By Ute Albrecht, Anas Fadli and Chandrika Ramadugu
Most commercially available citrus scion cultivars are highly attractive to the Asian citrus psyllid and susceptible to HLB. There are some citrus species, however, that are HLB tolerant or resistant. This may be because they are less attractive to the psyllids and/or because they restrict pathogen proliferation and therefore HLB disease development.
WHAT FINGER LIMES OFFER
Recently, there has been much excitement about finger limes (Microcitrus australasica), which are native to Australia and produce distinct, finger-shaped fruits. Although often still referred to as Microcitrus, these species are now recognized as belonging to the Citrus genus. The fruits of finger limes are not suitable for juicing but are popular in the gourmet food industry for flavoring and as a garnish (“citrus caviar”).
Different finger limes with a variety of visual and sensory characteristics are available in Florida to grow as specialty crops. Some of them produce high amounts of antioxidants, vitamin C, citric acid and sugars, thus offering the potential of valuable nutrient sources if introduced as alternative commercial crops.
Aside from their unique fruit, flavor and nutritional benefits, finger limes have gained much attention because they are tolerant or even resistant to HLB (See citrusindustry.net/2021/03/05/could-finger-limes-be-floridas-hero/). Their higher tolerance compared to standard juice cultivars may be associated with the production of special leaf volatile compounds that repel the disease vector and/or specific phytochemicals that are toxic to the HLB pathogen or enhance plant immunity.
Recently, researchers in California discovered a peptide with dual functionality (antimicrobial and immunity activating). Whether this peptide is effective against HLB in a disease-endemic environment such as Florida and how to apply it on a commercial scale is still being investigated. Another Microcitrus species that has demonstrated tolerance to HLB during six years of field testing is the large leaf Australian lime (Microcitrus inodora),which has similar characteristics to the finger lime.
WORKING TOWARD HLB RESISTANCE
Ultimately, the financially and environmentally most sustainable solution to HLB is developing genetic resistance or tolerance.
The breeding program at University of California, Riverside (UCR) has created novel citrus scion hybrids that incorporate the HLB resistant traits from Australian limes with the fruiting characteristics of citrus to produce fruit that is suitable for use in juice production and/or the fresh fruit market. These breeding efforts are complementary to the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) and U.S. Department of Agriculture (USDA) Agricultural Research Service breeding programs in Florida, which use other citrus relatives such as trifoliate orange (Poncirus trifoliata) to develop disease-resistant cultivars with orange-like fruits.
The first-generation (F1) of novel Microcitrus hybrids have undergone extensive testing regarding their resistance to HLB in a contained greenhouse environment. The most resistant candidates were further evaluated to select those that produce fruit with the most appealing appearance and most favorable juice quality using standard fruit-quality analysis methods and numerous taste panels. Metabolomics analyses to enhance flavor and develop databases for flavor prediction were also included in the selection process. Four of the most promising candidates that have undergone this rigorous selection process are now being field-tested in a collaborative effort between California, Texas and Florida, funded under a USDA National Institute of Food and Agriculture (NIFA) Emergency Citrus Disease Research and Extension Program (ECDRE) grant.
Researchers in Florida initiated experimentation in 2021. With the support of a commercial citrus nursery, the first set of these rigorously selected Microcitrus hybrids was produced on two rootstocks, Carrizo and US-942. Trees were planted in fall 2021 in two different locations. One location is near Fort Meade, a typical Central Florida Ridge site, and the other location is near Arcadia, a typical flatwoods site. Valencia and Lisbon lemon trees were included in each trial for comparison. The lemon scion was included because it is anticipated that some of these first-generation hybrids will produce fruit with more lemon-like characteristics. The F1 hybrids are also utilized as parents for breeding second-generation (F2) hybrids to further improve fruit-quality traits while maintaining HLB-resistance traits.
The trees in all field trials are monitored regularly for growth, health, psyllid colonization and leaf bacterial titers. Despite a slight setback in January/February when much of the flush was killed by a freeze, trees recovered quickly and are now growing rapidly. One of the hybrids, Wilking mandarin × Microcitrus australasica, was not affected and has been growing profusely on both rootstocks. Moreover, so far it seems that psyllids do not like to feed and reproduce on it. This hybrid is also the one expected to produce fruit with the most mandarin/orange-like characteristics.
Researchers are continuing to monitor the disease response and horticultural performance in the subtropical, HLB-endemic environment of Florida. Colleagues in Texas and California have established field trials with the same scion cultivars but with rootstocks representative of their respective industries. Sour orange and Bitters are used in Texas, and Carrizo and Alemow in California.
It will be interesting to compare the horticultural characteristics of these hybrids in the three different environments. It is anticipated that the first fruit for quality assessment will be produced no later than year two. In the meantime, the selection of improved F2 hybrids through a combination of standard and advanced breeding tools continues, hopefully, to provide the ultimate solution for HLB: genetic resistance.
Ute Albrecht (firstname.lastname@example.org) is an assistant professor and Anas Fadli is a postdoctoral research associate at the UF/IFAS Southwest Florida Research and Education Center in Immokalee.
Chandrika Ramadugu is a project scientist at UCR.
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