Photo above: Fusarium expressing itself in lettuce. The browning inside the root is as you find it for many species
Fusarium oxysporum f. sp. lactucae (Fol) – is a major threat in lettuce. The fungus can damage the lettuce via the root system in both protected and open field crops, resulting in a loss of yield. Four official variants of Fusarium in lettuce appear in different regions throughout the world; Fol: 1-4. Nunhems has found resistance to Fol: 1 and Fol: 4. This discovery has resulted in an assortment of resistant varieties which are available under the Fusarium Defense label.
Resistance levels in Fusarium Defense
Lettuce varieties with Fusarium Defense have proven resistance against Fusarium.
There are different levels of resistance
Susceptible varieties are unable to restrict the growth and/or development of Fusarium. This leads to major losses in yield and allows the further spread of the fungi.
Varieties with Intermediate Resistance (IR) restrict the growth and/or development of Fusarium to some extent. These varieties may, however, exhibit symptoms and/or damage under normal disease pressure.
Varieties with High Resistance (HR) significantly restrict the growth and/or development of Fusarium under normal disease pressure. However, HR varieties might show symptoms under extreme conditions because high resistance is not the same as immunity. Immunity is when a plant root system is not subject to fungal infection. There are no lettuce varieties with immunity to Fusarium.
You will find the strength of the resistance, high resistance (HR) or intermediate resistance (IR) clearly mentioned in the variety descriptions.
Fusarium in susceptible lettuce varieties
Fusarium wilt in lettuce is caused by the soil-borne pathogen Fusarium oxysporum f. sp. lactucae (Fol). The fungus attacks the plant's vascular system, blocking the transport of water and nutrients. This internal disruption triggers visible symptoms such as yellowing, stunting, and wilting, which can ultimately lead to total plant collapse. Externally, infected lettuce displays yellowing and wilting leaves. Internally, a cross-section of the upper roots and crown tissues typically reveals reddish-brown necrosis and subsequent decay, often accompanied by necrotic leaf veins. Because another pathogen, Pythium tracheiphilum, produces nearly identical early symptoms, laboratory testing is essential for an accurate diagnosis.
