Evaluation of Huanglongbing-Tolerant Citrus Hybrid Rootstocks for Resistance to <i>Tylenchulus semipenetrans</i>

The response of seven Poncirus trifoliata hybrid rootstocks tolerant to Huanglongbing (HLB) was evaluated for resistance against the citrus nematode (CN) Tylenchulus semipenetrans. HLB, caused by the bacterium ‘ Candidatus Liberibacter asiaticus’, is one of the most destructive diseases of citrus and has become established in citrus production areas across the United States. Trees affected by HLB decline faster when coinfected with soilborne pathogens. Thus, planting citrus rootstocks that can withstand multiple pathogens is the foundation of an integrated pest management (IPM) program in areas infested by HLB. The rootstocks that were screened for CN resistance were the best performing in Florida field trials under HLB pressure. The level of resistance of citrus hybrid rootstocks to T. semipenetrans was assessed in controlled conditions by comparison to standard rootstocks with documented resistance (Swingle citrumelo) and susceptibility (sour orange and Cleopatra mandarin) to CN. Typical symptoms caused by CN were reproduced on susceptible rootstocks (sour orange, Cleopatra mandarin, Ridge, US-802, US-812, US-1283, and US-1284) and included the characteristic development of the dark root phenotype, a decreased root and shoot growth, and the appearance of leaf chlorosis. Three hybrid rootstocks (US-897, US-942, and US-1516) displayed CN resistance levels comparable to Swingle citrumelo and did not express any visible root and shoot symptoms. Our results showed that the resistance mechanism was not caused by chemotaxis but rather by restricting female penetration into the roots and impeding its ability to reproduce and form juveniles. These results provide valuable information to help citrus growers make educated decisions about rootstock selection.