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17 Case studies of important IDM practices for horticultural crops with examples. 18 Implementation and impact of IDM modules- Benefits and Limitations. Lecture 1: Categories /Classification of plant diseases Plant Pathology is a branch...

17 Case studies of important IDM practices for horticultural crops with examples. 18 Implementation and impact of IDM modules- Benefits and Limitations. Lecture 1: Categories /Classification of plant diseases Plant Pathology is a branch of agricultural science that deals with the study of fungi, bacteria, viruses, nematodes, and other microbes that cause diseases of plants. Plants diseases and disorders make plant to suffer, either kill or reduce their ability to survive/ reproduce. Any abnormal condition that alters the appearance or function of a plant is called plant disease. The term ‘Pathology’ is derived from two Greek words ‘pathos’ and ‘logos’, ‘Pathos’ means suffering and ‘logos’ Means to study/ knowledge. Therefore Pathology means “study of suffering”. Thus the Plant Pathology or Phytopathology (Gr. Phyton=plant) is the branch of biology that deals with the study of suffering plants. It is both science of learning and understanding the nature of disease and art of diagnosing and controlling the disease. The science of plant pathology has four main objectives: 1. To study the living, non-living and environmental causes of plant diseases.(Etiology) 2. To study the mechanisms of disease development by pathogens.(Pathogenesis) 3. To study the interactions between the plants and the pathogen.(Epidemology) 4. To develop the methods of controlling the diseases and reducing the losses caused by them.(Control/ Management) Definition of plant disease : Disease is a malfunctioning process that is caused by continuous irritation which results in some suffering producing symptoms (American Phytopathological society & British Mycological society). Disease is an alteration in one or more of the ordered sequential series of physiological processes culminating in a loss of coordination of energy 3  utilization in a plant as a result of continuous irritation from the presence or absence of some agent or factor. Disease: 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, 1997). Disease Triangle: The interactions of three components of disease, i.e., the host, pathogen and environment, can be visualized as a disease triangle. The length of each side is proportional to the sum total of the characteristics of each component that favour disease. The interaction of susceptible host plant, virulent pathogen and favourable environmental conditions leads to the development of the disease. Disorder: Non-infectious plant diseases due to abiotic causes such as adverse soil and environmental conditions are termed disorders. The common characteristic of non- infectious diseases of plants is that they are caused by the lack or excess of something (temperature, soil moisture, soil nutrients, light, air and soil pollutants, air humidity, soil structure and pH) that supports life. Non- infectious plant diseases occur in the absence of pathogens, and cannot, therefore, be transmitted from diseased to healthy plants. Pathogen: An entity, usually a micro-organism that can incite disease. In a literal sense a pathogen is any agent that causes pathos (ailment, suffering) or damage. However, the term is generally used to denote living organisms (Fungi, bacteria, MLO’s, nematodes etc.,) and viruses but not nutritional deficiencies. 4  Parasite: Organisms which derive the materials they need for growth from living plants (host or suscept) are called parasites. Pathogenicity is the ability of the pathogen to cause disease Pathogenesis is the chain of events that lead to development of disease in the host (or) sequence of progress in disease development from the initial contact between the pathogen and its host to the completion of the syndrome Sign: The pathogen or its parts or products seen on a host plant. Symptom: The external or internal reactions or alterations of a plant as a result of a disease. Syndrome: The set of varying symptoms characterizing a disease are collectively called a syndrome. On the basis of their mode of perpetuation and mode of primary infection: 1) Soil borne diseases: In these diseases, the pathogens survive in soil or on infested plant debris lying in soil either as their resting spores or as mycelia strands and rhizomorphs. They all attack the root system of host plants. Eg: Damping off (Pythium sp.),Seedling blight (Phytophthora, Fusarium sp.) 2) Air borne diseases: Some pathogens infects the host plant through air and bring primary as well as secondary infection. Eg: Rusts, Powdery mildews.Loose smuts bring about secondary infection through air. 3) Seed borne diseases: Some pathogens survive as dormant mycelium in the seeds or other propagative structures of host plants. Eg. Loose smut of wheat (internally seed borne) On the basis of their cause, diseases are classified as: 1) Infectious plant diseases: These diseases are caused by living agents,the pathogen. All pathogens are parasitic on plants. These are characterised by the ability of the pathogen to grow and multiply rapidly. Ex: Powdery mildews, Rusts. 2) Non-infectious diseases: These diseases do not spread from plant to plant (non- infectious).These diseases are caused due to abiotic factors(non parasitic or physiological). Eg: Black heart of potato. 5 On the basis of production and spread of the inoculum: 1) Single cycle disease or simple interest disease: In single cycle disease the increase of disease is mathematically analogous to simple interest disease. 2) Multiple cycle or compound interest disease: In multiple cycle diseases, the increase in disease is mathematically analogous to compound interest of money. 3) Polyetic diseases: These are also polycyclic diseases but they complete their disease cycle in more than one year over years. Eg.: Cedar Apple Rust On the basis of plant parts affected: 1) Localized: If they affect only specific organs or parts of the plants. Eg.: Root Rot, Leaf spot. 2) Systemic: If entire plant is affected. Eg.: Downy mildew, damping off On the basis of group of causal organisms: 1) Fungal disease: Caused by plant pathogenic fungi. Eg. Anthracnose 2) Bacterial disease: Caused by plant pathogenic bacteria. Example: Citrus canker 3) Viral disease: Caused by plant viruses. Eg. Rice tungro disease 4) Phanerogamic phytopathogenic diseases: Caused by phanerogamic plant parasites. Eg. Striga, Cuscutta. 5) Nematode Diseases: Diseases caused by plant pathogenic nematodes. Eg. Ear cocle of wheat. On the basis of occurrence and consequent effects: Epidemic or Epiphytotic disease: A disease usually occurs widely but periodically in a destructive form is referred as epidemic or Epiphytotic disease. Ex: Late blight of potato – Irish famine (1845) Endemic: Constantly present in a moderate to severe form and is confined to a particular country or district. Ex: Club root of cabbage in Nilgiris Black wart of potato – Synchytrium endobioticum Onion smut – Urocystis cepulae 6 Sporadic disease: Occur at very irregular intervals and locations and in relatively fewer instances. Ex: Udbatta disease of rice, Angular leaf spot of cucumber – Pseudomonas lachrymans Pandemic: Diseases occurring throughout the continent or sub-continent resulting in mass mortality. Eg. Late blight of potato. On the basis of organs affected: 1) Fruit diseases: In these diseases fruits are mainly affected Eg. Apple scab. (Venturia inaequalis) 2) Root diseases: In these disease, root is mainly affected. Eg.Root rot of papaya.(Pythium aphanidermatum) 3) Leaf diseases: Disease is localised in the foliage. Eg. Leaf spot of cotton. ( Alternaria gossypii (Jacz.) 4) Seedling diseases: Seedlings are affected in which stem and root tissues rot. Eg. Damping off of seedling.(Rhizoctonia sp.) On the basis of host crop plants affected: 1) Cereal diseases: Disease which affect cereal crops eg. Wheat,barley and oat. 2) Pulses diseases: Diseases which affect pulses crop. Ascochyta blight of chickpea. 3) Millet diseases: Diseases which affect millets. Eg. Green ear disease of Bajra. 4) Vegetable diseases : Diseases which affect vegetable crops.Eg. Early blight of Tomato caused by Altrnaria solani. 5) Fruits diseases : Diseases affect fruit crops.Eg. Apple Scab 6) Ornamental plant diseases: Diseases affecting ornamental plants. Eg. Chrysanthemum stunt. 7) Forest diseases: Diseases affecting forest trees and plantation. Eg. Sudden Oak Death (Phytophthora ramorum) On the basis of symptoms produced on host plants: 1) Rusts : Caused by Basidiomycetes of the order Uredinales. Eg. Stem rust of wheat. 7 2) Smuts : Caused by fungus of order Uredinales,mass of black powedery spores and grains are not produced.Eg. Loose smut of wheat caused by Ustilago nuda tritici. 3) Wilts :In this disease, the vascular system of plant is affected.Eg. Bacterial wilt of cucurbits caused by Erwinia trachiephila. 4) Powdery mildews: It is a disease of foliage, stem, flower and fruit.Eg.Powdery mildew of grapes. Rots : In this disease, underground part of plant is infected.Eg. Rhizoctonia and Phytophthora root rot. 6) Blight : It is complete chlorosis ,yellowing and browning which results in death of the plant. Eg.Leaf blight of paddy. 7) Leaf spots :It may be caused by fungi or bacteria, spots are formed on leaves which results in complete yellowing and dropping of leaf. Eg.Septoria leaf spot in tomato 8) Canker : Canker is a dead area in bark or cortex of woody stem. Ex: Citrus canker 10 Anthracnose: an ulcer-like lesion that can be necrotic and sunken. These lesions can appear on the fruit, flowers and stems of the host - e.g. Apple Anthracnose of stems and or leaves (Cryptosporiopsis sp. Formally Pezicula sp.), or Dogwood Anthracnose (Discula distructiva) 11. Damping Off: it is a rapid collapse and death of very young seedling. Either the seed rots before emergence or the seedling rots at the soil line and falls over and dies. The most common genera involved are Fusarium, Rhizoctonia and Pythium. 12 Scab: localized lesion on host fruit leaves tubers and other plant parts. These infections usually result in a roughened, crust-like area on the surface of the host - e.g. Apple Scab (Venturia inaequalis) and Pear Scab (Venturia pirina). 8