Plant Disease is the deviation of a plant from its normal function and reduces the growth yield or causes death to the plant. Diseases can be noticed through signs and symptoms.
Symptom – are the expression of the disease caused by the manifestation of the physiological reaction of the plant due to harmful activity of the pathogen.
Sign – physical evidence of the presence of disease agent (e.g., mold or fungal spores, bacterial ooze).
Defined as sequential appearance of disease symptoms on a plant during the development of the disease or sum total of symptoms exhibited by a disease.
Fleck or necrotic spot, Uredial pustule,Telial pustule, Death of organ or plant
Local infection: An infection affecting a limited part of a plant e.g. leaf spot.
Systemic infection: Infection that spread point of infection to different parts of the plants e.g. wilts, virus infection, loose smut
Lesion: A localized necrotic or chlorotic areas of diseased tissue/ organ.
Local lesion: A localized spot produced on a leaf upon mechanical inoculation with a virus.
Types of symptoms
- Morphological symptoms
- Histological symptoms
Morphological: (Externally detectable symptoms caused by any pathogen e.g. blight, leaf spot
–Hyperplasia & Hypertrophy
Necrosis degeneration of protoplast followed by death of the tissue or organ or plant
- Plesionecrosis (Nearly dead): necrotic symptoms expressed before the death of the protoplast are called plesionecrosis.
E.g. yellowing, hydrosis, wilting
- Holonecrosis: necrotic symptoms expressed after the death of the protoplast are also called holonecrosis. In this the affected tissue turns brown in colour
E.g. Rots, spots, blights
Appearance of uniform or non-uniform yellowing of leaves due to infection.
it is the result of breakdown of the chlorophyll
e.g. by fungi (e.g., celery yellows), viruses (e.g., sugar beet yellows virus),
bacteria (e.g., coconut lethal yellowing),
protozoa (e.g., hart rot), spiroplasmas or phytoplasmas Wilt Browning of vascular tissues Wilt: A symptom characterized by loss of turgor, which results in drooping of leaves, stems, and flowers. e.g. bacterial wilt of tomato Pathological wilt: caused by pathogen-fungus, bacteria and viruses Physiological wilt: due to water stress and high temperature Hydrosis: appearance of water soaked trnasluscent diseased tissues whose intercellular spaces contains liquid. This type of symptoms precedes holonecrotic symptoms
Holonecrotic symptoms May develop on any part of the plant and generally the infected tissues turns brown.
- Holonecrotic plant disease symptoms can be divided into three categories
–Necrosis of the green plant parts
–Necrosis of the storage organs
–It is of woody tissues
- Necrosis of the storage organs
Necrosis of the green plant parts leads to formation of number of disease symptoms like:
- Restricted necrosis
–Speck or fleck
- Is the collapse and death of seedlings due to extensive necrosis of stem tissues before or after they emerge from the soil (pre-emergence and post-emergence damping-off, respectively).
- caused by Pythium and Rhizoctonia ,
Leaf spot: A wll defined or self-limiting grey,tan or brwon necrotic lesion on a leaf Angular leafspot of beans
Septoria leafspot of tomato Veinal necrosis and local lesions BCMNV Shot hole. When a necrotic tissue with in a leafspot cracks and fall off, leaving small holes in their place Shot hole of Cherry: Coccomyces hiemalis Purple Blotch of garlic: Alternaria porii Blotch: A necrotic area covered with brown fungus mycelium on leaves, shoots, and stems
Streaks & Stripe Maize Streak- MSV Barley stripe Drechslera graminea Necrosis of woody tissues
- Die back: extensive necrosis of shoots from top/ tip to down ward e.g. die back of chilli/ citrus
- Cankers: Necrosis of the bark tissues e.g. citrus canker
- Gummosis: Oozing of gum like secretions from the woody tissues: Gummosis of stone fruits
Die back: extensive necrosis of shoots from top/ tip to down ward e.g. die back of chilli/ citrus
- Formation of sharply delineated, dry, necrotic, localized lesions on the stem due to death of cambium tissue of bark, or (in non-woody plants)
Cankers Bacterial canker of tomato Stem canker Nectria canker on apple Gummosis: A plant disease in which the lesions exude a sticky liquid. ooze from bacterial canker (apricot) is milky in color Guttation: Exudation of water from plants, particularly along the leaf margin Extensive necrosis
Late blight of Potato
- A plant disease characterized by rapid and also extensive death (Necrosis) of plant foliage.
- A general term applied to any of a wide range of unrelated plant diseases. (e.g., chestnut blight, fireblight, late blight, halo blight)
Bacterial blight of beans Rice blast: Magnaporthe gresia Blast : it is sudden death of unopened bud or inflorescence
Formation of discrete, dark-colored, necrotic lesions on the leaves, stems, and/or fruits.
Caused by fungus:
Colletotrichum spp. Bean anthracnose:
C. lindemuthianum Chili anthracnose:
C. capsici Fire blight of apple: Erwinia amylovora Necrosis of the storage organs
Buckeye Rot of tomato Rot: The softening, discoloration, and often decay or disintegration of a succulent plant tissue as a result of fungal or bacterial infection.
Root & Fruit Rots ROOT ROT Leak: exudation of juice from tissues with soft rot is called leak Infected apple fruit exuding droplets of fire blight bacteria. Mummification Drying of the rotted organs resulting in shriveling and hardening like mummies brown rot fungus (Monilia fructicola) causes mummification the peach fruits Hyperplastic & Hypertrophic symptoms
- Wound tumors,
- Witches Broom
Hyperplasia: A plant overgrowth due to increased cell division. Hypertrophy: A plant overgrowth due to abnormal cell enlargement. Crown galls on peach; Agrobacterium tumefaciens
- An abnormal plant structure formed in response to parasitic attack by certain microorganisms (bacteria, fungi, viruses) or insects.
- Galls may develop either by localized cell proliferation or increase in cell size.
Tumor: An uncontrolled growth of tissue or tissues Crown gall/ tumor Hyperplastic & Hypertrophic Hyperplasia: A plant overgrowth due to increased cell division. Hypertrophy: A plant overgrowth due to abnormal cell enlargement.
A symptom caused by certain plant viruses in which there are small outgrowths on the plant Leaves of pea (Pisum sativum cv. Dark Skinned Perfection) infected with Pea enation mosaic virus (PEMV)
An abnormal form of plant growth characterized by profuse outgrowth of lateral buds to give a broom like appearance. Potato witches’ broom Groundnut witches’ broom Hypoplastic symptoms
- Atrophy/ Hypoplasis/ dwarfing/ Stunting
- Rosette: in this the internodes do not enlarge and leaves are clustered like petals of rose e.g. Peach rosette, ground rosette
- Albication: Complete repression of colour caused by viruses, bacteria, fungi, and iron deficiency
- Mosaic; appearance of dark green and light green areas on leaves e.g. Tobacco mosaic
- Chlorosis: failure of chlorophyll to develop fully.
- Atrophy or stunting or Dwarfing
- Corn stunt
The loss of chlorophyll from the tissues of a plant, resulting from microbial infection,
e.g. viral infection, the action of certain phytotoxins, the lack of light, to magnesium or iron deficiency, etc. Chlorotic tissues commonly appear yellowish
Chlorosis & Chlorotic ring spot by BCMV Rosette: An abnormal condition in which the leaves form a radial cluster on the stem. Groundnut rosettee caused by: Groundnut rosette umbravirus Mosaic BCMV yellow mosaic MYMV Golden mosaic CpGMV
Mosaic plant disease:
Appearance of dark green, light green pattern or sometimes chlorotic areas on leaves due to virus infection Histological Symptoms: (can be detected microscopic studies of the disease sample/ tissue) also called Pathological anatomy or Morbid anatomy e.g. Cuticle thickness, cell wall degeneration etc.
Tylose formation in xylem vessels due to wilt General Disease symptoms DOWNY MILDEW (Peronospora parasitica) Downy mildew: Formation of superficial cottony hyphal growth with spores generally on under side of the leaf Powdery Mildew Apple scab (Fruit)
Apple scab (Leaf) Scab: roughened, crust like diseased area on the surface of a plant organ (e.g., apple scab, potato scab, wheat scab).
COMMON SCAB (Streptomyces species)
(a) Vein clearing and Chlorotic ringspot
(b) symptoms due to BCMV in common bean
Vein clearing: A symptom of virus-infected leaves in which veinal tissue is lighter green than that of healthy.
Epinasty plant disease:
Downward curling of a leaf blade resulting from more rapid cell growth on the upper side of a petiole than on the lower side; often a hyperplastic symptom of plant disease Green Vein banding
Vein banding: A symptom of virus-infected leaves in which tissues along the veins are darker green than other laminar tissue. Golden mosaic of cowpea
Leaf curl of tomato
- Sclerotia that replaces the grain in a diseased inflorescence
- Disease of certain grasses and cereals, especially rye, caused by Claviceps purpurea
Typical Leaf roll of lower leaves Smut: Appearance of masses of dark, powdery, and sometimes odorous spores on inflorescence e.g. stinking smut of wheat, common smut of maize.
Mold : A downy fungal growth on rotted or decaying host tissue, usually consisting of mycelium. e.g. grey mold of chickpea White Mold/ rot of Beans
Rust: A plant disease giving a “rusty” appearance to a plant and also causal agents form rust-colored spores
Small blister-like eruptions on the leaf epidermis created by spores formed underneath and also push outward. WHITE RUST (Albugo candida)
Ring spot plant disease :
Appearance of single or concentric rings of discoloration or necrosis, the regions between the concentric rings being green. The center of the lesion may also be chlorotic or necrotic
Stem-pitting: A symptom of some viral plant disease characterized by depressions on the stem of the plant Stem pitting in apple
Sooty mold: Appearance of dark, spongy, hyphal mats on the surfaces of certain plants due to organisms that grow on honeydew. Dodder infection
Scorch: “Burning” of leaf margins as a result of unfavorable environmental conditions (high temperature)
When diseases attack your plants they lose value and causes loss to you controlling them. It is best to take immediate action when you notice the degree of damage caused to your plants on the field to enable you control them without too much cost incured on you.