Which of the following can cause infectious plant diseases?

The correct choice highlights a range of biological agents that are known to cause infectious diseases in plants. Parasitic plants, nematodes, bacteria, and phytoplasmas are all classified as pathogens that can infect host plants, leading to various diseases.

Parasitic plants can siphon nutrients from their host plants, weakening them and making them more susceptible to other diseases. Nematodes, tiny roundworms, can infect the roots of plants, leading to root damage and stunting growth. Bacteria can invade plant tissues and cause diseases such as bacterial blight or wilt, often leading to significant damage and loss. Phytoplasmas, which are a type of bacteria without a cell wall, can disrupt plant functions and lead to serious diseases, often transmitted by insect vectors.

In contrast, other choices include factors that do not directly cause infectious diseases. Insects may transport pests or pathogens but are not themselves pathogens. Environmental stresses like drought, nutrient imbalances, and weather variations can lead to stress in plants but do not cause infectious diseases in the way that biological agents do.