Host parasite relationship ,interaction & mechanism of

Host parasite relationship ,interaction & mechanism of infection

1. Host parasite relationship
interaction, &mechanism of infection
VaishaliS.Patil
Professor, Department of Botany
ShriShivajiCollege of Arts, Commerce & Science
Akola

2. The fungi obtain food from the host plant through haustoria. They
establish a close connection with their host plants.
Host-An plant on or in which a parasite or commensalorganism
lives.
Parasite-A living thing ie. Fungi that lives in or on another living
thing & gets food&sometimes shelter from it &usually causes
harmto it.
Host and parasite establish a sort of relationshipwhich effects
each others growth, metabolism, etc.
It follows the distribution and localization of parasite on or
within the host, then growth or multiplication of parasite, the
resistanceof host to the parasite and the parasite to the host.
Parasite enter into the tissues or to send haustoriainto the
surface cells. Germ tubes are produced from spores. This germ
tubes converted into mycelia. Haustoriaare produced by mycelia
which enter into the surface cells. This phenomenon of the
entrance of the pathogen into the host can be-

3. Haustorium

4. 1.The mechanism of infection-
a)Spore comes in contact of a suitable host plants it frequently
requires free water, as drops or a dew film for germination.
b)The germ tube grows in the direction of the host surface and attach
itself to the surface firmly by special structures the appressoria.
Appressoriafixed to the surface by sticky secretion. It gives rise to
one or more thin finely pointed infection threads or hyphae, which
directly pierce the cuticle and gain entry into the interior of the host
tissue.
The infection hyphanow meets with the epidermal walls,
consisting mainly of cellulose and secretes enzymes, which dissolve
the cell walls and gains further entry into epidermal cells and deeper
into host tissues by its enzymicactivities.
Entire process is mechanical in its early stage & chemical in its later
stage.
These enzymes are capable of dissolving cell walls and directly or
indirectly killing the cell protoplasm. The contents are organic acid,
eg.oxalicacid and toxins.

6. In rust, smuts & downy mildews the host tissue is not killed until a late stage of
development.
In process of infection in several plant pathogenic bacteria takes place
through hydathodesas well stomata to gain entry into the tissues,eg. Bacterium
amylovorum.
The process of infection of plant viruses gain their entry into host tissues
mainly through the help of sucking insects e.g.leafhoppers and thrips.
2.The path of infection.Parasitism is an association or a situation
in which two organisms of different taxonomic positions live together where
one enjoys all sorts of benefits (like derivation of nourishment, reproduction
etc. which are basic requirements for existence) at the expense of the other.
The benefited organism is called the parasite and the organism harbouring the
parasite is called the host.
Hosts are not hospitable to parasites. Instead they consider parasites as foreign
bodies and want to exterminate or overpower them by operating various
devices like: producing antibodies, increased peristalsis, diarrhoea, mucus
secretion, encystationby host tissues etc. Parasites to avoid host’s reaction for
existence develop many specialities like increased fecundity, polyembryony,
safe-habitat, production of special enzymes, a good deal of transmission etc.

7. The path of infection is concerned with the entry of parasite into host.
Direct Indirect
1.Entry
through
intact or
cuticularized
surface
Epidermis
2.Entry through
non cutinized
surface
(i.e.,through
special organs)
such as
a)flower
infection
b)seedling
infection
c)bud infection
d)needle leaves
e)root hairs.
3.Entry
through
natural
openings
such as
a)stomata
b)Lenticels
c)hydathodes
4.Entry through
artificial
openings
a)Entry through
injuries
b)Entry through
lesions
c)Entry through
wounds caused
by insects.

8. 1.Entry through intact or cuticularizedsurface &
Epidermis-
It takes place by plant pathogenic fungi e.g. Botrytis cinerea. It
induces the formation of a local internal swelling of underlying
cellulose wall, so that a small papilla projects from the under-side of
the wall into the cell. This papilla is being penetrated by the haustorial
tube and latter enters the epidermal cell and forms a haustorium.
2.Entry through non cutinized surface-a)flower infection-It
take place place
1. Through stigma-e.g. Sclerotiniacinereacausing blossom and twig blight of
stone fruit.
2.Through nectaries-e.g. Bacillus amylovorouscausing fire blight of apples;
Sclerotiniafruticolacausing brown rot of pears; Taphrinadeformanscausing
leaf curl of peaches.
b)Seedling infection-Seedling gives asylum to pathogen. e.g.smutsand bunts
of sorghum caused by Sphacelothecasorghi., S.cruenta; bunt of wheat by
Tilletiacaries;flagsmut of wheat by Urocystistritici.downymildew of cereals
by

9. By Sclerosporagraminicola, onion smut by Urocystiscepulea,
foot rots by Pythium,Fusarium,Rhizoctonia.
C)Bud infection-In this type infection takes place when
causal organism reaches the growing point.eg.scabof apples
by Venturiainaequalis; wart disease of potato by Synchytrium
endobioticum.
D)Needle leaves-The hyphaeof pathogens grow downward
in needle leaves. e.g. blister rust of pine, which uses the needle
points as avenues of entry of sporidiaand Cronartiumribicola.
E)Root-hairs-e.g. Myxamoebaeof club root of cabbage
(Plasmodiophorabrassicae) enter its host through root hair and
youngest epidermal cells. Other examples are Fusariumwilt;
Texas root rot by Phymatotrichumomnivorum; hallow stem
of sorghum by Rhizoctoniabataticola& nodule bacteria
Rhizoblumradicicolaof Leguminous plants.

10. 3.Entry through natural openings-
a)Stomata-e.g.uredosporesof Puccinia;downy
mildew of vine by Plasmoparaviticola.

11. b) lenticels-e.g. Powdery scab of potato by Spongospora
subterranea; common potato scab by Actinomyces
scabies;lateblight of potato by Phytophthorainfestans
;infection caused by Botrytis cinerea, Nectriacinnabarina.
C)hydathodes(water stomata)-Black rot of cabbage by
Xanthomonascampestris

12. 4.Entry through artificial opening-
a)Entry through injuries-Crown gall by Bacterium
tumefaciens. Injuries caused by farm operation, digging, hail
storms, sun burn etc. e.g. ripe rots by Rhizopus, Aspergillus,
Colletotrichum, Fusarium& bacteria.
Other examples are late blight of potato by Phytophthora
infestans, early blight of potato by Alternariasolani,
downy mildew of grapes by Plasmoparaviticola,
gummosis of citrus by Phytophthorapalmivora.
b)Entry through lesions-e.g. Dry rot of potato by
Fusariumcaeruleum.
c)Entry through wounds caused by insects(vector)-e.g.
Brown rot fungus Sclerotiniafructicola, Phomopsis
versoniana; red rot of sugarcane by Colletotrichum
falcatum.

13. How pathogens attack plants-
Activities of pathogens in plants are mostly chemical
in nature. Diseases caused by pathogens on plants
takes place between substances secreted(enzymes,
toxins, growth regulators, polysaccharides &
antibiotics) by the causal agents & those present
within the plant.
1.Enzymes-Plant pathogenic enzymes which are protein
molecule disintegrate the host cells, break down food
substances present in the cell. These enzymes break down
plant substances into smaller molecules which the pathogens
may absorb and utilize for growth and energy. e.g.
hydrolases. They break cell wall which consist of cellulose,
pectins, hemicelluloses, lignin, lipids, starch & nucleic acid.

14. Growth regulators-e.g. auxins, gibberllins,IAA&
cytokinins. It produced in the pathogens tend to alter the
normal growth pattern of the plant into new and unusual
directions thus an abnormal growth and diseased condition
of the plant is obtained. Pathogen produce non growth
regulator which caused abnormal growth and disease of
plant. It results in the development of overgrowths,
rosetting, excessive root branching, stem malformation,
stunting, defoliation, wilting, suppression of bud growth etc.
Toxins in plants-They act directly on living host protoplast
and kill the cells of the plant. They affect osmotic relations
or enzymatic reactions in plants. They also affect on
respiration of plants. e.g.1. Fusariumwhich produces
lycomarasmin, fusaricacid,dehydrofusaricacid causes wilt
disease to many plants such as tomato which epinasty,

15. plugging, browning of xylem vessels, necrosis, wilting,
drooping & finally death of the plant.
2.Pyricularia-e.g. Pyriculariaoryzaecauses blast disease of
rice. It causes yellowing, striping, stunting of seedlings, leaf
spots, collar rot of stem at the base of the leaf.
3.Some other toxins –Alternaricacid from Alternaria
solanicauses early blight of potatoes and tomatoes;
Colleotinfrom Colletotrichumfuscumcauses anthracnose
of Digitalis.
4.Host specific Toxins-e.g.victorinproduced by
Helminthosporiumcauses leaf blight of oats; Periconia
circinata, Alternariakikuchianaetc.
5.Mycotoxins-They are secondary fungal metabolites
cause intoxication to man & animals. e.g. Toxin produce by
Aspergillus, Penicillium& Fusarium.