The correct answer is: B. Bacteria
Fire blight is a bacterial disease that affects rosaceous plants, including apple, pear, quince, hawthorn, and mountain ash. It is caused by the bacterium Erwinia amylovora. Crown gall is a bacterial disease that affects many plants, including apple, pear, cherry, plum, peach, and grape. It is caused by the bacterium Agrobacterium tumefaciens.
Viruses are not able to cause plant diseases on their own. They require a vector, such as an insect, to transmit them from one plant to another. Fungi are able to cause plant diseases by attacking the plant’s roots, stems, leaves, and fruits. Protozoans are not known to cause plant diseases.
Here is a brief explanation of each option:
- Viruses are acellular, obligate parasites that replicate only inside living cells. They are not able to cause plant diseases on their own. They require a vector, such as an insect, to transmit them from one plant to another.
- Bacteria are single-celled organisms that are able to cause plant diseases by attacking the plant’s roots, stems, leaves, and fruits. Some examples of bacterial plant diseases include fire blight, crown gall, and black spot.
- Fungi are multicellular organisms that are able to cause plant diseases by attacking the plant’s roots, stems, leaves, and fruits. Some examples of fungal plant diseases include powdery mildew, rust, and blight.
- Protozoans are single-celled organisms that are not known to cause plant diseases.