Return to Resistance: Breeding Crops to Reduce Pesticide Dependence

Raoul Robinson
agAccess, Davis, Calif. 1996

Raoul Robinson, Ph.D. spoke at the University of California, Davis campus in March 1998 on "New Approaches to the Control of Crop Parasites." Robinson is a Canadian/British plant scientist with 40-years experience working in crop improvement. He is the author of numerous books including Plant Pathosystems (Collier-MacMillan, 1987) and Host Management in Plant Pathosystems (Springer-Verlag, 1976). His latest book, Return to Resistance: Breeding Crops to Reduce Pesticide Dependence, is written for a wider, less technical audience.

His UC Davis talk addressed the main topic of this recent book, the use of durable, horizontal host-plant resistance to manage parasites. He includes diseases, mites, insects, and nematodes in this category but excludes weeds as they are competitors and not parasites. In his talk and his book, he contrasts most current resistance breeding programs that are based on single gene, or vertical resistance, to programs that are based on a quantitative multi-gene horizontal resistance.

Vertical resistance is based on the "gene-for-gene" model of host parasite interactions, where every pathogen has a gene for parasitic ability and every host has a resistance gene. Robinson used a "lock-and-key metaphor" to describe vertical resistance, where the parasite gene is the key and the host gene is the lock. Either the key fits and the parasite gains entry to the host, or the lock holds and the parasite is excluded. Obviously the selection pressure in the parasite to overcome this kind of host resistance is intense. Indeed, others have described this kind of resistance breeding as "an arms race"--no sooner are new genes for resistance discovered and integrated into a plant, then the pathogen overcomes the resistance. Horizontal resistance, on the other hand, is quantitative; traits are results of mixtures of multiple gene products, therefore one observes a range of inheritance and genetic expression in the crop from a minimum to a maximum. This kind of resistance is more location specific, selected for over time and in the presence of local races of all pathogens, and is delimited by the crop's own range of cultivation.

Robinson and other breeders have shown that horizontal resistance can be developed and is durable in potatoes, coffee, corn, and beans. Importantly, not only is this kind of resistance long-lasting, but it greatly reduces or eliminates the need for pesticides. He cites examples of potato cultivars such as "Alpha," a Dutch cultivar grown in the Toluca Valley of Mexico, originally bred in Europe with vertical resistance to late blight (Phythopthora infestans). It now typically receives 20 to 25 fungicide applications a year, while cultivars with horizontal resistance such as "Tollocan" and "Rosita" are sprayed only one to two times a year. Much of this potato resistance breeding work in Toluca was done by John Niederhauser with support from the Rockefeller Foundation.

In addition to discussing late blight on potatoes in Mexico and a more general history of late blight during the great Irish Potato Famine, Robinson described various other case studies of the challenges and successes in breeding for horizontal resistance. He discussed resistance breeding against the tropical corn rust (Puccinia polysora) in Kenya and coffee berry disease (Colletotrichum coffeanum) in Ethiopia.

Robinson also addressed possible new directions and solutions to some of the current shortcomings in the reliance on vertical resistance in crop protection. He suggested that plant breeders, farmers, amateur breeders and environmentalists should seriously consider making use of horizontal resistance through what he calls "plant breeding clubs." People form these clubs out of concern about the quality of commercially available cultivars, or to contribute practically to developing cultivars less reliant on pesticides. A club might be linked to a university interested in teaching students with "hands-on learning," or club members might simply be interested in the possible patent royalties.

For more information on how to breed for horizontal resistance, why and how to form plant breeding clubs, and for a general overview of the history of breeding for host-plant resistance to parasites, see Robinson's new book. Return to Resistance: Breeding Crops to Reduce Pesticide Dependence can be ordered for $29.95 by calling Fertile Ground (agAccess) at (530) 756-7177.

For more information: R. Robinson, 445 Provost Lane, Fergus, Ontario, Canada, N1M 2N3.