20237002941315

Citrus Huanglongbing (HLB) is considered one of the most difficult challenges for several citrus industries worldwide. The financial impacts of HLB are staggering and may result in citrus being replaced by other fruit trees and alternative crops.

In Florida, where the disease has been well established for over ten years, the yield of citrus trees has been reduced to yields reflecting the Florida industry 70-100 years ago. The reduced yield is reflected in the high price of orange juice for the morning breakfast. The disease threatens the citrus crop in other citrus-growing states like California, Texas, and Arizona.

Finding long-term, sustainable solutions is imperative for continued citriculture.

Citrus HLB is a complex disease associated with a non-cultivable bacterial pathogen, spread by a psyllid insect vector that is difficult to control, and the disease has no known cure; infected trees decline, produce unmarketable fruit, and die in a few years. Management of the disease by preventing spread, control of the insect vector, and application of anti-microbial substances are good practices - but of limited value for a tree crop that can live for several decades.

Despite intensive research for the past fifteen years, tangible solutions to control HLB are not yet available. Novel approaches are needed to find practical methods to manage HLB in the citrus-growing states of the USA. Current methods of disease management are not financially affordable. Excessive use of pesticides to control the psyllid populations is environmentally unsafe; pesticides will lose effectiveness after prolonged use. Ecologically friendly biological control mechanisms that reduce the psyllid population will not be adequate to manage HLB since a single insect vector carrying the pathogen can spread the disease in a citrus grove.

When the citrus processing plants and juice factories are closed because of low yield and diseased fruit, it will be very difficult to revive this industry infrastructure. Long-term strategies for disease management are urgently needed to continue citrus cultivation in areas where the insect vector and the pathogen have been established.

Citrus is propagated mainly through grafting. The vegetative mode of propagation of a limited number of cultivars has resulted in innumerable cultivated types, but there is little genetic diversity in cultivated citrus. When new pathogens arrive, this genetic uniformity can result in disease epidemics and dire consequences for the crop. In addition, there is no documented resistance to HLB in cultivated citrus. However, other citrus-related wild plants may have resistance traits to HLB.

Based on our previous work, we determined that certain non-citrus taxa that are closely related to citrus have disease-resistance traits and these may be utilized to impart the much-needed HLB resistance to cultivated citrus types. Our approach is to generate novel hybrids of citrus using conventional breeding methods with sexually compatible citrus relatives having genetic resistance to the HLB pathogen. Towards this goal, we have conducted a breeding program since 2013 by crossing citrus accessions with disease-resistant/tolerant Australian lime species belonging to the genus Microcitrus. Crop relatives are utilized as sources of beneficial traits in other cultivated plants. Citrus is sexually compatible with many closely related genera; this biological peculiarity of cultivated citrus types was exploited in our breeding program to generate hundreds of intergeneric hybrids.

Citrus breeding is considered very difficult and time-consuming since the generation time can be 4 to 5 years. Since our breeding program has been in progress for ten years now, we have generated progeny from several different types of crosses. Evaluation of the novel hybrids in the field and in the greenhouse determined that HLB resistance traits can be inherited by a small proportion of the breeding progeny. However, since one parent of the cross bears fruit that is barely edible, many hybrids of the first-generation crosses have fruits that may not be acceptable as substitute varieties, even though they have disease-resistance traits.

We have now generated hybrids of the second generation by crossing the promising hybrids from the first generation to commercial types of citrus. A small percentage of the advanced hybrids is expected to have retained the HLB resistance traits and may have fruit qualities similar to commercial citrus types. We propose to utilize molecular approaches to develop markers that will aid in the selection of the breeding progeny from the second generation of hybrids to identify individuals with promising HLB resistance. If such hybrids can resist HLB in field conditions and if they have fruit traits that are deemed acceptable by the public, we will have novel hybrids with disease resistance and acceptable organoleptic attributes. Cultivar development of promising individuals will be required before large-scale cultivation can be adopted.

If novel hybrids with genetic resistance to HLB are developed, it will be possible to cultivate citrus despite having the pathogen and the insect vector in the area. Management of the groves that have resistant cultivars will be similar to the grove management done before HLB arrived. Financially, it will be feasible to cultivate citrus if elaborate, expensive management practices are not necessary. Another advantage is that the novel hybrids may have new flavors that may enhance the known citrus flavors that are generally associated with citrus fruits. We will look for the metabolomics profile of novel hybrids to identify the flavor-enhancing compounds in the hybrid fruits and compare them with standard citrus. Replacement of presently cultivated citrus varieties with novel hybrids will be beneficial to manage HLB.

The molecular markers we propose to develop in the project can be used in the future to make pre-selections from our breeding program in California. We envisage the development of multiple varieties resembling mandarins, sweet oranges, grapefruit, and lemons soon. The availability of HLB-resistant hybrids with citrus-like flavors (and some novel flavors) will be extremely beneficial to restoring the citrus industry in the United States. Citrus fruits provide valuable nutrients and constitute an essential part of a healthy diet. Restoring the citrus industries with useful, disease-resistant hybrids will result in job creation and improvement of economic situations in citrus-growing regions.

Regents Of The University Of California At Riverside

10.309 - Specialty Crop Research Initiative

Institute of Food Production and Sustainability (IFPS) [USDA - NIFA]

Riverside, California 92521-0001 United States

Single Zip Code

USDA-NIFA-SCRI-009843