Disease is defined as impairment of health or a condition of abnormal functioning.
When
the ability of the cells of a plant or plant part to carry out one or more of
these essential functions is interfered with by either a pathogenic organism or
an adverse environmental factor, the activities of the cells are disrupted,
altered, or inhibited, the cells malfunction or die, and the plant becomes
diseased.
Scientifically
speaking “disease is a malfunctioning
process that is caused by continuous irritation. Of curse, this process must
result in some suffering. And hence, disease is a pathological process.”
Another definition: Disease is a complex phenomenon and is an interaction
among the host, the parasite, and the environment. It cans also be defined as a
disturbance in the rhythmical equilibrium of a host in respect of structure, or
physiology or both and may lead to the death of a part of or the entire host or
reduce the economic value of its products.
What are causal agents of disease?
Disease may be caused by
·
Biotic Agents
(Bacteria, Fungi, Algae, Nematodes, Mycoplasmas etc)
·
Abiotic agents (Environmental
factors like Temperature, Rainfall, Humidity, High or low pH etc)
·
Mesobiotic agents
(These are Viruses)
BIOTIC
AGENTS
A.
FUNGI
Fungi are small,
generally microscopic, eukaryotic, usually filamentous, branched, spore-bearing
organisms that lack chlorophyll. Fungi have cell walls that contain chitin and
glucans (but no cellulose) as the skeletal components.
Their
majors groups are
i.
Oomycota
ii.
Zygomycota
iii.
Basidiomycota
The
various symptoms caused by fungi in
plants include, leaf spots, blight, canker, dieback, root rot, damping off,
basal stem rot, anthracnose, scab, decline,
clubroot, galls, warts, withes-broom, leaf curls, wilts, rusts, smuts,
mildews, are common diseases caused by fungi.
Examples
of diseases are numerous since fungi are
largest group of plant pathogens; late blight of potato, loose smut of wheat,
kernel bunt, rust of wheat, powdery mildews of various crops and plants.
B.
BACTERIA
These
are
generally single-celled prokaryotic microorganisms whose genetic material
(DNA) is not bound by a membrane and therefore is
not organized into a nucleus. Their cells consist of
cytoplasm containing DNA and small (70 S) ribosomes.
The
symptoms caused are similar to those
caused by fungi because they infect as fungi do. These include root rot, leaf
curl, scab, canker, soft rots of fruits of fruits.
Examples of
diseases include, bacterial leaf spots and blight of cereals and grasses,
bacterial spots of pepper and tomato and bacterial vascular wilts.
C.
PARASITIC
HIGHER PLANTS
More
than 2500 species of higher plant are found living parasitically on other
plants. Their main common characteristic is
that these parasites are vascular plants that have developed
specialized organs which penetrate the tissues of other
(host) vascular plants, establish connections to the host plant
vascular elements, and absorb nutrients from them.
These parasitic plants produce flowers and seeds and belong
to several widely separated botanical families. They vary
greatly in their dependence on their host plants. Some, e.g.,
mistletoes, have chlorophyll but no roots so they
depend on their hosts only for water and minerals.
Others, e.g., dodder
have
little or no chlorophyll and no true roots so they
depend entirely on their hosts for their
existence. Relatively few of the known parasitic higher
plants cause important diseases on agricultural crops
or forest trees.
Their
examples include, dodder, witchweed,
broomrapes, dwarf mistletoes of conifers, kudzu vine etc.
D.
PARASITIC
GREEN ALGAE
Green algae are
single-celled organisms that form colonies, or multicellular,
free-living organisms, all of which have chlorophyll b.
Algae
are the organisms, often microorganisms, other than typical land
plants, that can carry on photosynthesis. Algae are sometimes
considered as protists with chloroplasts.
Algae are the main
producers of photosynthetic materials in aquatic
ecosystems, including unstable areas such as muds,
sands, and intertidal aquatic habitats.
Forexample
Cephaleuros which is an intercellular
parasite in the leaves of a number of angiosperms such as Magnolia, Rhododendron, Camellia
sinensis (Tea) and Piper nigrum
(Pepper). Till now much work has been done on the “red rust of tea” caused by Cephaleuros virescens.
E.
NEMATODES
The
nematodes are roundworms which live in soil or water. Many of them are found
are free-living and other are parasitic on animals and plants. They are the only
plant parasite belonging to the animal kingdom studied in plant pathology. The
example of plant pathogenic species include; Meloidogyne (root-knot nematode),
Heterodera (cyst nematode), Globodera (cyst nematode).
ABIOTIC
AGENTS
Disease
may be incited by high or low termperature, unfavourable oxygen relations,
accumulation of injurious impurities, lightening injuries, mineral deficiencies
or excess, injurious atmospheric gases etc.
Characteristics
of diseases caused by abiotic agents
·
They
are caused by excess or loss of something that support life.
·
They
cannot be transferred from diseased to healthy plant since they occur in
absence of pathogen.
·
Infect
plant in all stages of their lives (e.g., seed, seedling, mature plant, or fruit)
·
Symptoms
vary from slight to severe and plants may even die.
·
Symptoms
vary in kind and severity depending upon the particular environmental factor
involved and with the degree of deviation from normal.
The
diseases may be caused due to chemical injury, nutritive deficiency, freeze
damage, russetting of fruit, wilting due
to water stress, flooding etc.
MESOBIOTIC
AGENTS
VIRUSES
A
virus nucleoproteins that multiplies only inside living cells and have ability
to cause disease.
Symptoms
are usually produced on leaves but sometimes on fruits, stem and roots as well.
The viral infection always including dwarfing or stunting of entire plant and
reduction in total yield. The effects may be slight or severe. The common
symptoms are local lesions, ring spots and mosaics, chlorosis, vein clearing,
vein banding, bunchy top, hypertrophy, atrophy, curling, crinckling of leaves.
