qPCR-based quantification reveals early biomass differences of Alternaria japonica and Sclerotinia sclerotiorum in two field pennycress accessions

In the American Midwest, farmers are looking for a new partner to join the familiar rhythm of corn and soybean fields. They are turning their attention to a humble, often overlooked plant called field pennycress. This small, hardy weed, which has grown wild in the region for centuries, is now being domesticated as a cover crop—a living blanket planted during the winter to protect the soil, prevent erosion, and capture carbon before the main crops arrive. Its seeds hold promise for producing clean biofuels and high-protein animal feed, offering a way to farm more intensively without using extra land. However, as scientists work to turn this wild weed into a reliable crop, they face a familiar enemy: disease. Just like the vegetables and grains it grows alongside, pennycress is vulnerable to fungal infections that can rot its leaves, stems, and seeds. To save this potential crop, researchers must first understand exactly how these invisible invaders attack and how different varieties of the plant defend themselves.

The challenge lies in the invisibility of the early stages of infection. Fungi often begin their work long before a farmer or a scientist can see a spot or a wilt on a leaf. Traditional methods of checking for disease rely on the human eye, waiting for brown spots to appear or for white, fuzzy mold to spread. But by the time these signs are visible, the fungus has already established a significant presence inside the plant tissue. This delay makes it difficult to tell which varieties of pennycress are naturally strong and which are weak, a crucial distinction for breeders trying to develop a resilient crop. To solve this, a team of researchers at the Donald Danforth Plant Science Center and CoverCress Inc. developed a new way to measure the enemy's presence, not by looking at the damage it causes, but by counting the fungus itself while it is still hidden.

The researchers focused on two specific fungal enemies that plague brassica crops, the family to which pennycress belongs. One is Alternaria japonica, which causes black spots on leaves and pods. The other is Sclerotinia sclerotiorum, responsible for a disease known as white mold, which turns plant tissue into a soft, white rot. The team collected samples of these fungi from pennycress plants growing in fields in Illinois. They grew the fungi in the lab to confirm their identity, using genetic sequencing to ensure they were working with the correct species. Once verified, they set out to test how these fungi interacted with two different types of pennycress: a variety known as MN106, which showed signs of resistance, and another called 2032, which appeared much more susceptible to disease.

To see how the plants reacted, the scientists performed a series of controlled experiments. They took leaves and pods from both types of pennycress and placed them in humid containers. They then introduced the fungi to these plant parts, either by placing a small piece of fungal growth directly onto the tissue or by spraying the plant parts with a suspension of fungal fragments. In some tests, they also inoculated the stems of the plants. Over several days, they watched closely for signs of infection. When they looked with the naked eye, the results were often ambiguous. On the leaves, the resistant and susceptible plants looked surprisingly similar in the early days of infection. The only way to see a clear difference was to stain the leaves with a blue dye that highlights dead plant cells, revealing that the susceptible plants were indeed suffering more damage, but this method was still somewhat imprecise and difficult to measure accurately.

The breakthrough came when the researchers turned to a molecular tool called quantitative polymerase chain reaction, or qPCR. Instead of waiting for the fungus to kill plant cells and create visible spots, this technique allowed them to detect the genetic material of the fungus directly from the plant tissue. They extracted DNA from small discs of leaf and pod tissue and used specific genetic primers—molecular tags designed to find only the DNA of the target fungi. By comparing the amount of fungal DNA to the amount of plant DNA, they could calculate exactly how much fungus was growing inside the plant, even when the plant looked perfectly healthy on the outside.

The results of this molecular counting were revealing. The qPCR assay showed that the susceptible variety, 2032, was accumulating significantly more fungal biomass than the resistant variety, MN106, much earlier than the eye could detect. For the black spot fungus, the difference in fungal growth was measurable within three days of infection, long before the lesions were large enough to be easily distinguished by sight. For the white mold fungus, the difference was even more pronounced; the molecular test could detect significantly more fungus on the susceptible plants just one day after infection, whereas visual signs of the disease did not appear until two or three days later. This sensitivity allowed the researchers to see the true nature of the plant's defense, proving that the resistant variety was successfully limiting the growth of the fungus from the very beginning.

The study also highlighted how the fungi behaved differently on different parts of the plant. While both fungi attacked leaves and pods, the white mold fungus was also able to infect the stems, a detail that was confirmed through the molecular tests. The researchers found that the susceptible plants not only allowed the fungi to grow faster but also provided a more hospitable environment for the fungi to produce their resting bodies, which can survive in the soil for years. The molecular data provided a clear, quantitative picture of these interactions, showing that the difference between a resistant and a susceptible plant is not just about how bad the disease looks at the end, but about how effectively the plant stops the fungus from taking hold in the first place.

This work establishes a new standard for evaluating pennycress as a crop. By using these DNA-based tools, breeders can now screen thousands of plant varieties quickly and accurately, identifying those with the strongest natural defenses without waiting for the disease to fully develop. The researchers confirmed that the susceptible variety, 2032, carries a specific genetic trait that makes it vulnerable, a finding that opens the door to breeding that weakness out of future crops. Ultimately, this research provides the essential toolkit needed to protect pennycress from fungal diseases, ensuring that this promising new crop can fulfill its potential as a sustainable solution for American agriculture. The ability to see the invisible battle between plant and fungus gives scientists the power to win the war for a healthier, more resilient food system.