Citrus Huanglongbing (HLB)
The prevalent species of HLB bacteria, 'Candidatus Liberibacter asiaticus' (Las), is Gram- negative and has a significantly reduced genome (1.26 Mb). As an obligate and insect-transmitted plant pathogen, Las attacks all citrus species and citrus hybrids in the genus of Citrus, and causes a systemic disease of citrus by residing in the phloem of the plant hosts. The first report of HLB in the United States was in the southern tip of Florida in 2005 and over the decade that followed, HLB has become endemic in Florida. The disease has spread or been introduced into other citrus-producing states, particularly in Texas and California.
Citrus breeding via mutant selection
The recent history in citrus cultivar development, where many new rootstock and scion cultivars have been the result of deliberate breeding efforts, is in stark contrast with the evolution of citrus cultivars over a longer period of time. In the past, new cultivars were developed from the observation and propagation of naturally occurring mutations with some perceived benefit. The history of grapefruit cultivar diversity, arising from the single introduction in 19th century Florida of a very seedy, white fleshed grapefruit clone, and diversification via somatic mutations that followed a path toward near seedlessness and increasing levels of red pigmentation in flesh and rind, has been well documented historically. The diversification of sweet orange cultivars has followed a longer historical path going back a few thousand years, but nonetheless via the same mechanism of naturally occurring somatic mutations. Researchers have exploited the use of irradiation to induce mutants for various traits, and several successful cultivars such as Star Ruby grapefruit and Tango mandarin have resulted. The University of Florida Citrus Research and Education Center (UF-CREC) sweet orange improvement program has released several improved cultivars in recent years by exploitation of stable somaclonal variation. The diversity of Satsuma and Clementine mandarin cultivars available globally likewise have all come from somatic mutations. And, until the second half of the 20th century, all rootstocks used globally were not the results of planned breeding programs. Two rootstocks released in Florida in the 1960s with resistance to burrowing nematode (Radopholus similis) were selected out of seedlings in orchards devastated by "spreading decline" and more recently, citrus seedlings have been shown to contain varying levels of resistance to diseases such as citrus canker. Overall, this illustrates the long history of finding resistance to citrus disease problems through careful observations of trees performing differently from their neighbors.
HLB may be yet another citrus disease problem that can be mitigated using these traditional techniques. In fact, bud sport selection is fast becoming the most promising short-term approach for selecting HLB tolerant/resistant mutants since the millions of infected trees in Florida provide a high probability of obtaining HLB-tolerant/resistant plants from commercial citrus varieties. Initial proof of this concept for HLB tolerance/resistance was uncovered by us on Duncan grapefruit (one of the most susceptible citrus varieties) in the greenhouse at the U.S. Horticultural Research Laboratory in Fort Pierce, FL. This phenomenon was also observed by citrus grower Daniel Scott in his commercial grove in Fort Pierce where over three hundred Ruby Red and Marsh grapefruit volunteer seedlings (ca. 12-25 years old trees) have out- competed their mother plants and have survived many biotic stresses from soil pathogens to nematodes. More importantly, some of these volunteer seedlings appear disease-free even though they have been under high HLB pressure for many years. In this proposal, we will take advantage of these same traditional breeding methods by selecting variant citrus plants or bud sports with HLB tolerance/resistance.