‘Preemptive’ breeding: More than a step ahead of crop disease outbreaks

These plant pathogens — which are noted for their twin-shaped appearance — could be transmitted by whiteflies to cultivated crops, including beans. At that time, these viruses were “not yet considered pathogens of economic importance for the agriculture sector” in Colombia.

Soon after that survey though, Morales et al found symptoms of leaf crumpling in certain bean crops in some parts of the Valle del Cauca Department. They identified the cause of the “unexpected outbreak” as a member of Geminiviridae family: the bean leaf crumple virus (BLCrV).

BLCrV, as it name implies, causes leaves to wrinkle and shrinks the pods, reducing product quality. It can prevent the plant from flowering and eventually kills the whole bean crop. It is particularly devastating to Andean beans but has a lesser effect on Mesoamerican beans, as CIAT Plant Breeder Bodo Raatz found out during field trials not a long time ago.

Andean and Mesoamerican beans make up the common bean family. The Andean kind includes dark and light red kidney, white kidney, and cranberry beans. Pinto, great northern, small red, pink, black, navy, and small white beans belong to the Mesoamerican gene pool.

Raatz conducted the trials to evaluate breeding lines of Andean beans. They were done on a site that also had Mesoamerican bean crops and that was found to be infested with whiteflies. He noted that as the Andean beans withered, some Mesoamerican bean plants continued to grow unharmed. Tests overseen by CIAT Lead Virologist Wilmer Cuellar confirmed that BLCrV was the culprit.

Concerned over the possibility that the virus will also appear elsewhere and become a far-reaching outbreak, the Bean Program decided to start crossing Andean beans with Mesoamerican lines that have resistance to the pathogen.

“Call it ‘preemptive’ breeding, if you like,” said Raatz.

The concern that the virus will spread to other places is valid for several reasons, according to Cuellar and Raatz.

Whiteflies thrive in warm regions and are typically found in the lowland. But climate change is making highlands, the ideal site for cultivating Andean beans, warmer.

“Some people in the Colombian mountains said they had seen beans with crumpled leaves, and some of the samples brought to us were positive for the virus,” Cuellar said. “So it’s possible that whiteflies are moving up, but we need to study this further to confirm that they are in higher altitudes now.”

Or it could spread to neighboring countries. Apart from Colombia, the Andean beans are a staple of Argentina, Chile, Ecuador, and Peru.

Raatz estimated that by the end of 2018, CIAT will have breeding lines for beans that are resistant to BLCrV. To develop these lines, he and his team use both phenotypic and molecular breeding methods.

The phenotypic method focuses on plant traits, such as if the plant displays symptoms of the disease or not during field trials. Molecular breeding homes in on the DNA and allows scientists to see if a plant has the resistance gene that provides immunity to, in this case, BLCrV.

Raatz is also using molecular breeding to develop beans with resistance to a new strain of the bean common mosaic virus, known as BCMV isolate 1755a, which he described to be “extra dangerous.” Unlike BLCrV that requires transmission by whiteflies, BCMV belongs to a different group of viruses that can enter and “hitchhike” in the seed to infect germinating plants.

BCMV isolate 1755a, added Raatz, was discovered in the United States in samples that probably originated from China. But he thinks the virus will eventually reach Africa as the Asian country exports a lot of beans to the continent.

If so that would be alarming. Like Latin Americans, Africans are big consumers of beans.

“We’re doing preemptive breeding so that we’re ready for any disease outbreak,” said Raatz.

The breeding lines for beans with resistance to BCMV isolate 1755a are expected to be delivered to partners by 2019, he said.