Citrus A B blight: update on research C D By Nian Wang and Ron Brlansky itrus blight is an old and mysterious disease that causes Cthousands of trees to become unproductive every year in Florida. Citrus blight can occur on all common citrus cultivars, on trees of all root- stocks and on seedling trees, but is not known to affect other plants. Trees on Typical citrus blight symptoms. A. Left: healthy Hamlin sweet orange; right: Ham- rootstocks of rough lemon, Rangpur lin with citrus blight. B. Shooting of sprouts inside of the canopy. C. Off-season lime, trifoliate orange and Carrizo flowering in the citrus blight diseased trees to the left. D. Citrus blight diseased citrange are especially susceptible, trees showing twig dieback, and overall tree decline. whereas those on sour orange and sweet orange are most tolerant. Once trees on PROBABLE SYNERGETIC with blockage of water and soluble any rootstock develop the disease, they INTERACTION OF HLB mineral nutrients (caused by the xylem- usually decline within a short time. AND BLIGHT plugging due to blight) as well as the Trees affected by blight grow With the introduction of HLB into blockage of photosynthetic products normally until they reach 5 to 6 years Florida citrus groves, citrus blight has to non-photosynthetic parts (caused by of age or older, then begin to display been getting less attention, even though phloem-plugging due to HLB). Trees zinc deficiency in the leaves, block- the problem still exists. In fact, the might decline faster when both diseases age of xylem tissues, twig dieback rapid spread of HLB has compounded are present. and overall tree decline, rendering an the problem that the citrus industry SEARCH FOR THE CAUSAL has with the losses due to citrus blight. economical failure of the trees. Major AGENT OF CITRUS BLIGHT physiological changes occur, such as The presence of both diseases in off-season flowering, longer flowering groves is now a common occurrence. The causal agent of blight remains periods and shooting of sprouts inside Both diseases cause severe decline in unknown; however, a variety of of the canopy. It is common to have the health and productivity of trees, bacterial, fungal and viral pathogens less water uptake into the trunk of the although the symptomology differs. have been suggested. Early studies affected plants due to xylem blockage. Co-infection has severe consequences suggested that Physoderma, a fungus, was present in blighted trees, but the associated structures found were actu- ally fibrous or filamentous plugs in xylem vessels. Soil-borne organisms and soil problems also have been suggested as causes of blight. Fusarium solani, Blue Goose has the people, a fungus that causes root rot in beans experience and resources and other root problems in many plant species, has been extensively studied to improve your return on your investment in relation to the disease. However, it has never been conclusively demon- Growers Cutting Heavy equipment strated that this fungus is capable of • Installation services • Services for Agricultural and causing the disease in the field. • Grove care It also was proposed that blight was • Hedging Commercial construction • Crop manage- • Excavation • Grading • Land clearing caused by a variant of citrus tristeza ment • Topping • Waterway maintenance – Water virus that affected trees on rough • Property • Skirting control structure installation lemon rootstock. management • Cross Hedging Xylella fastidiosa, the causal agent of Pierce’s disease of grapes, was FORT PIERCE Real estate reportedly detected in blight-affected 772-461-3020 Main Office citrus trees and was suggested as a 9901 Okeechobee Rd. 34945 • Leasing Services causal agent. However, the quantities ARCADIA CLEWISTON • Agricultural sales and purchases 863-993-9661 863-677-1125 of bacteria usually associated with 6057 Nichols St. 34266 2550 Witt Rd.. 33440 other diseases caused by X. fastidiosa 28 CITRUS INDUSTRY • June 2013 are not present in blighted citrus, and caused by an unknown systemic infec- that a specific organism is responsible the discovery of X. fastidiosa as the tious agent. Lack of the knowledge of for a particular disease, there are cause of citrus variegated chlorosis the causal agent continues to hinder the severe limitations. The criteria can eliminated the bacterium’s role as the development of reliable and specific only be met with those organisms cause of citrus blight. diagnostic procedures for citrus blight that can be cultured, and all uncultur- An idaeovirus, similar to raspberry and thus efficient management and or able pathogens which include various bushy dwarf virus, also has been asso- control of the disease. important plant pathogenic organisms ciated with citrus blight but pathoge- must be proven with revised methods. nicity studies were not completed. KOCH’S POSTULATES IN THE The application of nucleic acid-based In addition to the list of biotic agents, ERA OF GENOMICS methods for microbial identification is the effect of various abiotic factors such Determining the causal agent of now available for potential pathogens. as soil pH, liming and fertilization, and any plant disease is not always an easy These new methods have been con- other management practices in relation task. To determine which, if any, of vincingly used for associating various to the disease have been extensively the organisms associated with diseased uncultured organisms for their associa- studied to determine their role in the plants is responsible for the symptoms, tion with plant diseases. For example, expression of disease symptoms. Koch’s postulates should be fulfilled. the three Candidatus Liberibacter spe- Blight has been attributed to clay Koch’s postulates include four steps: cies associated with huanglongbing and layers in the soil, to liming of groves (1) regular association; (2) isolation in phytoplasmas found in many diseased and to “modern agricultural practices.” pure culture of the organism from the plant species have been identified. However, the disease has never been diseased samples; (3) inoculation of reproduced in the field by changing the organism in the susceptible host to NEXT-GENERATION SEQUENCING cultural practices. reproduce disease symptoms; and (4) FOR IDENTIFYING THE CITRUS However, symptoms and character- re-isolation of the same organism from BLIGHT PATHOGEN istics associated with citrus blight can the experimentally infected host. One of the new methods used for be reproduced by root graft inocula- Despite the importance of Koch’s determining the sequence of undiscov- tions, which suggests that the disease is postulates in providing rigorous proof ered pathogens is called next-genera- tion sequencing. Such a technique may be used for identification of the causal agent of citrus blight. This recently developed sequencing technique can provide sequencing information through a large number of short nucleic acid sequences, and coupled with the correct information on the sequence, significantly improve the researcher’s ability to investigate various potential causal agents from a plant sample. Predictably, next-generation sequencing recently has been used to uncover unknown etiological agents such as two new viruses causing citrus leprosis. Another advantage of using these techniques for the detection of unknown or uncultured pathogens is the ability to fully sequence the genome from infected tissues in cases “UPDIKE CITRUS SERVICES LLC where the pathogen is present in sig- nificantly greater numbers as compared and sister company, SUNNY to other associated microorganisms. FLORIDA PEACH COMPANY. Currently, with support from the Providing Florida growers Citrus Research and Development Foundation, the authors are collaborat- with a variety of services ing on identification of the putative to give that extra compe- causal agent of citrus blight using next-generation sequencing. Data has titive edge in today’s ever been acquired and is under analysis, S D RA OW TU and the sequence information has the NTO N AL changing market.” W potential to identify the causal agent. • CUSTOM CITRUS/PEACH CARE The long-term goal of this research is to develop new detection methods that • CONSULTING • MARKETING • HARVESTING will have the potential to allow us to D. Clinton Updike, II determine the sources of the pathogen LICENSED & BONDED FRUIT BUYER/DEALER and any potential vectors. With this information, we can then determine the 863-537-5121 appropriate management strategies. [email protected] Nian Wang is an assistant professor and P.O. Box 9 •Alturas, FL 33820 Ron Brlansky is a professor at the Uni- Citrus and Peach Production versity of Florida’s Citrus Research and Education Center in Lake Alfred. 30 CITRUS INDUSTRY • June 2013.