This group of infectious disease-causing organisms comes from single-celled organisms that usually enter through plant wounds (e.g., crown gall, soft rot, certain cankers).

• A. Bacteria
• B. Viruses
• C. Nematodes
• D. Mites

Answer: (A)

Single-celled microorganisms that commonly infect plant tissue after wounds are bacteria. Bacteria are independent cells that can invade through cuts or abrasions in plant tissue and spread, causing diseases such as crown gall, soft rot, and certain cankers. They produce enzymes that break down cell walls or manipulate plant tissues, leading to the characteristic symptoms like tissue decay or gall formation.

Viruses aren’t cells at all and need a host cell to replicate, so they don’t fit the description of a single-celled organism. Nematodes and mites are multicellular organisms (worms and arthropods, respectively) that cause disease in different ways, not by being single-celled bacteria. That combination of being single-celled and entering through wounds to cause those specific diseases points to bacteria.