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

The correct answer is D, 1, 2, 3 and 4. Birds, blowing dust, rain and wind can all carry the agents of plant disease from one plant to another. Plant pathogens such as fungi, bacteria and viruses cannot move on their own over long distances, so they depend on carriers. Wind lifts light fungal spores high into the air and can carry rust and blight spores over hundreds of kilometres, which is how wheat rust spreads across regions. Blowing dust carries spores and infected soil particles onto healthy leaves. Rain splashes bacteria and spores from diseased leaves and soil onto nearby plants and helps them enter through wounds. Birds carry spores and bacteria on their feet and feathers, and spread the seeds of parasitic plants such as mistletoe in their droppings. Option A is wrong because it leaves out dust and wind, two of the commonest carriers. Option B is wrong because it leaves out birds and dust. Option C is wrong because it leaves out rain, which is a major means of spreading bacterial diseases. Exam tip: plant diseases spread through wind, water, soil, seeds, insects, birds and human tools.