Pre-symptomatic fibrous root decline in citrus trees caused by huanglongbing and potential interaction with Phytophthora spp.

Authors: GRAHAM, JAMES - University Of Florida; JOHNSON, E - University Of Florida; Gottwald, Timothy; IREY, M - Us Sugar Corporation

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/18/2018
Publication Date: 9/1/2013
Citation: Graham, J.H., Johnson, E.G., Gottwald, T.R., Irey, M.S. 2013. Pre-symptomatic fibrous root decline in citrus trees caused by huanglongbing and potential interaction with Phytophthora spp.. Plant Disease. 97:1195-1199.

Interpretive Summary: Citrus huanglongbing (HLB) also sometimes referred to as citrus greening is acknowledged to be the most serious disease threatening citrus worldwide. HLB has become endemic in the state of Florida, and has swept across the state until nearly every area is infected. Florida citrus growers believe the disease is a threat to their continued existence. The disease has recently been found in Texas and California, prompting grave concern by those states as well. This research demonstrates for the first time that HLB causes severe root loss of citrus trees even before those trees become visually symptomatic with disease symptoms. Losses range from about 27 to 40% of the total feeder root structure of trees. Phytophthora is a fungus like organism that also attacks and kills feeder roots of citrus trees. This work also demonstrates that there is an interaction between HLB and Phytophthora, synergistically exacerbating root loss and therefore decline of infected citrus trees. The importance of the work is that it points to root health as a major part of the decline and problem associated with HLB infection. It points to new avenues of research wherein dealing with root health and finding means to promote root health, could be beneficial in combating HLB to some extent.

Technical Abstract: Huanglongbing (HLB) caused by Candidatus Liberibacter asiaticus (Las) was first detected in Florida in late 2005 and is now widely distributed throughout the commercial citrus-growing regions. In recent seasons, concurrent with freeze and drought episodes, symptomatic HLB-infected trees were much more affected by the extremes of temperature and moisture than trees without HLB. Symptoms exhibited by the stressed trees were excessive leaf loss and premature fruit drop even when HLB-infected trees were managed with enhanced nutritional programs which are thought to improve tree health of HLB-infected trees. This stress intolerance may be due to a loss of fibrous roots. To assess root status of HLB-affected trees, blocks of 2,307 three-yr-old Hamlin orange trees and 2,693 four-yr-old Valencia orange trees were surveyed visually and by real time PCR to determine Las infection status. The incidence of Las-infected trees (pre-symptomatic PCR+, visually negative and symptomatic PCR+, visually positive) trees was 89% for the Hamlin block and 88% for the Valencia block. HLB+ trees had a 30 and 37% reduction in fibrous root mass density for pre-symptomatic and symptomatic trees, respectively, compared to HLB – trees. In a second survey, 10- to 25-yr-old Valencia trees were identified within 3-6 months of canopy expression as HLB symptomatic (PCR+, visually positive) or non-symptomatic (PCR-, visually negative) in orchards located in the central ridge, south-central and southwest flatwoods. Pairs of HLB+ and HLB- trees were evaluated for PCR status, fibrous root mass density and Phytophthora nicotianae progagules in the rhizosphere soil. HLB+ trees had 27-40% lower fibrous root mass density and in one location higher P. nicotianae per root. However, Phytophthora populations per cm3 soil were high on both HLB+ and HLB- trees. Fibrous root loss results primarily from HLB damage which may be interacting with P. nicotianae.