Wild spinach offers path to breed disease resistance into cultivated varieties

Researchers at Washington State University's Mount Vernon Northwestern Washington Research & Extension Center demonstrated in a new paper that some strains of wild spinach are resistant to Fusarium wilt, a fungal disease that is a persistent problem for growers of commercial spinach seed, and they identified regions of the plants' genome associated with that resistance.

The findings are important for seed growers in western Washington and Oregon, where a significant portion of the world's spinach seed is grown and where the pathogen has long been a problem due to the acidic soils.

"We were very, very pleased we found some excellent resistance when we did the screening and then we followed up with the DNA sequencing and looking at where that resistance might be lying," said Lindsey du Toit, a plant pathologist who has worked on fighting disease in seed crops for 25 years at WSU's Mount Vernon NWREC.

Though the new paper, published this month in Scientific Reports, identified several varieties of wild spinach associated with resistance to Fusarium wilt, more study is needed to understand the genetic nature of the resistance. However, seed companies don't have to wait to apply the findings -- they can begin breeding hybrids with the wild spinach varieties that showed resistance.

"You don't necessarily have to understand the mechanism of resistance in order to use it," du Toit said. "This is a tool that's available immediately to breeding programs."

Spinach consumption has been growing dramatically around the world. In the U.S., the per-capita consumption of the vitamin-rich vegetable has more than doubled in the past 20 years, with a particularly strong market for baby leaf spinach.

Most of the domestic crop is grown in hot, dry regions such as California, Texas and Florida. But growing spinach seed requires a rare combination of seasonal conditions -- long, dry summers that aren't too hot. As a result, around a fifth of the world's spinach seed is grown in the Pacific Northwest.

But those crops have little resistance to Fusarium wilt, which afflicts spinach by entering through the roots and blocking their ability to take up water. Seed growers have tried to manage this problem by rotating spinach crops on long timeline -- a decade or more between plantings -- and taking other measures to treat the soil with calcium carbonate to reduce the acidity.

Even so, the prospect of an expensive "wipeout" of an entire crop has remained a continual threat.

In the current study, du Toit and a former post-doctoral researcher in her lab, Sanjaya Gyawali, screened 68 varieties of wild spinach from the region where the plant originated -- Uzbekistan and Tajikistan -- and compared them to 16 cultivated varieties. Researchers from the University of Arkansas also participated in the study.

They found strong resistance to the pathogen in several wild varieties. They then identified the chromosomal locations associated with the most powerful resistance. Those locations -- known as quantitative trait loci -- can be used by breeders to introduce more resistance to Fusarium wilt into commercial lines using marker-assisted selection, a technique that uses DNA markers to select for desirable traits.

The work was funded in part by the Specialty Crop Research Initiative of the U.S. Department of Agriculture's National Institute of Food and Agriculture. The project was also supported by WSU CAHNRS Hatch Projects, and the Alfred Christianson Endowment in Vegetable Seed Science.