Concept:Crop diseases damage plants, reducing their productivity and the market value of their produce.Explanation:Statement I is true because diseases often lower the quality of farm products.Statement II is true because diseased leaves lose their ability to carry out photosynthesis effectively.Statement III is false because disease control measures actually increase the cost of production, not reduce it.Statement IV is true because diseases lead to a fall in the total yield of the produce.Thus, the true statements are I, II and IV.Answer:D. I, II and IV only