General Plant attacker of the month November: fungus in lettuce - Fusarium oxysporum f. sp. lactucae

01/11/2020

Fusarium oxysporum f.sp. lactucae is a fungus that causes a wilting disease in head lettuce (butter lettuce). Enduring spores of the fungus infect the roots from the soil and grow through the vascular bundles into the pit, where growth retardation and/or wilting occurs due to blockage, among other things, (Figure 1).

Spot with wilted heads of lettuce between normal heads
Figure 1. Spot with wilted heads between normal ones

When cutting the crop you can see the yellowish-brown to red discoloration of the vascular tissue in the pit (Figure 2). Since 2015 a new variant ("physio 4") of this fungus was introduced in Flanders; the former varieties had no resistance to this fungus. This variety has spread quite rapidly in the outdoor under glass lettuce sector in almost all of Flanders. The introduction and spread probably occur via contaminated planting material, contaminated soil on tools or shoes, and possibly via the air. Commercially interesting new varieties with sufficient resistance against the new variety are not yet sufficiently available. Alternative lettuce varieties and alternative crops (endive, pak choi, lamb's lettuce, etc.) are either not or less susceptible to this fungus, but it is still being investigated whether they can harbor latent infections and therefore maintain the presence of the fungus in the soil. Current control is mainly based on soil disinfection (e.g. by steaming) but there is a need for more sustainable solutions.

yellow-brown discoloration in lettuce
Figure 2. Yellow-brown discoloration of the vascular tissue when cutting a severely damaged crop - © PSKW

ILVO works together with PSKW, PCG, Inagro and UGent in the VLAIO research project FoSSy to increase our knowledge about this organism and to develop a package of measures and techniques to adequately control the fungus. Concretely, we want to gain insights into the pathways of introduction and spread and into the (combined) effectiveness of the use of partially resistant varieties, new soil disinfection techniques, alternative crops and hygiene measures. ILVO is mainly involved in the development of molecular detection technology that allows to specifically detect the new variant in soil and (alternative) host plants.

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Kurt Heungens

Plant health and plant-pathogenic fungi expert