Presymptomatic Fibrous Root Decline in Citrus Trees Caused by Huanglongbing and Potential Interaction with Phytophthora spp.

Abstract

Huanglongbing (HLB), associated with ‘Candidatus Liberibacter asiaticus’, 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 good nutritional and irrigation practices recommended to support health of HLB-affected trees. This stress intolerance may be due to a loss of fibrous roots. To assess root status of HLB-infected trees on ‘Swingle’ citrumelo rootstock (Citrus paradisi × Poncirus trifoliata), blocks of 2,307 3-year-old ‘Hamlin’ orange trees and 2,693 4-year-old ‘Valencia’ orange trees were surveyed visually and with a real-time polymerase chain reaction (PCR) assay to determine ‘Ca. L. asiaticus’ infection status. The incidence of ‘Ca. L. asiaticus’-infected trees (presymptomatic: ‘Ca. L. asiaticus’+, visually negative; and symptomatic: ‘Ca. L. asiaticus’+, visually positive) trees was 89% for the Hamlin block and 88% for the Valencia block. ‘Ca. L. asiaticus’+ trees had 30 and 37% lower fibrous root mass density for presymptomatic and symptomatic trees, respectively, compared with ‘Ca. L. asiaticus’– trees. In a second survey, 10- to 25-year-old Valencia trees on Swingle citrumelo or ‘Carrizo’ citrange (C. sinensis (L.) × P. trifoliata) rootstock were sampled within 3 to 6 months after identification of visual HLB status as symptomatic (‘Ca. L. asiaticus’+, visually positive) or nonsymptomatic (‘Ca. L. asiaticus’-, visually negative) in orchards located in the central ridge, south-central, and southwest flatwoods. Pairs of HLB symptomatic and nonsymptomatic trees were evaluated for PCR status, fibrous root mass density, and Phytophthora nicotianae propagules in the rhizosphere soil. ‘Ca. L. asiaticus’+ trees had 27 to 40% lower fibrous root mass density and, in one location, higher P. nicotianae per root but Phytophthora populations per cubic centimeter of soil were high on both ‘Ca. L. asiaticus’+ and ‘Ca. L. asiaticus’– trees. Fibrous root loss from HLB damage interacted with P. nicotianae depending on orchard location and time of year.

Diverse DNA extraction methods and PCR primers for detection of Huanglongbing-associated bacteria from roots of ‘Valencia’ sweet orange on sour orange rootstock

“…L. asiaticus in the roots prior to leaf symptom development could be very instrumental in planning Phytophthora prevention measures as it was reported that prior infection of Ca. L. asiaticus promotes Phytophthora growth and subsequent damage to fibrous roots causing root decline (Graham et al, 2013); this is very important in Texas where Phytophthora inoculum is widespread (Timmer, 1972). Moreover, Trivedi et al (2010Trivedi et al ( , 2012 reported that the relative composition of the rhizosphere microbial community is affected and changed due to HLB infection.…”

“…This study also revealed that there was an enhancement of LB-induced symptoms by P. parasitica in citrus trees. Moreover, it was recently reported that HLB causes pre-symptomatic fibrous root decline, which was observed independently of Phytophthora (Graham et al, 2013). Detection of Ca.…”

Efecto del HLB sobre el rendimiento de limón mexicano en Colima, México

“…The lower production of photosyntates, together with the reduction of its transport to other organs, especially the root, is likely to affect the overall vigor of the tree. Although the effect of disease in root density has not been studied, it is presumed that the amount of tree roots decreases in tres affected by the disease, as suggested by Graham et al (2013).…”

“…Es probable que la menor producción de fotosintatos, junto con la reducción en su transporte hacia otros órganos, especialmente la raíz, afecte el vigor general del árbol. Aunque no se ha estudiado el efecto de la enfermedad en la densidad de raíces, se presume que la cantidad de raíces disminuye en árboles afectados por la enfermedad, como lo sugiere Graham et al (2013).…”

Integration of soil applied neonicotinoid insecticides and acibenzolar-S-methyl for systemic acquired resistance (SAR) control of citrus canker on young citrus trees

“…In Florida citrus production, the neonicotinoid insecticides imidacloprid (Admire ProÔ, Bayer Crop Science) and thiamethoxam (THIA, Platinum Ô , Syngenta Crop Protection) are applied as soil drenches to non-bearing trees for the control of citrus leafminer (Phyllocnistis citrella) and the associated infection of leafminer galleries by Xcc (Rogers, 2012). Systemic neonicotinoids are even more important for reducing transmission by the psyllid vector (Diaphorina citri) of Candidatus Liberibacter asiaticus (Las), the causal agent of Huanglongbing (HLB) disease (Graham et al, 2013b). HLB was first detected in Florida in late 2005 and is now widely distributed throughout the commercial citrus-growing regions (Gottwald, 2010).…”