Interaction of Fusarium Wilt Race 4 with Root-Knot Nematode Increases Disease Severity in Cotton.

Fusarium wilt, caused by f. sp. , is a severe disease of cotton ( spp.). Strains of the wilt pathogen in the United States, such as race 1, require the presence of nematodes such as southern root-knot nematode () to cause appreciable disease. The exception is the race 4 strain of the wilt pathogen, which can attack cotton without concomitant infection by plant-parasitic nematodes and was first identified in California in 2001 and in Texas and New Mexico since 2017. The effects of the interaction between and race 1 or race 4 on wilt severity and nematode reproduction on two cultivars, Acala 44 and FM 966, and a cultivar, Pima S-4, were directly compared in growth chamber assays. All three cultivars were susceptible to Suppression of nematode reproduction by the wilt pathogen was detected only for race 4 on all three cultivars on a per plant basis but not on a per gram root tissue basis. The control, alone, and race 1 alone treatments caused no symptoms. Inoculation with race 1 and caused moderate wilt symptoms in 'Acala 44' and 'FM 966' and mild symptoms in 'Pima S-4'. However, race 4 treatment caused severe wilt in 'Pima S-4' and moderate wilt severity in 'Acala 44' and 'FM 966'. The symptom severity of 'Acala 44' and 'FM 966' further increased in the presence of . Thus, race 4 is not only capable of causing wilt in the absence of but can also interact with the nematode to further increase disease severity. Though control of wilt caused by race 1 can be achieved mainly through breeding for nematode resistance, it will be imperative to incorporate both southern root-knot nematode and race 4 resistance to effectively control the disease should race 4 expand into southern root-knot nematode-infested fields.