Consider the following : 1. Birds 2. Dust blowing 3. Rain 4. Wind blowing Which of the above spread plant diseases ?

States that seeds may be dispersed by being eaten by birds and then passed out with faeces, showing birds transport viable plant material and biological matter between locations.

A student could infer that if birds move seeds and faeces, they might also move pathogen-bearing material (spores, infected tissue, bacteria) on or in their droppings to new plants or soils.

Explains that pollen and seeds are carried by animals such as birds, demonstrating birds regularly contact flowers/fruits and move reproductive plant material among plants.

Combine this with the fact that contact with infected flowers/fruits can transfer pathogens (e.g., spores or contaminated pollen) to other plants visited by the same bird.

Defines vectors as animals (insects) that spread pathogens, giving the general rule that animals can act as carriers to transmit disease-causing agents.

By analogy, a student can reason that other animals (birds) that contact plants could similarly act as vectors for plant pathogens under the right circumstances.

Notes that crop diseases can spread widely across areas and require region-wide chemical control, implying the existence of mechanisms (including mobile carriers) that move pathogens over distances.

A student could link this pattern of regional spread to mobile animals (like birds) as one possible dispersal mechanism to investigate further.

Describes how exotic plants spread rapidly in new habitats when 'natural' enemies are absent, illustrating that movement of organisms into new areas can enable spread and establishment of biological agents.

Using that pattern, a student might consider birds as agents that move plant-associated organisms (including pathogens) into new habitats where they could establish.