Control of Plant Diseases PDF

# Control Of Plant Diseases Unlike the treatment of human and animal diseases, the plant diseases are treated aiming to protect the population rather than individual plants. However, some plants especially trees, ornamentals and some virus-infected plants are treated individually. Control measures...

# Control Of Plant Diseases Unlike the treatment of human and animal diseases, the plant diseases are treated aiming to protect the population rather than individual plants. However, some plants especially trees, ornamentals and some virus-infected plants are treated individually. Control measures are applied basically to minimise or to reduce the losses by the disease. There are various kinds of control measures, but the selection of a specific kind to be applied requires thorough knowledge of the intricate relationship between the agents (or organisms) that cause the disease and the affected host plants. Plant disease control have been classified into three broad categories: 1. Prophylactic (preventive) measures. 2. Therapeutic (curative) measures. 3. Immunisation measures. ## 1. PROPHYLACTIC (PREVENTIVE) MEASURES These are the preventive measures applied in advance before the disease spreads in the given area. These measures are aimed to prevent healthy plants to come in contact with the pathogen. Prophylactic measures are of three kinds: 1. Exclusion, 2. Eradication and 3. Protection. ### (i) Exclusion: This includes some regulatory measures in order to keep the pathogen away from entering the area where the susceptible hosts are growing or to reduce the intensity of pathogen to its minimum extent. It is achieved by certain regulatory methods such as; Quarantines, inspections, etc. **Plant quarantine:** Plant quarantine is the legally forced restriction on the movement of diseased plant material or of fungi, bacteria or viruses, that cause diseases in plants. The legislation has been placed on the statute-book in most agriculturally advanced countries. The legislature in India had passed a 'Destructive/Insects and Pests Act' in 1914 and amended it from time to time to impose restriction of shipments of diseased plant materials or pathogens into the country. The plant quarantine stations are working at every major sea and air port to check the entry of plant materials carrying pathogens which can cause disease in India. Every plant material or seed packets imported from another country must be accompanied by a certificate from an officer authorized by the Ministry of Agriculture stating that it is free from disease or pathogen. Even the cultures of fungi and bacteria are not allowed to be brought into India unless they are permitted by authorized Mycologist of the Government of India. Earlier, when there were no such restrictions and the plant quarantine was not in practice, the pathogens were disseminated from one country to another through transportation of seeds, seed stocks, cuttings, grains, fruits, etc. For instance, late blight of potato caused by _Phytophthora infesten_ had its original home in South America from where it was transported to USA then Europe and then to India. Similarly, Citrus canker was transported from Asia to the Western Countries. ### (ii) Eradication: Eradication is the elimination of pathogen after it has become established in the area where host is growing. It is done by the following methods → **(a) Field sanitation and rogueing:** Sanitation is the elimination of inoculum present in plant, field, or warehouse, whereas rogueing is the removal of diseased plants and their destruction. Both sanitation and rogue-ing can be done by mechanical or chemical methods. Mechanical methods include tillage, removal and burning of diseased plant debris, deep ploughing and soil inversion. Chemical methods include use of certain herbicides) (such as arsinates, chlorates, dilute sulphuric acid, dinitrophenol, etc.) and other chemicals to disinfect the fallen plant debris. Destruction of diseased plant materials or crop residues after harvest is helpful to control club root disease of crucifers and late blight of potato. Under such conditions when an orchard or nursery get infected by a disease; it is better to eradicate the whole plantation by burning (e.g., Citrus canker, each yellow, etc.) To avoid such cases it is advisable to remove individual diseased plants as early as possible to minimise further spread of the disease. is useful in some viral diseases and whip smut of sugarcane. **(b) Eradication of plant parts:** Infected plant parts such as, cankers woody branches, lesions of fire blight of apple and pear trees should be eradicated by removal and burning or treated with effective fungicides. **(c) Eradication of overwintering/oversummering hosts:** Some pathogen overwinter/oversummer on wild perennial plants or grasses. These are called collateral hosts. Such plants should be removed from the area. This method is useful in controlling some viral diseases and downy mildews. **(d) Eradication of alternate hosts:** Sometimes removal of alternate hosts helps to prevent and check the spread of a disease caused by a heteroecious pathogen. For example, black stem rust of wheat is caused by _Puccinia graminis var. tritici_ which is a heteroecious pathogen. It completes its life cycle requiring two hosts – wheat and barberry. The disease has been partly controlled in USA by the removal of barberry bushes. ### (iii) Protection: The preventive control measures, not included under exclusion and eradication, are grouped under protection. These are basically preventive measures applied to discourage the spore germination, penetration and disease development. It is done by adopting the following methods - **(a) Crop rotation:** It is generally observed that continuous cultivation of a particular crop in a field causes accumulation of the inoculum of a pathogen. To avoid this intensification of pathogen, non-susceptible crops are planted for the next 3 or 4 seasons. This plantation of alternating crops of susceptible and non-susceptible hosts is called crop rotation. Crop rotation is one of the most effective and old practice of plant protection. Obligate parasites, which are host specific, are controlled by this method. Soil pathogens which inhabit soil and have narrow host range can also be controlled by this method. Many diseases such as, wilt of arhar caused by _Fusarium udum_, foot rot of betel (_Piper betel_) caused by _Phytophthora nicotianae var. parasitica_, red rot of sugarcane, bacterial blight of paddy, angular leaf spot of cotton, etc., are controlled by this method. **(b) Selection of good quality planting material:** The materials for planting such as, seeds, cuttings, tubers, grafts, setts, etc., should be free from disease. These materials should be obtained from a place where disease is not prevalent. They should be healthy and effectively treated. More precaution should be taken before sowing that the material for planting is mature, undamaged and have a high germinating capacity. **(c) Conditions for germination, emergence and growth:** Plants should not be sown at a date when pathogen is prevalent in the area. Sowing should be done at proper time when the conditions for germination and growth are favourable. Rapid germination and early vigorous growth of seedlings avoid infection. **(d) Method and depths of sowing:** Planting materials should be sown in the field using proper methods and appropriate depths. Faulty mechanical sowing should be avoided. **(e) Use of fertilizers:** Appropriate fertilizers should be used at appropriate times. Some fertilizers protect host plants from diseases. For example, nitrogen fertilizers, when added in soil, protect plants from smuts. **(f) Avoid injury:** Injury of plant parts should be avoided in order to check the entry of pathogens. ## (2) THERAPEUTIC (CURATIVE) MEASURES These are the curative measures applied for the treatment of diseased plants and plant parts. Such measures are helpful to relieve the plant from sufferings and repair the damages. Therapeutic measures are of two types: 1. Physical therapy and 2. chemical therapy. ### (i) Physical therapy: This involves treatment of plant, plant parts and soil by physical agents such as, temperature, radiations, etc. These are applied as follows: **(a) Soil sterilisation by heat:** The soil is sterilized by steam either in special containers in which the steam is supplied under pressure or on special benches in which steam is allowed to pass through the soil. The soil is heated at 82°C or above for about 30 minutes. This method is used in seed beds soil and green houses to kill the soil inhabiting pathogens. **(b) Hot water treatment:** Seed borne smut diseases are eliminated by immersing infected seeds in hot water at 54°C for 10 minutes (e.g., loose smut of wheat caused by _Ustilago tritici_). This treatment is also affective for roots and rhizomes infected internally by pathogens (such as, _Verticillium_). **(c) Hot air treatment:** This treatment is given to remove excess of moisture from plant organs and protects them from fungal and bacterial attack. Several viral infected dormant plant materials are treated with hot air at temperatures ranging from 35°C to 54°C. Hot air treatment of plant organs also hastens healing of wounds and protects them from infection of some weak pathogens. **(d) Refrigeration (low temperature treatment):** This is the most common method used to prevent post harvest diseases of perishable fruits and vegetables. The storage plant parts are kept under low temperature to avoid secondary infection. Low temperature inhibits the activity of pathogens and prevents the secondary infection. The most effective temperature is slightly more than freezing point. **(e) Treatment of radiations:** Many pathogens of post harvest diseases of fruits and vegetables are killed by radiations such as, ultraviolet radiations, X-rays, y-rays, a-particles and ẞ-particles. ### (ii) Chemical therapy: The chemical substances which are used to protect the plants are commonly called pesticides. Depending on the kind of pathogen or organism they destroy, the chemical substances are known as fungicides (used against furgi), bactericides (used against bacteria), insecticides (used against insects), nematicides (used against nematodes), rotenticides (used against rotifers or wheel animalcules), acaricides (used against mites and ticks), herbicides (used against herbs), etc. **Fungicides:** The fungicides are considered core chemicals that kill the fungus spores and mycelium. The fungicides can be used as eradicants or in controlling the disease after infection as curative agents. Most of the fungicides are not absorbed and translocated by plants and, therefore, they are effective only in those areas where they are applied. Since fungicides are absorbed and translocated by plants and, therefore, used as eradicant (chemotherapeutant). The fungicides are applied to plants in different forms. Sometimes different fungicides are used to control diseases of different plant parts (such as, foliage, seeds, tubers, bulbs, wounds, stored fruits and vegetables). Some are used as disinfectants of soil and warehouses. On the basis of their effects and activities, the fungicides may be called: **(a) Protectants:** The fungicides which are applied on host in advance before the pathogen attacks it. They kill the pathogen before they penetrate the host. **(b) Eradicant (Therapeutic):** These fungicides are applied on host after it is infected by the pathogen. They kill the pathogen after it has invaded the host. The two major kinds of fungicides, which are widely used, are - topical fungicides and systemic fungicides. **(a) Topical fungicides:** These are the chemical substances applied to the infected parts of the diseased plant. They destroy the pathogen in situ, confined to a particular spot or organ of the plant. For example, the powdery mildews, which are exoparasites and confined to the host surface, are destroyed by spray of topical fungicides. The seeds internally infected with fungi and bacteria are treated with appropriate concentrations of topical fungicides (viz., dithiocarbamate, phenol, formaldehyde vapours, etc.) which on penetration kill the pathogen in situ. This kind of chemotherapy is known as topical chemotherapy. **(b) Systemic fungicides:** These are the chemical substances which provide immediate external protection to the host plant against fungal pathogens. They are also absorbed by roots or by foliage leaves and translocated upward through the xylem capillaries along the transpiration stream where they provide internal protection without phytotoxicity. Thus, the systemic fungicides may be both protective as well as eradicants in their action. The systemic fungicides chase the pathogens to the farthest leaflet of the host. They are evenly distributed within the plant and kill the pathogen by their direct toxic action without harming the host. (Some of the common systemic fungicides are listed below - 1. Oxanthiins - These include carboxin and oxycarboxin. Carboxin is sold as vitavax, which is used as seed treatment against damping off diseases and smuts. Oxycarboxin is sold as plantvax which is used against rust. 2. Benzimidazoles - These include benomyl and thiabendazoles. Benomyl is sold as benlate which is used against powdery mildews, _Cercospora_ leaf spots, smuts of wheat, apple scab, blast of rice, fruit rots, etc. Thiabendazole is used against post-harvest diseases caused by fungi imperfecti. 3. Pyrimidines - These include triarimol, ethirimol and dimethirimol, which are used against powdery mildews. 4. Dithane R-24 - It is sold as INDAR and used against leaf rust of wheat. 5. Antibiotics - Some antibiotics such as streptomycin te racyclines and cycloheximide are used as systemic fungicides) **General classification of fungicides** The fungicides may be classified into two major groups - 1. inorganic and 2. organic fungicides. 1. **Inorganic fungicides**: These are the earliest fungicides used to control crop diseases. These include mainly >**(a) Copper fungicides**: >**(i) Bordeaux mixture:** It is the most common fungicide first discovered by Millardet in 1885 at University of Bordeaux, France. It is prepared by mixing copper sulphate, lime (calcium hydroxide) and water in different proportions. The most commonly used proportion is 4 : 4 : 50 (i.e. 4 lb: 4 lb : 50 Gallons). The Bordeaux mixture is very effective against many fungal and bacterial leaf spots, blights, anthracnose, downy mildews and cankers. >**(ii) Fixed (insoluble) copper:** These fungicides contain basic copper sulphate (Basicop), basic copper chloride, copper oxides or cupric hydroxide. These are used as dust or sprays. They are less effective than Bordeaux mixture but used against the same diseases. >**(b) Sulphur fungicides ;** >**(i) Elemental sulphur** - Elemental sulphur of different particle size is used as dust or spray. It is effective against powdery mildews and some rusts, leaf blight and fruit rots. >**(ii) Lime sulphur** - It is prepared by boiling hydrated lime and sulphur together and used against powdery mildews, anthracnose, etc. >**(c) Mercuric chloride** - Solution of 1:10000 proportion is used as surface sterilant but due to high toxicity to man it is not widely used in fields. >**(d) Sodium hypochloride** - 0.35% of calcium or sodium hypochloride is used as a mild fungicide for surface sterilization of grains. 2. **Organic fungicides:** These are the most common and widely used fungicides. These include: >**(a) Formaline** - The most common and oldest organic fungicide is formaldehyde. 40% aqueous solution of formaldehyde is commonly known as formaline. It is used as soil sterilant. >**(b) Carbamates** - These are organic sulphur compounds viz., thiuram disulphides, metallic dithio carbamates and ethylene bisdithiocarbamates. Some of the common carbamates are - >>(i) Ziram (Zinc dimethyldithiocarbamate) >>(ii) Ferbam (Ferric dimethyl dithiocarbamate) >>(iii) Zineb (Zinc ethylene bisdithiocarbamate) >>(iv) Nebam (Disodium ethylene bisdithiocarbamate) >>(v) Maneb (Manganous ethylene bisdithiocarbamate) >>(vi) Thiram (Tetramethyl thiuram disulphide) Ziram and Ferbam are commonly used against damping off and several foliage and fruit diseases. Thiram is used as seed dressing as well as soil spray for _Pythium_, _Rhizoctonia_ and other damping off diseases. Zineb and Nebam are used against many foliage and flower pathogen. >>(c) Quinones - A few quinones such as chloranil (sold as spergon) and dichlone (sold as phygon) are used as organic fungicides. >>(d) Benzene compounds - >>(i) Dinitro-o-crosol - used as spray for diseases of some fruit and ornamental trees. >>(ii) Hexachlorobenzene (HCB) – used as seed treatment. >>(iii) Pentachloronitrobenezne (PCNB) – used against soil borne diseases. >>(iv) Dichloran - used as foliar, fruit and soil fungicide. >>(v) Dinocap - used against powdery mildews. >>(vi) Diazoben - used against damping off and root rots. >>(vii) Chlorothalonil - used as broad-spectrum fungicide. >>(e) Heterocyclic compounds - These include captan, folpet, captafol, glyodin, dyrene, etc. >>(f) Antibiotics - A large number of antibiotics are known which act as bactericides and fungicides. These compounds are absorbed and translocated by host and, therefore, act as systemic fungicides. Some of the common examples are (i) Streptomycin - (sold as agrimycin, phytomycin, agri-step, etc.) used as spray against bacterial pathogens causing spots, blights, wilts, rots, etc.; (ii) tetracyclines are used against diseases caused by bacteria and mycoplasmas; (iii) Cycloheximide (sold as actidione) used against many fungal diseases including leaf spots and powdery mildews. ## (3) IMMUNISATION MEASURES The immunisation measures are based on improving the immunity (i.e., ability of exemption from infection by a pathogen) in host plants by methods of: (i) biological control or by (ii) development of resistant varieties.. ### (i) Biological control: Use of living organisms or viruses against a pathogen in order to eliminate the disease is called biological control. It has been observed that many microorganisms parasitise, antagonise or compete with many pathogenic microorganisms. These properties have been exploited by pathologists in controlling many plant diseases. The biological control operates in the following ways - **(a) Antibiotics:** These are the chemical substances secreted by some microorganisms which inhibit the growth and activities of other microbes. They are toxic mostly against bacteria, mycoplasma and some fungi. **(b) Cross-protection:** It is a method in which a mild strain of virus or a microorganism is inoculated in host plant. These provide protection of host plants from those viruses and microorganism which may cause much more severe damages. It is called cross-protection. For example, bean and sugar-beet plants, when inoculated with a mild virus, exhibit a greater resistance against some rusts and powdery mildews. Similarly, the seeds, germinating seeds or the roots of nursery stock, when treated with a suspension of a strain of _Agrobacterium radiobacter_, do not easily get infected by the virulent strain of _A. tumefaciens_. It provides protection of apple and other trees from a soil-borne crown gall disease caused by _Agrobacterium tumefaciens_. **(c) Hyperparasitism:** This method is applied where the microorganism or viruses parasitise the other pathogenic microorganisms. These include bacteriophages, mycoparasites, etc. :- >**Bacteriophages** - The control of bacterial disease is carried by the use of bacterial viruses which parasitise the bacterial cells. >**Mycoparasitism** – This method is applied in controlling the fungal diseases. In this case, the mycelium and spores of some pathogenic fungi are attacked and parasitised by the other fungi. For example, the conidia of some powdery mildews are parasitised by _Ampleomyces_. Similarly, the uredia and uredospores of some rusts (e.g., _Melampsora_ spp.) are attacked by _Cladosporium_ spp. >**Parasites of nematodes:** Some soil fungi, protozoa, bacteria and viruses attack the plant-parasitic nematodes and thus, control the plant diseases caused by them. **(d) Antagonism:** One pathogen interacting and suppressing the activity of another pathogen, is called antagonism. Several such examples have been observed in plant systems and are being utilized to control the serious plant diseases. For example, the stem rot of carnation caused by _Fusarium roseum_ is controlled by the application of _Bacillus subtilis_ to stem cuttings or to soil. Many naturally occurring antagonists present in the soil near the host plants are activated by specific soil treatments. For example, the growth and activity of _Trichoderma_, an antagonist of many soil borne pathogens, is activated by special soil treatments to control soil borne diseases. **(e) Competition:** Many organisms complete with the other organisms for the substances which are in short supply. It is called competition. The microflora present on the phyllosphere competes with pathogenic microorganisms landing on aerial parts of the host and thus, weaken the activity of pathogen. This phenomenon is being utilized as a possible means of biological control. ### (ii) Disease resistance: It is the inherent ability of a plant to prevent or restrict the establishment and subsequent activities of a potential pathogen. A particular host may be resistant against all races of a pathogen (i.e., horizontal resistance) or may be effective against a few races of a pathogen but not against others (i.e. vertical resistance). Control measures by the development of new varieties resistant to diseases are being applied in almost all the economically important plants. If a new variety with high yield is produced by breeders and if it is not resistant to the local pathogens and pests, it can not be recommended to the farmers. Breeding for disease resistance is achieved by the usual methods such as, introduction, selection, hybridisation, mutation, polyploidy, budding and grafting. **(a) Plant introduction** - A variety taken from its centre of origin and grown (or introduced) at a new place having different environmental conditions, is known as plant introduction. By this method, the plant breeders introduce a disease resistant variety, taken from its place of origin, to a place where the disease is prevalent. If it is still resistant and has no effect on yield, the variety is recommended. **(b) Selection** - Plants having disease resistance are selected and then grown along with the susceptible plants in the field. They are then infected with the pathogens and tested for disease resistance. Only those plants exhibiting disease resistance are selected and tested for several years by cultivating them in different seasons and at different localities. If they exhibit resistance in all conditions, they are selected and recommended to the farmers. **(c) Hybridisation** - This method is widely used for the development of disease resistant varieties. By this method a variety susceptible to disease is artificially crossed with a variety with disease resistance. The hybrids of F1 generations are tested. If the character of resistance is introduced in the hybrids, they are selected for further investigation. **(d) Grafting** - In case of asexually propagated plants, where sexual reproduction does not occur, the hybridization technique is not possible. In such cases, a graft of disease resistant plant is made on a susceptible plant and the new variety is evolved. **Examples of some resistant varieties** 1. **Resistant varieties of wheat:** Sonara 64 (suitable for cultivation under late monsoon condition); K 65 (developed in 1965 in Kanpur, good for bakeries); Lerma Rojo64A (resistant to black and yellow rust); Sharbati Sonara (developed at IARI, suitable for late sowing); Kalyan Sona (resistant to loose smut, stinking smut and tolerant to powdery mildew and _Alternaria_ leaf blight); Sonalika (resistant to rusts); Chhoti Lerma (S331) (recommended for cultivation in North Indian plains and hills of South India); Safed Lerma; Hira; Lal Bahadur, etc. 2. **Resistant varieties of rice:** IR-8; IR-20; Jaya ; Padma (dwarf variety); Hansa (early maturing variety) ; Pankaj ; Kavery; 3. **Resistant varieties of arhar (_Cajanus cajan_):** NP 15 and NO38 are resistant to wilt disease. 4. **Resistant varieties of potato:** Kufri Naveen, Kufri Jeevan, Kufri Alanakar, Kufri Khasi Garo and Kufri Moti are resistant to Late blight disease. 5. **Resistant variety of tobacco:** Ambalema variety of tobacco is. resistant to mosaic disease. 6. **Resistant variety of Lady's finger:** Pusa Swami of variety of Lady's finger is resistant to yellow vein mosaic disease. # Questions 1. What are the important measures adopted for the control of plant diseases caused by fungi and bacteria ? 2. Write short notes on the following : - > (a) Plant Quarantine