LAUSR

Fusarium wilt, caused by Fusarium oxysporum f. sp. vasinfectum, is a severe disease of cotton (Gossypium 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 (Meloidogyne incognita) to cause appreciable disease. The exception is the race 4 strain of the wilt pathogen that can attack cotton without concomitant infection by plant-parasitic nematodes that was first identified in California in 2001, and in Texas and New Mexico since 2017. The effects of the interaction between M. incognita and race 1 or race 4 on wilt severity and nematode reproduction on two G. hirsutum cultivars, Acala 44 and FM 966, and a G. barbadense cultivar, Pima S-4, were directly compared in growth chamber assays. All three cultivars were susceptible to M. incognita. Suppression of nematode reproduction by the wilt pathogen was detected only for race 4 on all three cultivars on per plant base but not on per gram root tissue base. The control, M. incognita alone, and race 1 alone treatments caused no symptoms. Inoculation with race 1 and M. incognita 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 M. incognita. Thus, race 4 is not only capable of causing wilt in the absence of M. incognita, but also can 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.