1a. Objectives (from AD-416):
1. To screen citrus species and related species in the USA and China for resistance to the Asian citrus psyllid. 2. To identify plant traits which impart resistance to the psyllid, traits that could be used to develop commercial citrus varieties with psyllid resistance for the purpose of reducing incidence of huanglongbing disease.
1b. Approach (from AD-416):
Subject candidate citrus species and related species in free-choice infestation studies and determine species that are avoided by the psyllid for food and reproduction. Evaluate these apparently resistant plant species in no-choice studies to confirm they are avoided by the psyllid. Plant resistant species in the field and monitor them over time to determine if they develop huanglongbing.
3. Progress Report:
This project is related to inhouse project objective 3c: Investigate basic biology and ecology of the Asian citrus psyllid and its natural enemies in Florida citrus. The objective of this project was to discover citrus germplasm with resistance to the Asian citrus psyllid. Research during the project showed that germplasm in the following major groups of Rutaceae did not have any significant resistance to infestations of the Asian citrus psyllid (ACP): sweet oranges, citrons, pomelos, limes, lemons, sour oranges, papedas, mandarins and hybrids among these groups including grapefruit. However, most (but not all) accessions of Poncirus trifoliata as well as a number of its hybrids (×Citroncirus) were shown to have natural resistance to ACP. Two types of resistance have been identified. One of these resistant types (antixenosis) greatly reduces infestation levels of the psyllid, a resistance trait that may be related to volatiles used by the psyllid to find and infest plants or the presence of a volatile that repels the psyllid. The other resistant type (antibiosis) results in reduced longevity of psyllids, possibly related to the presence of toxic secondary plant metabolites. P. trifoliata can be crossed with Citrus species using traditional breeding methods, thus it may be possible to identify traits conferring resistance and to breed these traits directly into Citrus using traditional breeding or other methods. Volatiles imparting antixenosis may have value for managing or monitoring ACP. Clear differences exist in the volatile profiles of susceptible and resistant plants. A new project is being planned to analyze the amino acids, sugars, flavenoids, carrotenoids, isoprenoids, and sterols in phloem contents of germplasm that differs in psyllid susceptibility. This will provide information about the underlying reasons why some germplasm is psyllid-resistant and help identify traits that can be used in citrus breeding programs to confer psyllid resistance. Poncirus trifoliata accessions (CRC code at the National Clonal Germplasm Repository, University of California, Riverside) identified as having antixenotic resistance to ACP: 838, 1498, 1717, 2552, 2554, 2861, 2862, 3151, 3206, 3207, 3209, 3210, 3212, 3213, 3215, 3217, 3218, 3219, 3330A, 3330B, 3338, 3411, 3412, 3484, 3485, 3486, 3547, 3548, 3549, 3571, 3586, 3588, 3882, 3888, 3938, 3939, 4006, 4007, 4009, 4017, and 4138 (48 accessions evaluated). ×Citroncirus (trifoliate hybrids) accessions with antixenosis resistance: 275, 1438, 1447, 1448, 1459, 1463, 2618, 3205, 3348, 3771, and 3881 (34 accessions evaluated). Poncirus trifoliata and ×Citroncirus accessions with antibiosis resistance: 276, 275, 1438, 1447, 1448, 1463, 3330B, 3348, 3881, 3889, and 3969 (17 accessions evaluated).