“The power of Px Defense lies in its combination of resistant sources”

How Px growth impacts the plant

As a phytopathologist – in other words, a specialist in plant diseases – Denis has unrivalled knowledge of just how severely powdery mildew can affect melons. Denis: “When spores of Podosphaera xanthii (Px) form colonies on the leaves, they are visible as white spots. The fungus spreads by absorbing nutrients from the plant through its mycelium. This directly affects plant development, resulting in substantial reductions in fruit yield and quality.”

Carried over long distances

The harmful effects of powdery mildew are a worldwide problem. “Px affects not only melon crops, but also watermelons, courgettes and cucumbers. The powdery mildew spores are usually carried by the wind, sometimes over distances of many kilometres. This allows the fungus to spread rapidly, especially in areas of dense cultivation,” explains Denis.

Official identification of races 6 and 7

What makes Px particularly difficult to control is that new harmful variants continue to emerge. For example, the International Seed Federation (ISF) officially described the latest races – 6 and 7 – in 2023. Denis: “Most varieties were resistant against races 1, 2, 3, 3.5 and 5. However, a fungus is constantly evolving, mainly through natural mutation. If a mutated variant survives on a resistant variety somewhere, it can multiply rapidly. That’s how races 6 and 7 developed.”

Testing sources of resistance

The search for new sources of resistance against races 6 and 7 was already in full swing at that time. “As phytopathologists and pre-breeders, we’re always on the lookout for potential sources of resistance. We test how resistant each source is, and where exactly in the DNA the relevant trait is located,” continues Denis. Identifying the location is what pre-breeder Ingrid does. For races 6 and 7, she investigated which specific genes were involved in the resistance – the source of which had been found in a wild melon. “I identified which genes made the difference,” she explains.

Developing a marker

Ingrid then created what is known as a genetic marker, which acts as a kind of flag within the DNA sequence. “A molecular marker helps breeders introduce the desirable trait more quickly,” says Ingrid. “We can test which offspring have the resistance and which don’t, based on their marker genetic profiles. This improves the efficiency of the breeding process.”

Powerful resistance

All that research has led to two new Galuna varieties (34 GL0186 RZ and 34 GL0188 RZ) with Px Defense. A label which is only given when including intermediate resistance against races 6 and 7. These varieties for late summer crops are comparable to the variety Galuna Hiron RZ. They will soon be followed by varieties for other cultivation seasons and melon types.

Ingrid is delighted with this result. “We can now help melon growers control powdery mildew in a more natural way. It is a powerful resistance, partly because it’s combined with resistances against other races. I’ve just seen the field trials, where susceptible varieties and the new resistant varieties are being grown side by side. The striking contrast speaks for itself.”

Preventing spores from spreading

Although the results are very impressive, the fungus can still be present in a mild form. That’s why vigilance remains necessary, according to Denis: “As much as possible, it’s essential to prevent powdery mildew spores from spreading. For example, growers should destroy crop residues after harvesting, as Px can survive on them. The fungus must be given as little opportunity as possible to adapt. We continue to monitor powdery mildew closely, in active collaboration with growers, so that we can respond quickly to new developments. Our work is never done.”