It invades the plants through the roots or through small wounds. It causes yellowing of the leaves at the base, and the roots start to wither and die. There are no therapies for treating it. In recent years, the rises in temperature in autumn and spring, as well as in the soil, have de-seasonalised the disease, making it spread to other more unusual times, other than just in the summer. It is moving from high regions to lower ones. It propagates very quickly; it is produced through physical means and can live in the soil for up to 20 years.
It is called Fusarium oxisporumf.sp. lactucae. It is a fungus in the soil of which four strains have already been described; two of these are present in Spain, Fol:1 and Fol:4. And it is on the way to being one of the main problems for lettuces. “The situation is very serious. There are no effective soil disinfection methods, and it is affecting farms in Lorca, Cartagena, Almendricos… there are already occasional problems regarding fusarium in the winter,” comments Andrés Román Re, a Lettuce Crop Specialist at Rijk Zwaan.
The seed company is working on developing varieties with high resistance to fusarium that are encompassed under the Fusarium Defence umbrella. At present, it has three commercial varieties available of iceberg: Laurinas RZ (launched commercially two campaigns ago), and two new developments: Limanas RZ and Antanas RZ. Rijk Zwaan is using these to open a transplant window ranging from February to October.
They all have a good base shape without ribs and they behave well against tip burn and sprouting, in addition to high resistance to Fol:1 and intermediate resistance to Fol:4
Laurinas RZ is early, it grows fruit with important weight per head and good quality. Limanas RZ offers an extensive transplant window that maintains the quality and uniform calibre. And Antanas RZ’s excellent shape without ribbing stands out, with great tolerance to sprouting and tip burn.
In a pre-commercial phase, the company has materials with resistances in the 12 typologies they work with. “Although at present Fusarium affects iceberg lettuces most, in other types we are seeing that, in spite of having intrinsic genetic resistance, when the pressure of the fungus is very high, they are affected.”
