36. Which one of the following organisms is responsible for crown gall disease in plants?

• (a) Xanthomonas campestris
• (b) Rhizobium etli
• (c) Agrobacterium tumefaciens
• (d) Erwinia stewartii

  Agrobacterium tumefaciens causes crown gall disease in plants. This bacterial pathogen transfers T-DNA from its Ti plasmid into plant cells, inducing uncontrolled growth of tumors at wound sites. The correct answer is (c) Agrobacterium tumefaciens.​

  Option Analysis

  Crown gall disease features tumor-like galls on plant roots and stems, primarily in dicots.

  • (a) Xanthomonas campestris: Causes black rot in crucifers like cabbage via polysaccharide production, not galls.​

  • (b) Rhizobium etli: Forms symbiotic nitrogen-fixing nodules in beans, unrelated to tumors.​

  • (c) Agrobacterium tumefaciens: Transfers T-DNA encoding auxin and cytokinin synthesis, causing galls; key in genetic engineering.​

  • (d) Erwinia stewartii: Induces Stewart’s wilt in corn via vascular blockage, no gall formation.​

  Crown gall disease in plants, caused by Agrobacterium tumefaciens, leads to tumorous galls on roots and stems, impacting crops like roses and grapes. This plant pathology highlights bacterial gene transfer mechanisms central to biotechnology. Understanding crown gall disease plants Agrobacterium tumefaciens aids in precise disease control.​

  Disease Mechanism

  Agrobacterium tumefaciens harbors a Ti plasmid with T-DNA that integrates into the plant genome post-infection at wounds. This triggers opine and hormone production, driving gall formation. The process exemplifies natural genetic engineering, now harnessed for transgenics.​

  Symptoms & Impact

  Galls appear as rough, woody swellings at the crown, disrupting nutrient flow and weakening plants. Young trees suffer most, with severe cases causing dieback. Economically, it affects orchards and ornamentals globally.​

  Incorrect Organisms Explained

  Other options fail as gall inducers:

  Organism	Disease Caused	Key Difference from Crown Gall
  Xanthomonas campestris	Black rot (crucifers)	Vascular necrosis, no tumors ​
  Rhizobium etli	Nitrogen fixation (beans)	Symbiotic nodules, beneficial ​
  Erwinia stewartii	Stewart’s wilt (corn)	Leaf wilt, bacterial streaming ​

  Prevention Strategies

  Prevent wounds during planting and apply bacteriocins like K84 for biocontrol. Remove galls early; avoid wet soils favoring the pathogen. Chemical dips with streptomycin help nurseries.​