"Improving spinach starts with changing the way we grow it"

By Esther van den Bergh

Yellow patches and fuzzy growth on spinach are often signs of downy mildew, the crop's most common and costly disease. It is caused by a fungus-like pathogen (Peronospora effusa). Resistant spinach varieties, created to recognize the pathogen and block it, are used to manage the disease. Nevertheless, the pathogen has successfully overcome spinach’s defense more than 20 times. PhD candidate Melanie Mendel investigated how downy mildew infects spinach. According to her, making spinach stronger also means changing how we grow it. Mendel defends her dissertation today.

The problem with how we grow spinach

Worldwide, ten to sixteen percent of global crop production is lost to pests and pathogens. This threat is expected to grow as high plant densities, intensive irrigation, and monoculture farming create increasingly favorable conditions for disease. Spinach is no exception, and after five years of studying its main enemy, Mendel sees the problem as bigger than the pathogen alone.

The current way spinach is grown creates an ideal environment for downy mildew.

"We should not only focus on simply building a stronger spinach plant, but it is essential to create a less favorable environment for the pathogen," she argues. "The current way spinach is grown creates an ideal environment for downy mildew. Plants are grown close to each other, and the spinach plants are often genetically very similar." To make spinach less prone to downy mildew, Mendel suggests exploring alternative cultivation strategies that make it more difficult for the pathogen to spread, such as growing different spinach varieties together or combining spinach with other crops.

Stronger spinach, smarter pathogen

But why is a stronger plant alone not enough? "It seems that the more we invest in making a resistant spinach, the more the downy mildew fights back," Mendel says. Spinach resistance works like an alarm system: resistant plants recognize the pathogen and block it. The pathogen, however, behaves like a burglar that disables the alarm. It uses small molecules, or 'weapons', that either keep it hidden from the plant's alarm or switch it off. Previous research mostly looked at the defense of spinach, but Mendel decided to investigate how the pathogen infects the plant, as little is known about this process. This was not easy: the downy mildew can only survive on spinach, and both the pathogen and the spinach proved difficult to study.

Zooming in on a microscopic pathogen

The DNA of downy mildew is like the burglar's toolbox: it contains many possible 'weapons', but it does not reveal which ones are used during the break-in. To find out, Mendel looked at which 'weapons' the pathogen activates during the early stages of infection. But it was hard to tell which leaves were successfully infected: "Downy mildew is microscopically small, so you cannot see it until the plant is heavily infected," Mendel explains.

Technician Joyce Elberse came up with a clever solution. She first infected the leaves and then removed a small piece from each leaf. She then tracked whether the remaining part of the leaf developed clear signs of infection. Only the pieces from leaves that turned out to be infected were studied further. Thanks to this trick, the researchers ended up with a shortlist of 'weapons' the pathogen uses.

Researchers should also invest effort towards developing experimental tools for plants and crops in which existing tools do not work well.

Putting the 'weapons' to the test

The next step was to find out what each 'weapon' does to spinach's alarm system. However, getting the 'weapons' one by one into a spinach leaf proved rather difficult. Various delivery methods that work for other pathogens or in other plant species did not always work in spinach. "As a first test, we brought the 'weapons' of a bacterial tomato pathogen into spinach, which worked well," Mendel says. "Unfortunately, we had no time to adapt the same system for the downy mildew 'weapons', so that remains a task for future research."

The challenge of studying spinach

Looking back on her research, Mendel reflects on the challenges of studying spinach in the laboratory. "Nowadays, we have a lot of fancy molecular tools in plant biology, but applying them to spinach proved rather difficult," she says. She sees this as a bottleneck that affects several other crops as well. "Most plant research focuses on a few model plants, which have many tools available. Researchers should also invest effort towards developing experimental tools for plants and crops in which existing tools do not work well."

But growing healthy spinach may also require looking beyond the laboratory. "Right now, spinach is put under a lot of pressure by its growing environment." When the same burglar keeps breaking into your house, you can keep installing new alarms, or you can look at the neighborhood the house is in: a street where every house has the same lock. "Improving spinach starts with changing the way we grow it," Mendel concludes.