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Natural resistance

 Photo courtesy of Benoit Bizimungu/AAFC.

Potato breeders have long turned to exotic or wild varieties for their rich and varied genetic material. Wild species contain a great diversity of traits, which relate to a myriad of aspects of potato cultivation and end-product desirability. The ability to resist disease is a trait of particular interest.

In a new project, Dr. Benoit Bizimungu is now hard at work in harnessing the natural resistance found in wild potato species to help better control late blight. Dr. Qin Chen, who helped develop somatic hybrids, and technicians Ron Gregus, Evelyn Lyon and Susan Smienk have assisted Bizimungu during this breeding project at the Agriculture and Agri-Food Canada research centre in Fredericton.

Late blight (P. infestans) is arguably the most devastating and dangerous potato disease, and because most – if not all – commercial potato cultivars are susceptible to the fungus, farmers must apply fungicide to their fields several times each growing season. Over recent years, more virulent strains of late blight have emerged, strains that are harder to control with fungicides, and this means breeding efforts to create varieties with better resistance are more important than ever. Planting varieties with boosted resistance also means that growers may be able to reduce the amount of fungicide applied to fields in the future.  

Most breeding programs around the world in this vein have focused on using a wild potato relative called Solanum demissum as their main late blight resistance source. Bizimungu explains at least 11 resistance genes (R1-R11) have been identified in S. demissum, and four of them (R1, R2, R3 and R10) have been introgressed into commercial cultivars, but strains of the pathogen that are able to overcome the defences related to these genes have emerged.

Breeders are therefore turning to new sources of resistance, and working to develop germplasm with polygenic resistance (resistance that is stronger, longer-lasting and more durable). In the quest to create varieties less susceptible to late blight, the more durable resistance that can be had, the better. “Durable resistance means that different resistance mechanisms are present in the plant, allowing it to fend off disease in several different ways,” Bizimungu explains.

Plant defence mechanisms against late blight work in two ways. One is the presence of physical and chemical barriers on the plant surface that help prevent the fungus from penetrating the plant. The other set of plant defences come into play after successful pathogen penetration has occurred. These latter defences inside the plant include the production of hypersensitive response lesions, as well as the production of various substances that negatively affect the pathogen, such as reactive oxygen species, a glucose residue molecule named callose and various proteins. Each of these defences work in different ways. Reactive oxygen species, for example, fend off late blight through their antimicrobial activity and through their important role in cross-linking cell wall proteins.

Resistance from the wild
Solanum species are found growing in the wild over a large geographic area – from the southwestern United States to many parts of South America. For this project, Bizimungu is employing three sources of resistance recently found in Mexico (a region with heavy late blight disease pressure) that have the potential to offer natural protection to some strains of late blight. These three wild species are Solanum bulbocastanum, S. chacoense and S. pinnatisectum, with the last one identified in particular as having distinct genetic late blight resistance.

With the aid of molecular markers, Bizimungu and his team have crossed these strains to produce 16 new advanced potato selections. Many more selections with improved resistance are expected to come out of the breeding program in the future. The breeding process began with adding highly virulent late blight inoculum to plants in the lab. The selections that showed the best resistance were then planted and assessed in the field in the presence of natural late blight infection pressure. The best performers from those field trials are currently being hybridized into fresh market and processing selections with acceptable agronomic performance. The progeny is being assessed in various stages at two Canadian locations under different conditions: the Vauxhall research substation in Alberta under irrigation production and the Benton Ridge substation in New Brunswick under rain-fed cultivation.

Of the three wild species, Solanum pinnatisectum presented a special challenge since crossing it with commercially cultivated potato plants (via sexual hybridization) is not possible. The team therefore had to use somatic hybridization, which involves the fusing of somatic (body) cells of parent plants. Successful somatic hybrids were subsequently backcrossed with adapted cultivars or elite breeding clones and the progeny was screened for resistance to P. infestans – and also to see if they possess the capacity for sexual crossing. Resistant backcross progenies are being grown at the Vauxhall research substation in Alberta and are being assessed for field performance in terms of late blight resistance and agronomic potential.

New lines from all three wild species will be field-tested over the next few years, Bizimungu explains, at which point they will either be used as parents to further develop improved cultivars or be released commercially. “Three potatoes with improved, moderate resistance to some strains of late blight in controlled tests are currently under active evaluation by industry for potential commercialization.” 

July 13, 2015  By Treena Hein