Evolutionary clash: Plant pests and diseases battle humankind for control in the fields

Maize and wheat farmers worldwide are facing the emergence and spread of new or modified strains of deadly crop diseases and pests, including insects and micro-organisms such as fungi and viruses. Chemical controls are costly and potentially harmful to human and environmental health. Genetically-bred crop resistance and pesticides work for a time, but the organisms evolve to overcome those restraints or spread to susceptible crop varieties at new locations.

The International Maize and Wheat Improvement Center (CIMMYT) applies science and partnerships to draw resistance genes from maize and wheat landraces, global seed collections, and other crop genetic resources. The genes are used to breed and test new, resistant varieties, whose seed is then increased and made available to farmers. Scientists also wield modern systems to track pest and pathogen evolution and movements. They analyze pest and pathogen interactions with crops and create combinations of new genes that offer longer-lasting resistance. The diseases and pest described here are examples of those that have made the news recently.

The Ug99 race of wheat stem rust, a fungal disease, emerged in eastern Africa in the late 1990s and has spawned 13 new strains, spreading to 13 countries. Ug99 is highly-virulent for nearly all popular wheat varieties. The national research programs in Ethiopia and Kenya have supported the yearly screening of as many as 50,000 wheat lines from breeding programs worldwide under strong natural Ug99 infections, allowing rapid development of new, resistant varieties. Enough seed has been multiplied so many countries in the projected path of Ug99’s spread are safe from serious outbreaks. Another stem rust race group known as TKTTF has spread to over a dozen countries in Africa, Asia, and Europe, since its detection in Turkey in 2005.

Long confined to South America, the mysterious fungal disease known as wheat blast suddenly appeared in Bangladesh in 2016, causing 25-30 percent losses on 15,000 hectares of wheat and threatening to spread quickly throughout South Asia’s vast wheat lands, where no varieties are resistant. CIMMYT and CGIAR Research Program on Wheat (WHEAT) partners are at the center of an urgent global response to monitor, characterize and control blast and, especially, to develop and deploy resistant wheat varieties.

A moth from the Americas that appeared in Africa in 2016 and whose larvae feed on numerous crops, the fall armyworm is able to destroy as much as 70 percent of a maize harvest and, once adult larvae are established, is not easily controlled by pesticides. Scientists from CIMMYT and the International Institute of Tropical Agriculture (IITA) are working with partners worldwide on integrated approaches including chemical and biological controls, resistant varieties, agronomic management, and tracking and early-warning systems.

Involving a deadly alliance of two viruses and first reported in eastern Africa in 2011, MLN disease kills plants before they can grow, and the pathogens are transmitted by insects or contaminated seed. Serious damage to the region’s maize has led many farmers to stop growing the crop. Progress to counter MLN includes the production and distribution of resistant hybrids.

Text: Clyde Beaver, Bianca Beks, G. Michael Listman,

Contributors: Hans Braun, Dave Hodson, Jennifer Johnson, B.M. Prasanna, Ravi Singh

Photos: CIMMYT archives

Graphics: Gerardo Mejía, Bosen Zhou

Editors: Bianca Beks, G. Michael Listman, Julie Mollins, Geneviève Renard

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