First report of Fusarium nygamai causing pigeon pea wilt across Uttar Pradesh

In the fields of Uttar Pradesh, India, a silent struggle plays out between a vital food crop and an invisible enemy. The crop is pigeonpea, a hardy legume that serves as a crucial source of protein and nutrition for millions of people across Asia and Africa. It is a resilient plant, yet it faces a devastating threat from a disease known as wilt. This condition is caused by fungi that live in the soil and invade the plant's internal plumbing. Once inside, these microscopic invaders clog the water-conducting tubes, causing the leaves to turn yellow, the stems to darken, and the entire plant to collapse. For decades, farmers and scientists have known that a specific fungus, long identified as Fusarium udum, is the primary culprit behind these losses. However, the full story of what is happening underground has remained incomplete, leaving growers without a complete picture of the threat they face.

A team of researchers from universities and research institutes across India set out to uncover the true identity of the fungi attacking pigeonpea crops in Uttar Pradesh. They traveled to sixteen different districts, collecting samples from sick plants that showed the classic signs of wilting. In their laboratories, they carefully extracted the fungi from the roots and stems of these dying plants. From the soil and the infected tissue, they isolated ninety-seven distinct strains of the fungus. To understand which of these were the most dangerous, the scientists looked at the tools the fungi used to break down the plant. They measured the activity of two specific enzymes, which act like molecular scissors, cutting through the tough cell walls of the plant to allow the fungus to enter and feed. They found that while many strains were present, only five showed a particularly high ability to produce these destructive enzymes.

The researchers then put these five aggressive strains to the test. They grew pigeonpea seeds in controlled environments, some of which were known to be resistant to disease and others that were highly sensitive. When they introduced the fungi, the results were stark. One strain, which the team named Rj, proved to be the most destructive, causing the highest rate of infection and the most severe wilting in both the resistant and sensitive plants. To identify exactly what this deadly strain was, the scientists turned to genetic analysis. They examined the DNA of the fungi, comparing specific genetic sequences against a global database. The results revealed a surprising discovery: the most virulent strain, Rj, was not the expected Fusarium udum. Instead, it belonged to a species called Fusarium nygamai. This was the first time this particular fungus had ever been identified as a cause of wilt in pigeonpea. While Fusarium udum was still present and harmful, Fusarium nygamai emerged as a new and potent threat, capable of causing the highest disease severity index among the tested strains.

The study went further to see what happens when these two different fungi attack a plant at the same time. The researchers inoculated pigeonpea plants with Fusarium udum alone, Fusarium nygamai alone, and a combination of both. The plants infected with the combination of the two fungi suffered the worst fate. The disease spread faster and the plants wilted more severely than when either fungus acted alone. This suggests that the two pathogens work together to make the disease much worse, a phenomenon known as a synergistic effect. Inside the infected plants, the scientists observed the physical damage caused by this attack. The internal tissues of the stems turned brown and collapsed, and the water-conducting vessels were blocked by fungal strands. This blockage prevented water from reaching the leaves, leading to the rapid death of the plant.

The impact of this infection was also visible in the plant's chemistry. The infected plants showed a sharp drop in chlorophyll, the green pigment essential for making food, causing the leaves to fade and die. At the same time, the plants produced higher levels of proline, a compound they generate in response to stress, and increased levels of harmful molecules called reactive oxygen species, which indicate cellular damage. The resistant varieties of pigeonpea showed some ability to fight back, but even they suffered significant growth reductions in height, root length, and seed production when attacked by these fungi. The findings confirm that the landscape of pigeonpea disease is more complex than previously thought. It is not just one fungus causing the problem, but a mix of species, including the newly identified Fusarium nygamai, which can act alone or in combination with others to devastate crops. This discovery provides farmers and scientists with a clearer understanding of the enemy, offering a necessary foundation for developing better strategies to protect this essential food source.