This nematode also is cosmopolitan and probably accounts for more crop loss worldwide than any other species. Despite the importance of M. incognita in agriculture, its pathogenicity on hemp has hardly been examined. Whittle and Drain (1935) were the first to experimentally study this species on hemp, as Heterodera radicicola. Cotton was a good host, but not tobacco or peanut, which identifies this population as M. incognita Race 3 (Taylor and Sasser, 1977). Pogosyan (1960) and Nirula and Kumar (1964) reported M. incognita on C. sativa in Armenia and India, respectively, but did not provide details on hemp type or symptoms. In a study by Kotcon et al. (2018), the fiber cv. ‘Canda’ supported much greater galling and reproduction than did ‘Felina 32’. In recent greenhouse experiments the fiber type ‘Delores’ was a very compatible host for M. incognita Race 3, with numerous galls, egg masses, and high reproduction (up to 1 million eggs/ root system) (Bernard and Chaffin, 2020; Hansen et al., 2020) (Fig. 1). The reproductive factor (RF) was 36‒81. A CBD-dominant cultivar, ‘Wife’, had only a few galls and minor reproduction (Rf = 0.2) (Bernard and Chaffin, 2020). However, the CBD-dominant cultivar ‘Charlotte’s Web’ had a high Rf value (39.6) and other CBD-dominant cultivars ranged from 2.4 to 17.5. Van Biljon (2017) found that most of 10 tested cultivars were tolerant or susceptible to M. incognita Races 2 and 4, but that three of the 10 were resistant to Race 2, including ‘Futura 75’. This cultivar was a good host for M. incognita Race 3 (Bernard and Chaffin, 2020). Clearly there is a wide variation in susceptibility of hemp to M. incognita, both by cultivar and by nematode isolate, which could provide a useful tool for management of this nematode.

Evidence is beginning to appear that M. incognita is a threat to field production of hemp crops. This nematode caused galls on industrial hemp in a North Carolina production field, and although the infection was light it was associated with plant stunting (Thiessen et al., 2020). In Alabama, growth of cultivars ‘Boax’ and ‘Otto2’ was reduced and root systems exhibited galling demonstrated to be caused by M. incognita Race 3. In a subsequent greenhouse test this nematode isolate successfully caused galls and reproduced on ‘Maverick’ hemp, a CBD-dominant cultivar, with an Rf of 2.1 (Lawaju et al., 2021).

The reproduction factor (Rf) is a useful means of estimating the suitability of a host for nematode increase, but comparisons from one study to another must be made cautiously. In the Alabama study of Lawaju et al. (2021), 500-cm 3 pots were used, with an initial inoculum (Pi) of 2,500 eggs; the experiment was run for 45 days and resulted in an Rf of 2.1 on ‘Maverick’. In contrast, Bernard and Chaffin (2020) used 1000-cm 3 pots with a Pi of 5,000 eggs and ran their experiments for 60 days, with an average Rf on ‘Delores’ of 45 and on ‘Wife’ of 0.2. Several other factors prevent a close comparison of the two protocols. The two nematode isolates could have differing virulence on hemp and a priori knowledge of a hemp cultivar’s susceptibility is necessarily unknown, making Rf comparisons difficult. Comparative and cooperative studies are needed to better understand the complexity of root-knot nematode–hemp interactions.