Process of infection 

Process of infection can be grouped into three stages, i.e., pre-penetration, penetration and post-penetration.

Fig : Stages in the development of infection or disease cycle

1. PRE-PENETRATION: Depending upon the plant pathogen activity, the plant

pathogens are classified in to 2 categories

• In fungal pathogens, pre-penetration includes spore germination and growth of the resulting germ tube on the surface of the host plant.
• Bacteria have no dormant structures; hence no pre- penetration activity except for multiplication in infection drops on the natural openings.
• However, nematodes show some orientation towards root surface before actual penetration.
• In Rhizoctonia solani, the fungus on coming in contact with root surface, first forms infection cushions and appressoria and from these multiple infections takes place by means of infection pegs.
• In Armillariella mellea, the fungus hyphae form the rhizomorphs (aggregation of hyphae into rope like strands) and only these can cause infection.

Fig : Rhizomorphs                                        Fig : Appressorium

2. PENETRATION:

• Pathogens penetrate plant surfaces by direct penetration or indirectly through wounds or natural openings.
• Bacteria enter plants mostly through wounds and less frequently through natural openings.
• Fungi, nematodes and parasitic higher plants enter through direct penetration and less frequently through natural openings and wounds.

A)  Indirect Penetration :

1. Wounds:

• Wounds caused by farm operations, hail storms, or insect punctures, etc., will help in the entry of different plant pathogens into the host cells.
• Rhizopus, Gloeosporium, Aspergillus, Penicilium, Colletotrichum, Diplodia, etc.
• Weak parasites enter through the wounds caused by hail storms and freezing

2. Natural openings :

a) Stomata:

• In Puccinia graminis tritici, the uredospore germinates and forms a germ tube which on approaching stoma swells at the tip to form an appressorium in the stomatal aperture.
• From the appressorium a blade like wedge grows through the stomatal slits and swells inside to form a sub-stomatal vesicle from which the haustori penetrating the cells are produced.
• Mycosphaerella musicola forms a small structure called stomatopodium over the pore of the stoma after growing for few days on the surface of the leaf.
• A hypha then arises from it which grows into the sub-stomatal chamber and swells to form a vesicle, which in turn gives rise to hyphae which invade palaside tissues.

b) Lenticels: Sclerotinia fructicola (Brown rot of fruits), Streptomyces scabies (Scab of potato), Phytophthora arecae (Mahali disease of arecanut).

c) Hydathodes: Xanthomonas campestris pv. campestris (Black rot of crucifers)

B) Direct penetration:

a) Breakdown of physical barriers.:

• Fungi penetrate host plants directly through a fine hypha produced directly by the spore or mycelium or through a penetration peg produced by an appressorium.
• These structures exert pressure on the surface which results in stretching of the epidermis which becomes thin. Then the infection peg punctures it and effects its entry.

b) Breakdown of chemical barriers:

The host is provided with defense mechanisms against invasion which include

i) presence of cuticular layer on the epidermis,

ii) lack of suitable nutrients for the pathogen in the host cells,

iii) presence of inhibitory or toxic substances in the host cells,

iv) exudation of substances toxic to pathogen or stimulatory to antagonists of the pathogen.

Eg : I) The glands in leaf hairs of begal gram contain maleic acid which is antifungal and provide resistance to infection by the rust fungus (Uromycesciceris arietini).

ii) Protocatecheuic acid and catechol in the red scales of onion provide resistance to onion smudge pathogen, Colletotrichum circinans.

     To overcome these physical and chemical barriers, the fungi produce various enzymes, toxins organic acids and growth regulators.

Through non-cutinized surfaces:

a) Seedlings: Grain smut of jowar (Sphacelotheca sorghi), Loose smut of jowar (Sphacelotheca cruenta), Downy mildew of jowar and bajra (Sclerospora graminicola), Wheat bunt disease (Tilletia caries, Tilletia foetida).

b) Root hairs: Wilt causing fungi (Fusarium sp.), Club root of cabbage (Plasmodiophora brassicae), Root rot of cotton (Phymatotrichum omnivorum).

d) Flowers: Loose smut of wheat (Ustilago nuda tritici), Long smut of jowar (Tolyposporium ehrenbergi), Bunt of rice (Neovossia horrida), Ergot of rye(Claviceps purpurea)

e) Leaves: Basidiospores of white pine blister rust fungus (Cronartium ribicola) germinate and grow down into branches and leaves, where aecia are produced.

d) Nectaries: Fire blight of apple (Erwinia amylovora)

e) Stalk ends: Penicillium italicum, Theilaviopsis paradoxa (Post harvest disease fungi)

Through cutinized surfaces:

a) Cuticle: Leaf spot of spinach (Cercospora beticola), early blight of solanaceous plants (Alternaria solani), Tikka disease of groundnut (Cercospora personata).

3. POST PENETRATION :

Invasion and colonization:

• Infection is the process by which pathogens establish contact with the susceptible cells or tissues of the host and derive nutrients from them.
• A parasitic relationship is formed between host cytoplasm and parasite cytoplasm.
• During infection, pathogens grow and multiply within the plant tissues.
• Invasion of plant tissues by the pathogen, and growth and reproduction of the pathogen (colonization) are two concurrent stages of disease development.
• During establishment, pathogen produces different substances which include enzymes, toxins, growth hormones and polysaccharides which will help in colonization of the host.
• In ectoparasites (Rhizoctonia solani) the main body of the pathogen lies on the surface of the host with only feeding organs (haustoria) penetrating the tissues.
• The endophytic parasites or endoparasites grow subcuticularly (Diplocarpon  rosae, black spot of rose), in parenchyma tissues (most fungal and bacterial pathogens as well as many nematodes) or in vascular tissues (vascular wilt parasites).

• Some pathogens are endobiotic, i.e., mycelium is not produced and the thallus is entirely present within a host cell Ex: Synchytrium endobioticum.

The time interval between inoculation and appearance of disease symptoms is called the incubation period.