One of the following diseases is caused by fungi

Analyze the question: We need the one disease caused by a fungus. The other diseases are caused by different types of pathogens.

Explain the concept: Plant diseases are caused by different pathogens:

• Viruses cause mosaic diseases (patchy yellow and green leaves). Examples: cassava mosaic, tomato mosaic, groundnut rosette, and cocoa swollen shoot.
• Bacteria cause diseases like cassava bacterial blight and bacterial wilt.
• Fungi cause rusts, smuts, mildews, and some blights. Examples: coffee leaf rust (Hemileia vastatrix), maize smut, cocoa black pod (Phytophthora), and late blight of potato.

Step-by-step solution:

• A. Cassava mosaic: caused by a virus (cassava mosaic virus, spread by whiteflies). ✗
• B. Coffee leaf rust: caused by the fungus Hemileia vastatrix. Rust diseases are fungal, and they produce orange, powdery spore masses on the undersides of leaves. ✓
• C. Leaf blight of cassava rosette: the option is oddly worded. Cassava blight is typically bacterial (Xanthomonas), and rosette diseases are usually viral (e.g., groundnut rosette). ✗
• D. Tomato mosaic: caused by tomato mosaic virus. ✗

Answer: The correct option is B. coffee leaf rust.

Exam tip: Remember these keywords:

• "Mosaic" = virus
• "Rust" and "smut" = fungus
• "Blight" can vary, so check the specific crop.