Plant Pathology

E.T. Butler is recognised as the Father of Indian Plant Pathology. He initiated exhaustive studies of Indian fungi and plant diseases before 1910, laying the foundation for systematic plant pathology in India. K.C. Mehta is noted for his outstanding contribution to cereal rust research in India, while B.B. Mundkur established the Indian Phytopathological Society.

The three wheat rusts and their causal organisms are: Black rust (Stem rust) → Puccinia graminis var. tritici; Brown rust (Leaf rust) → Puccinia recondita; Yellow rust (Stripe rust) → Puccinia striiformis. In UP, the typical sequence of appearance is Leaf rust > Stripe rust > Stem rust.

Loose smut of wheat is an internally seed-borne disease. The pathogen (Ustilago nuda var. tritici) survives as dormant mycelium within the embryo of the seed. This is why surface seed treatments are ineffective; systemic fungicides (e.g., Vitavax / carboxin) or hot-water seed treatment are used to eradicate the internal mycelium.

The Bengal Famine of 1943 was precipitated largely by a severe epidemic of Brown spot of rice, caused by Helminthosporium oryzae (now Bipolaris oryzae). The toxin 'Cochliobolin' produced by this fungus also contributes to its virulence. This disease and the resulting crop failure were major contributors to the famine that caused millions of deaths.

The Kresek phase is a characteristic early symptom of Bacterial Leaf Blight (BLB) of rice, caused by Xanthomonas oryzae pv. oryzae. In this phase, young seedlings or tillers wilt and roll completely, turning greyish-green to yellow, and eventually die. BLB is also the only disease in rice classified as a typical vascular wilt. High humidity, strong wind, and rainy weather favour rapid spread; temperature below 20°C does NOT favour it.

Option E is INCORRECT because Hill bunt (stinking smut) of wheat is caused by Tilletia foetida (and T. caries), NOT Neovossia indica. Karnal bunt (partial bunt) is a separate disease caused by Neovossia indica (syn. Tilletia indica). All other options are correct facts about Karnal bunt. The name derives from Karnal (Haryana), where it was first observed.

Martinus Beijerinck coined the term 'contagium vivum fluidum' (living contagious fluid) in 1898, describing the tobacco mosaic pathogen as a fluid living entity that could reproduce only inside living cells. Ivanowski (1892) had earlier shown the agent passed through porcelain filters but did not challenge the idea that it was a bacterium. Adolf Mayer (1886) first described Tobacco Mosaic Disease. These discoveries collectively marked the beginning of virology.

The second-stage juvenile (J2) of root-knot nematode (Meloidogyne spp.) is the infective stage that penetrates plant roots and initiates infection. The first moult occurs within the egg; the J2 hatches and migrates through soil to enter roots near the growing tip, where it induces formation of giant cells and characteristic root-knot galls. Nematodes feed on host plants through a specialised piercing organ called the stylet.

Bordeaux mixture is prepared by mixing copper sulphate (CuSO₄) and slaked lime [Ca(OH)₂] in water, making it a copper-based inorganic fungicide. It also has bactericidal properties and is used for tree surgery (along with Chaubatia paste). It is a typical contact/protective inorganic fungicide — NOT systemic. Karathane is used for powdery mildew; dithiocarbamates include Dithane M-45 (Mancozeb).

Orobanche (Broomrape) is a complete (holo) root parasite — it lacks chlorophyll and obtains all its nutrition from the host's roots. Cuscuta is a complete stem parasite (holoparasite on stem). Loranthus is a semi-stem parasite (has some chlorophyll). Striga (Witch weed) is a semi-root parasite (has chlorophyll, partial parasite on roots). These distinctions are frequently tested in competitive agriculture exams.

MLOs (Phytoplasmas) are characterised by: (i) absence of rigid cell wall (hence pleomorphic), (ii) resistance to penicillin (which targets cell walls) but susceptibility to tetracycline, (iii) inability to grow on cell-free media — they are obligate intracellular parasites, (iv) transmission by leafhoppers (vectors), not soil/water. Diseases caused by MLOs include Little leaf of brinjal, Sugarcane grassy shoot, Aster yellows, and Witches' broom.

Powdery mildew fungi (Class: Ascomycetes; Order: Erysiphales) are obligate parasites whose mycelium is ectophytic — it grows on the outer (epidermal) surface of the host. They penetrate individual epidermal cells only through haustoria for nutrient absorption. This ectophytic growth gives the characteristic white powdery appearance on leaves. Control includes Karathane (dinocap) @ 0.06% spray. The causal organism of powdery mildew of wheat is Erysiphe graminis var. tritici.

Ergot of bajra (pearl millet) is caused by Claviceps microcephala (distinct from Claviceps purpurea which causes ergot of rye/wheat). The infection begins with the 'Honeydew stage' where a sticky, sweet, spore-bearing liquid (Sphacelia stage) is produced attracting insects for dispersal. This later hardens into a sclerotium (sclerotial stage). The sexual fruiting body is a Perithecium (not Cleistothecium). Ergotin is the alkaloid of ergot fungi; LSD is a synthetic derivative. The most important symptom is transformation of kernels into dark sclerotial bodies.

Trichoderma harzianum and T. viride are well-established biocontrol agents used against soil-borne fungal pathogens, particularly Fusarium oxysporum causing wilt diseases of chickpea, lentil, and pigeonpea. They can be applied both as soil treatment and seed treatment. They work through mycoparasitism, antibiosis, and competition for nutrients. Trichoderma is NOT effective against viruses, bacteria, or nematodes.

In the heteroecious rust Puccinia graminis tritici, the life cycle alternates between two unrelated hosts: (1) PRIMARY host = wheat — where Uredial (Stage II) and Telial (Stage III) stages occur; (2) ALTERNATE host = Berberis vulgaris (barberry) — where Pycnial (Stage 0) and Aecial (Stage I) stages occur. Basidiospores (Stage IV) produced from germinating teliospores infect the Berberis (alternate) host. In India, appearance of wheat rust takes place through dissemination of Uredospores, as the alternate host is largely absent in wheat-growing plains.