Cultivar Resistance and Alternative Crops to Manage Fusarium Wilt in Lettuce Caused by <i>Fusarium oxysporum</i> f. sp. <i>Lactucae</i> Race 4 and the Emerging Race 4+

ABSTRACT Fusarium wilt, caused by Fusarium oxysporum f. sp. lactucae (Fol), poses a severe threat to butterhead lettuce ( Lactuca sativa L.) production in Europe. New isolates have broken through resistance to Fol race 4 in existing lettuce cultivars. Pathogenicity tests on a differential cultivar set and genotyping revealed that the new isolates are variants of Fol race 4, designated here as race 4+. The new race differs in its profile of Secreted in Xylem ( SIX ) genes: while both races contain SIX9 and SIX14 , only Fol race 4 contains SIX8 . Fusarium spp. adapt quickly, which signals an urgent need to develop an effective integrated pest management (IPM) strategy for Fusarium wilt. Crop rotation can play a key role in this strategy, especially during warmer periods when Fol is most aggressive. Susceptibility to Fol races 4 and 4+ and root colonisation potential were assessed in various lettuce types and alternative vegetable crops. Most lettuce types showed either susceptibility or intermediate resistance, but coral lettuce was resistant. Most alternative crops were asymptomatic when grown in soil infested with both Fol 4 and Fol 4+ and also did not support root colonisation. However, in soil artificially infested with high Fol 4 concentrations, lamb's lettuce ( Valerianella locusta ) was symptomatic. In soil naturally infested with Fol 4 and Fol 4+, lamb's lettuce was asymptomatic, but latent growth of Fol 4 (and potentially Fol 4+) was observed at levels comparable to susceptible butterhead lettuce. The results suggest that effective management of Fusarium wilt during warmer periods could involve the cultivation of resistant alternative crops such as rocket or celery. During colder periods, a rotation of resistant coral lettuce cultivars may further reduce the disease pressure and allow the occasional culture of a susceptible butterhead lettuce cultivar.