How Much of Our Food Crops Are Lost to Disease?

How much of our food crops are lost to disease?

Plant disease is new to the curriculum. This is a simple and easy starter to encourage students to understand the impact that plant disease has both on the world’s poorest farmers and on consumers in the UK.

Fill a bowl with your choice of ‘crop’. This should ideally be a foodstuff that looks relatively close to its unprocessed form, such as rice and coffee beans. It should also be a foodstuff that your students can relate to – cocoa beans and coffee beans go down well.

The full bowl represents the total potential harvest. Ask a student to pour away (into another bowl) the amount that they think is lost to plant disease. Students tend to underestimate losses. You can ask the rest of the class if they think the student’s guess is right.

Then correct the amount yourself, either by pouring away more ‘crop’, or by adding it back in. Show students the amount of the crop lost to disease, and how much the farmer is left with. Discuss what impact they think this has on the price of goods in the supermarkets. Students may have noticed, for example, that chocolate bars are getting smaller, and that prices are rising.

By pouring away the crop, students gain a more visceral understanding of a farmer’s losses.

We have selected three crops as an example.

Rice – impact of all plant diseases – up to 30% loss

Current estimates of rice yield lost to disease: up to 30% (plus up to 10% lost to pests). (Crop losses due to diseases and their implications for global food production losses and food security Serge Savary, Andrea Ficke, Jean-Noël Aubertot & Clayton Hollier)

Rice blast, caused by a fungus, is a particularly severe disease. Rice is the staple food for more than half of the world’s population. Rice is a vitally important crop for the world’s poorest farmers, particularly in Asia. As a global trend, the poorer someone is, the more of their calories will come from rice. Increases in rice prices in recent years have seen riots around the world.

Coffee – impact of coffee rust disease – up to 60%

Current estimates of the impact of coffee rust are high. Central American farmers report up to 60% of their crops lost to coffee rust. (The Impact of Coffee Rust, Oxfam Impact Reports, 2014)

Coffee rust is a fungus (Hemileia vastatrix). Orange patches cover the leaves, resulting in defoliation and lack of photosynthesis. Coffee rust is endemic in all coffee-producing countries. Low levels of rust are manageable, but severe rust can make it uneconomic to grow coffee. Farmers can spray with fungicides - but this is particularly costly for smallholder farmers. In the past, rust did not affect the highest altitude plantations, but possibly due to global warming, rust has now increased its spread. Farmers must make the choice between growing cultivars more in demand by consumers (usually with more ‘Arabica’ genes) or a rust-resistant but lower-priced cultivar (with more ‘Robusta’ genes in it).

Cocoa beans – impact of frosty pod – up to 100%

Frosty pod can cause losses of up to 100% of a farmer’s crop, resulting in cocoa plantations being given up altogether. (Frosty Pod of Cacao: A Disease with a Limited Geographic Range but Unlimited Potential for Damage, W. Phillips-Mora and M. J. Wilkinson, 2007)

Frosty pod is a fungus that eats the insides and outsides of the cocoa pod. Currently confined to Southern and Central America, if it reaches Africa, the world cocoa production could be devastated.