Preharvest Fungicide Timing Improves Postharvest Seed Quality and Storage Stability by Suppressing Grain Mould in Sorghum (Sorghum bicolor (L.) Moench)

In the humid, rain-soaked regions of West Africa, sorghum is more than just a crop; it is a lifeline. This hardy grain feeds millions, provides fodder for livestock, and serves as a foundation for local economies where other cereals struggle to survive the heat and drought. However, the very conditions that allow sorghum to thrive—warmth and moisture—also invite a silent enemy: grain mould. This is not a single disease but a complex invasion by microscopic fungi that colonize the developing seeds while they are still on the stalk. When these fungi take hold, they do more than just discolor the grain; they eat away at the seed's internal life, turning what should be a viable plant for the next season into a dead, shriveled husk. For farmers who rely on saving their own seeds or storing grain for months until the next harvest, this loss is catastrophic. The challenge has long been how to stop these fungi before they destroy the harvest, but a critical question remained unanswered: does when a farmer sprays a protective chemical matter just as much as what they spray?

Researchers at the Council for Scientific and Industrial Research in Ghana set out to solve this puzzle by watching how the timing of a simple spray affects the grain long after the harvest is in. They worked in the Upper East Region, a landscape defined by a single, intense rainy season that creates the perfect breeding ground for fungal pathogens. In their fields, they planted two different types of sorghum: one is a well-known, reliable variety called Kapaala, and the other is a newer, high-yielding experimental line named ADV23012. To test the timing, they divided their plots into groups and applied two different fungicides—one organic and one synthetic—at three distinct moments in the plant's life: when the flowers first opened, when the seeds were forming a milky liquid inside, and when the seeds were hardening into a dough-like state. They also left some plots completely untreated to see how the grain would fare without any help.

The results revealed a clear and decisive pattern. The moment the sorghum flowers is the most critical window for protection. When the researchers applied fungicides at this early stage, the grain remained remarkably clean. In the best-case scenario, using the organic spray on the Kapaala variety during flowering, the grain mould affected less than two percent of the panicles, and the severity of the infection was barely noticeable. In stark contrast, the untreated plots were ravaged, with nearly half of the grain showing visible mould and severe discoloration. The study showed that waiting until the seeds were already forming or hardening to apply the spray was far less effective. By the time the grain reached the dough stage, the fungi had often already established a foothold, and the chemical sprays could not undo the damage that had begun weeks earlier.

This difference in field performance had a profound ripple effect that lasted long after the harvest. The researchers stored the seeds in bags under normal, warm laboratory conditions for six months, mimicking how a farmer might keep grain for the dry season. The seeds that had been protected at the flowering stage remained alive and vigorous. After half a year, the Kapaala seeds treated at flowering still had an 83 percent germination rate, meaning they could still grow into new plants. The untreated seeds, however, had largely died, with a germination rate dropping to just 26 percent. The seeds that had been sprayed late in the season fell somewhere in between, but they still suffered more than those protected early. The study confirmed that the fungi which attack the grain in the field do not simply disappear at harvest; they persist inside the seed, slowly eating away at its vitality during storage.

Beyond just keeping the seeds alive, the timing of the spray influenced how well the grain held up against other threats. The seeds from the early-sprayed plots lost less moisture and suffered far less damage from insects and bugs that typically infest stored grain. The untreated seeds, already weakened by fungal infection, became easy targets for pests and dried out much faster. When the researchers looked inside the seeds to identify the invaders, they found that two types of fungi, Fusarium and Curvularia, were the most common culprits in the damaged grain. These fungi were significantly more abundant in the untreated and late-sprayed seeds, confirming that the early intervention had successfully blocked the primary invasion.

The findings offer a straightforward lesson for farmers in tropical climates: the battle for grain quality is won before the harvest is even gathered. By synchronizing the application of protective sprays with the flowering stage, farmers can prevent the initial fungal infection that leads to a cascade of problems later. This approach does not just save the current harvest; it preserves the seed's ability to grow in the future and keeps the grain stable in storage. While the specific results varied slightly between the two sorghum varieties, the rule held true across the board: early protection is superior to late intervention. For communities that depend on the resilience of their crops, this simple adjustment in timing transforms a routine agricultural practice into a powerful tool for securing food and seed supplies against the relentless pressure of humid, tropical weather.