Cassava root rots in the humid tropics: fungal niche inversion and early-warning threshold in West Africa

In the humid, rain-soaked landscapes of West Africa, cassava is more than just a crop; it is a lifeline. This starchy root vegetable feeds millions and supports the livelihoods of countless rural families, serving as the foundation for local economies and cultural traditions. However, beneath the soil, a silent battle is constantly being fought. The very conditions that make cassava thrive—warmth and moisture—also invite a host of invisible enemies: soil-borne fungi. These microscopic organisms attack the plant's roots, causing them to rot and decay, often destroying the harvest before it can be dug up. For decades, farmers and scientists have tried to understand how to predict when and where these diseases will strike. The prevailing wisdom suggested that the answer lay in the local terrain: if the ground was wet and poorly drained, the disease would be bad; if the soil was dry and well-drained, the plants would be safe. This simple logic guided how farmers decided when to harvest their crops, with a general rule of thumb suggesting that cassava could be safely left in the ground for about a year before the risk of rot became too high.

But in the rugged, mountainous west of Côte d'Ivoire, where the land rises sharply from low river valleys to high peaks, this simple rule is breaking down. A recent study conducted by researchers from the University of Man and the University of Félix Houphouët-Boigny reveals that the story of cassava disease is far more complex than just wet versus dry soil. By mapping the health of nearly two thousand cassava plants across three distinct regions, the team discovered that the altitude of a field and the specific type of fungus attacking it are locked in a delicate, shifting dance. They found that a difference in temperature as small as one or two degrees Celsius—caused simply by the change in height from the valley floor to the mountain top—can completely flip which fungus is in charge. This discovery challenges the old ways of thinking about crop protection, suggesting that a field on a dry hilltop in a warm valley might be in more danger than a wet field on a cool mountain.

The researchers set out to test this idea by visiting 192 different cassava plots scattered across three administrative regions: Tonkpi, a highland area; Guémon, a hilly zone; and Cavally, a low-lying plain. They walked through these fields during the rainy season, examining 1,920 individual plants that ranged in age from eight to twenty-five months. For each plant, they dug up a few roots to check for signs of decay, scoring the damage on a scale from zero, for healthy roots, to four, for roots that were completely destroyed. They also took samples of the rotting tissue to the lab to identify exactly which fungi were causing the damage. The team was looking for a pattern: how did the severity of the disease change as the plants got older, and how did this change depending on whether the plot was on a summit, a slope, or in a lowland valley?

What they found was a striking reversal of expectations. In the low-lying Cavally region, where the air is warmer, the disease was rampant. Here, the roots were frequently attacked by a fungus called Sclerotium rolfsii, which thrives in heat and moisture. This pathogen was so aggressive that it caused severe rot in over 80 percent of the plants in the lowlands, often destroying the entire root system within just eight months of planting. The damage was so extensive that farmers in these areas could expect to lose nearly half of their potential harvest. In contrast, the Tonkpi region, which sits at a higher altitude and is therefore slightly cooler, told a different story. Even in the low, wet valleys of Tonkpi, the disease was much milder. The dominant fungus here was a different species, Lasiodiplodia theobromae, which is less aggressive in cooler temperatures. In these cooler lowlands, the plants remained healthy for much longer, and the overall damage was significantly lower.

The most surprising discovery, however, was what the researchers call a "topographic risk inversion." Usually, farmers assume that a dry, well-drained hilltop is the safest place to grow cassava, while a wet valley is the most dangerous. Yet, in this study, a dry hilltop in the warm Cavally region was actually more dangerous than a wet slope in the cool Tonkpi region. The warmth of the low-altitude Cavally valley was so intense that it overpowered the protective benefit of the dry soil, allowing the aggressive fungus to thrive even on the summits. Conversely, the cooler air of the high-altitude Tonkpi slopes acted as a shield, keeping the disease in check even when the soil was wet. This means that the location of a farm on a map is not enough to predict its safety; one must also know the temperature of that specific location.

This temperature difference, though seemingly small, drives a complete change in the "fungal niche," or the specific role and dominance of different fungi in the ecosystem. In the hot lowlands, the heat favors the rapid, destructive Sclerotium rolfsii. In the cooler highlands, that same heat is absent, allowing the slower, less destructive Lasiodiplodia theobromae to take over. The study showed that this shift happens with a temperature difference of just one to two degrees Celsius. Because of this, the "safe" time to leave cassava in the ground changes dramatically depending on where you are. In the hot lowlands of Cavally, the disease reaches a critical warning point—where the crop is no longer safe to leave in the soil—after only eight months. Waiting until the traditional twelve-month mark would result in massive losses. However, in the cool highlands of Tonkpi, the plants can remain in the ground safely for much longer, well beyond twenty months, without suffering the same level of decay.

The implications of these findings are profound for how farmers manage their crops. The old, one-size-fits-all rule of harvesting cassava after a year is no longer sufficient. Instead, the researchers propose a new, spatial decision-making guide that takes into account both the age of the plant and the specific topography and altitude of the field. For farmers in the warm lowlands, this means harvesting early, perhaps as soon as eight months, to save their crop from total ruin. For those in the cooler highlands, they have the luxury of waiting longer, allowing the roots to grow larger and the crop to be stored in the ground for as long as needed. This approach offers a practical, chemical-free way to protect the harvest, relying on the natural variations of the landscape to guide timing rather than expensive fungicides.

Ultimately, this study highlights that the health of a crop is not just about the soil it grows in, but about the complex interaction between the land, the air, and the microscopic life that inhabits them. As the climate continues to warm, these delicate balances may shift again, potentially allowing the more destructive fungi to move into areas that were once safe. By understanding these dynamics now, scientists and farmers can better anticipate these changes and adapt their strategies to ensure that cassava remains a reliable source of food and income for the communities that depend on it. The work serves as a reminder that in the humid tropics, the map of risk is not drawn by the rivers and hills alone, but by the invisible lines of temperature that run through them.