Q.52 Identify the correct statement(s) with respect to plant disease.
(A) Hairy root disease in tobacco is caused by Agrobacterium tumefaciens
(B) Loose smut of barley is caused by Ustilago nuda
(C) Stem rust of grape is caused by Plasmopara viticola
(D) Fire blight in pear is caused by Erwinia amylovora
Correct answer: (D) and (B)
Fire blight in pear is caused by Erwinia amylovora, and loose smut of barley is caused by Ustilago nuda. These statements align with established plant pathology facts.
Option Analysis
(A) Hairy Root Disease
Hairy root disease in tobacco results from infection by Agrobacterium rhizogenes, not Agrobacterium tumefaciens. A. tumefaciens causes crown gall disease through T-DNA transfer leading to tumors, whereas A. rhizogenes induces prolific root growth via the Ri plasmid.
(B) Loose Smut of Barley
Loose smut of barley is indeed caused by Ustilago nuda, a fungal pathogen that infects seeds systemically. Spores replace grain heads with black powdery masses during heading, favored by cool, wet conditions at flowering.
(C) Stem Rust of Grape
Stem rust of grape is not caused by Plasmopara viticola; this oomycete causes downy mildew on grape leaves and fruits. Stem rust typically affects cereals via Puccinia species, while grape rust involves Phakopsora euvitis.
(D) Fire Blight in Pear
Fire blight in pear is caused by the bacterium Erwinia amylovora, which spreads via insects and rain, overwintering in cankers. It kills blossoms, shoots, and branches, thriving at 65–80°F with moisture.
Introduction to Key Plant Diseases
Plant disease causative agents like Agrobacterium rhizogenes for hairy root disease tobacco and Erwinia amylovora for fire blight pear impact crops severely. Understanding pathogens such as Ustilago nuda in loose smut barley aids in disease management and exam success.
Bacterial Pathogens in Focus
Hairy Root Disease Tobacco: Agrobacterium rhizogenes transfers Ri plasmid T-DNA, causing hairy roots in tobacco via auxin overproduction. A. tumefaciens differs by inducing galls.
Fire Blight Pear: Erwinia amylovora enters via flowers, producing ooze that spreads bacteria, leading to blighted shoots.
Fungal Pathogens Detailed
Loose Smut Barley: Ustilago nuda infects embryos in seeds, producing spore-filled heads at maturity. Control uses seed treatments.
Stem Rust Grape Misconception: No true stem rust exists for grapes; Plasmopara viticola causes downy mildew with sporulation on leaf undersides. Rust-like symptoms come from Phakopsora euvitis.
Exam-Relevant Insights
For CSIR NET, note distinctions: bacterial (Erwinia amylovora), agrobacterial (A. rhizogenes), and smut fungi (Ustilago nuda). Accurate identification prevents crop losses and scores points.
