Step 1: Disease Identification.
Tristeza disease is a highly destructive global citrus disease. Its Spanish name, "Tristeza," translates to sadness, reflecting the decline observed in infected citrus trees.
Step 2: Causal Agent.
The disease is caused by Citrus tristeza virus (CTV), a member of the genus Closterovirus. This phloem-limited virus is primarily transmitted by aphids, notably Toxoptera citricida.
Step 3: Option Verification.
- Bacteria: Incorrect. Citrus canker is a bacterial disease, not tristeza.
- Fungus: Incorrect. Tristeza is not fungal in origin.
- Virus: Correct. Citrus tristeza virus is the causative agent.
- Mycoplasma: Incorrect. Phytoplasmas cause diseases such as little leaf, not tristeza.
Step 4: Determination.
Therefore, Tristeza disease in citrus is caused by a virus.