Influence of Climatic Factors on the Development of Cashew Powdery Mildew

In the sun-drenched orchards of Northeastern Brazil, the cashew tree is a vital source of livelihood, producing nuts and fruit that feed local industries and reach global markets. Yet, these trees face a persistent threat from a common fungal disease known as powdery mildew. Unlike many other plant diseases that thrive in wet, soggy conditions, this particular fungus is surprisingly comfortable in drier air. It targets the youngest, most tender parts of the tree—the new leaves, the flower clusters, and the developing fruit—often causing them to wither and drop before they can mature. For farmers, this means a direct loss of income. Understanding exactly how this fungus grows and spreads is the first step toward protecting the crop, but the specific environmental triggers that allow it to flourish in the tropical climate have remained somewhat unclear.

A team of researchers set out to map the precise conditions that allow the cashew powdery mildew fungus to survive and reproduce. They began by testing whether the moisture level in the air, known as relative humidity, was the key factor driving the fungus's ability to infect new tissue. They collected fungal spores from four different parts of the cashew tree: young leaves, flower clusters, immature nuts, and mature nuts. These spores were then exposed to air ranging from completely dry to fully saturated with moisture. Surprisingly, the amount of water in the air did not matter much; the spores could germinate and form the tiny structures needed to attack the plant across almost all humidity levels tested. What mattered far more was where the spores came from. Spores harvested from the flower clusters were the most vigorous and successful at starting an infection, while those from mature nuts were the least effective. This suggests that the fungus is particularly adapted to colonizing the fresh, new growth of the tree.

With the source of the infection identified, the researchers turned their attention to temperature and light, two factors that change constantly in an orchard. They found that a temperature of 23 degrees Celsius was ideal for the fungus to grow and develop its infection structures. When it came to light, the color of the spectrum played a significant role. Spores exposed to green and red light grew well, but blue light acted as a barrier, slowing down their development. The length of the day, or photoperiod, also influenced the fungus. A cycle of 12 hours of light followed by 12 hours of darkness allowed the fungus to reach its peak performance for germination and infection. However, when the researchers looked at how much the fungus reproduced—how many new spores it created—the results shifted. The highest number of new spores was produced when the plants were kept under a 17-hour day at 23 degrees Celsius, while continuous darkness or continuous light resulted in very little reproduction.

The study also explored how light intensity affects the disease in a more realistic setting, moving from controlled laboratory chambers to field conditions where plants are exposed to varying amounts of sunlight. The researchers grew young cashew seedlings and placed them in cages that blocked different amounts of light, ranging from total darkness to full, unfiltered sunlight. They observed that the disease became much more severe when the plants were shaded. In the darkest conditions, the fungus spread aggressively, covering large areas of the leaves and producing massive amounts of spores. Conversely, when the plants were exposed to full sunlight, the disease progression was significantly suppressed, and the fungus produced very few spores. This indicates that bright, direct light acts as a natural defense mechanism, inhibiting the fungus's ability to reproduce and spread.

These findings paint a clear picture of the cashew powdery mildew fungus as a creature that thrives in the shade and on young plant tissue. It does not rely on high humidity to start an infection, but it is highly sensitive to the quality and quantity of light it receives. The research suggests that the fungus is most dangerous when it can hide in the cooler, shaded parts of a dense canopy, away from the harshness of direct sun. By understanding these specific preferences, farmers can make informed decisions about how to arrange their trees and manage their orchards. Keeping the canopy open to allow more sunlight to reach the lower branches could naturally reduce the severity of the disease, offering a simple, ecological way to protect the harvest without relying solely on chemical treatments.