection from seed borne diseases

The seed treatment fungicides (seed protectant) from a protective coat on the seeds so as to protect the seeds from soil borne pathogens when sown in soil as well as seed borne pathogens. .

            Protection from soil borne diseases

Physical method

1.       Soil solarization: Irrigate the soil up to a depth of 10 cm and cover the soil with polythene sheet (100 m) after 2 days of irrigation. Leave the sheet undisturbed upto 4-6 weeks. Remove the cover and disturb the soil for further sowing or planting. 

2.       Streaming: In foreign countries, stream is passed through upper soil layers with perforated tubes fixed below the soil layer.

3.       Direct flaming: Burning plant residues and wood is effective manage the crop diseases. E.g. Red rot of sugarcane

4.       Hot water: E.g. Hot water treatment of Cabbage seeds @ 50⁰ C for 30 min against the black rot of cabbage.

Chemical method: Carbendazim @ 0.1 % effectively prevent cotton root rot. Soil drenching with 1 % Bordeaux mixture against Coconut wilt, Pepper wilt, Betel vine wilt.

Biological method: Bio control agents also protect the seeds from infection by the pathogen, present in the soil.

 

Immunization

Improving or inducing the resistance of the host. The best way of controlling the disease is by the selection and planting of resistance varieties (Cheapest method of plant disease control).

Methods of developing resistant varieties

1. Introduction
2. Pure line selection from the susceptible populations
3. Breeding by crossing and hybridization
4. Mutation using X ray, UV rays, Gamma rays and chemical mutagenic agents.

Sl.      No.	Characters	Vertical resistance	Horizontal resistance
1.	Controlled by genes	Major genes	Minor genes
2.	Stability	Instable	Permanent
3.	Resistance	Specific pathogens or races	Many Pathogens
4.	Other names	Monogenic, R gene, Oligogenic, specific resistance, Perpendicular resistance, Physiological resistance, seedling resistance, hypersensitivity or qualitative resistance	Polygenic, general or nonspecific resistance, polygenic, minor gene, mature plant, adult, quantitative resistance, partial or field resistance or tolerance
5.	Resistance power	Complete resistance	Incomplete or partial resistance
6.	Pathotype specific	Race specific	Race non specific
7.	Stage of expression	Seedling to Maturity	As adult plant
8.	Selection and evaluation	Relatively easy	Relatively difficult
9.	Risk of Boom and Burst	Present	Absent
10.	Suitable for hosts	Annuals not perennials	Both Annuals and Perennials
11.	Suitable for pathogen	Immobile pathogens and not mobile pathogens	Both mobile and immobile pathogens
12.	Efficiency	Highly efficient against specific races	Less efficient against all races
13.	Existence of gene for gene hypothesis	Yes	No
14.	Resistance confers	VR confers complete but impermanent protection	HR confers incomplete but permanent protection
15.	Mathematical model	Vertical axis of the graph	Horizontal axis of the graph

                                                            Resistance

	Structural
						Functional

 

                                Structural resistance

1. Waxes and cuticles :  Potato leaves with hair are resistant to late blight.

Linseed varieties are resistant to Melampsora lini have thick cuticle.

Barberry species resistant to wheat smut have thick cuticle and more waxy layer.

2. Cell wall / Endodermis : Potato varieties resistant to Pythium have more root fiber. In rice varieties resistant to blast fungus have more Silicic acid.

                                    Functional resistance

·         Irregular blossom opening is the main factor for absence of ergot disease in wheat.

·         Short or less time stomata opening citrus varieties are resistant to citrus canker disease for avoid the entry of pathogen.

                                    Chemical resistance

·         Older beans are resistant to anthracnose because of alkaline pH.

·         Grape vine varieties resistant to powdery mildew have more tannic acid.

                                    Nutrition

·         The ground nut varieties are resistant to tikka disease have more riboflavin content.

·         High sugar diseases : Rust and Powdery mildew

·         Low sugar diseases : Early blight

                              Toxic principles

·         Red coloured onions are resistant to smudge disease due to catechol, protocatechuic acid.

·         Gram varieties are resistant to Ascochyta rabiei due to the presence of mallic acid

·         Potato varieties are resistant to scab and late blight contain more chlorogenic acid solanin

·         Lime varieties are resistant to die back due to the presence of isopimpinellin

·         Tomato varieties are resistant to Septoria and Alternaria due to the presence of Tomatin

                              Active structural (Direct Protection)

1.Callose deposition : Barley varieties are resistant to powdery mildew

2. Lignification: Lignin are formed around the hyphae and thereby further penetration is avoided. E.g. Take all disease

3. Wound barriers: After infection in plant cells, corky layers are formed to prevent further invasion.

4. Hypersensitivity / PCD : it’s a quick defense mechanism of plants after the infection process.

5. Phytoalexins : Capsidiol, Gossypol, Pisatin

                                    Important resistant varieties

Rice blast: ADT 36, ADT 37, ADT 39

Rice brown spot : CO44

Rice Tungro: IR 50, CO 45

Ground nut tikka and rust : ALR 1, ALR 3

Black gram yellow mosaic: VBN 1, VBN 3

Sugar cane red rot: COC 99061, CO 86249

Potato late blight: Kufri Jyothi

Bhendi yellow vein mosaic: Arkha anamika, Parbhani kranti

Angular leaf spot of cotton: MCU 10

 

Avoidance

It involves avoiding disease by planting at time when, or in areas where inoculums is absent or ineffective due to environmental conditions.

1. Proper selection of geographical area: Cumbu ergot (wet areas)

2. Selection of field: Sugar cane red rot, downy mildew of cumbu

3. Choice of time of sowing: Late sowing are increases the incidence of wheat stem rust disease.

4. Disease escaping varieties: Early maturity varieties are resistant against wheat stem rust disease

5. Selection of seed and planting stock : Potato viral diseases

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Therapy

Reducing severity of disease in an infected individual.

Chemotherapy: chemicals are used for chemotherapy are systemic fungicides and antibiotics.

Heat therapy (Luthra and Sattar Solar Heat treatment – Wheat Loose smut)

Tree surgery (Chaubattia paste)

 

Prophylaxis

Exclusion (Quarantine and other legislative measures)

Eradication (Cultural eradication and Chemical eradication)

Protection (Cultural manipulation, Physical and chemical protections)

Immunization

Genetic resistance

Therapy (Physical and chemical therapy)