Huanglongbing (HLB) and Phytophthora foot and root rot are diseases that affect citrus production and profitability. The symptoms and physiological changes associated with these diseases are diagnosed through expensive and time-consuming field measurements. Unmanned aerial vehicles (UAVs) using red/green/blue (RGB, true color) imaging, may be an economic alternative to diagnose diseases. A methodology using a UAV with a RGB camera was developed to assess citrus health. The UAV was flown in April 2018 on a grapefruit field infected with HLB and foot rot. Ten trees were selected for each of the following disease classifications: (HLB-, foot rot–), (HLB+, foot rot–), (HLB-, foot rot+) (HLB+, foot rot+). Triangular greenness index (TGI) images were correlated with field measurements such as tree nutritional status, leaf area, SPAD (leaf greenness), foot rot disease severity and HLB. It was found that 61% of the TGI differences could be explained by Na, Fe, foot rot, Ca, and K. This study shows that diseased citrus trees can be monitored using UAVs equipped with RGB cameras, and that TGI can be used to explain subtle differences in tree health caused by multiple diseases.

Huanglongbing (HLB) is the most devastating and widespread citrus disease. All commercial citrus varieties are susceptible to the HLB-associated bacterium, Candidatus Liberibacter asiaticus (CLas), which resides in the phloem. The phloem is part of the plant vascular system and is involved in sugar transport. To investigate the plant response to CLas, we enriched for proteins surrounding the phloem in an HLB susceptible sweet orange variety, Washington navel (Citrus sinensis (L) Osbeck). Quantitative proteomics revealed global changes in the citrus proteome after CLas inoculation. Plant metabolism and translation were suppressed, while defense-related proteins such as peroxidases, proteases and protease inhibitors were induced in the vasculature. Transcript accumulation and enzymatic activity of plant peroxidases in CLas infected sweet orange varieties under greenhouse and field conditions were assessed. While peroxidase transcript accumulation was induced in CLas infected sweet orange varieties, peroxidase enzymatic activity varied. Specific serine proteases were upregulated in Washington navel in the presence of CLas based on quantitative proteomics. Subsequent activity-based protein profiling revealed increased activity of two serine proteases, and reduced activity of one protease in two C. sinensis sweet orange varieties under greenhouse and field conditions. The observations in the current study highlight global reprogramming of the citrus vascular proteome and differential regulation of enzyme classes in response to CLas infection. These results open an avenue for further investigation of diverse responses to HLB across different environmental conditions and citrus genotypes.

Citrus Huanglongbing (HLB) or citrus greening, is the most destructive disease for citrus worldwide. It is caused by the psyllid-transmitted, phloem-limited bacteria “CandidatusLiberibacter asiaticus” (CLas). To date, there are still no effective practical strategies for curing citrus HLB. Understanding the mechanisms againstCLas can contribute to the development of effective approaches for combatting HLB. However, the unculturable nature ofCLas has hindered elucidating mechanisms againstCLas. In this review, we summarize the main aspects that contribute to the understanding about the mechanisms againstCLas, including (1)CLas virulence targets, focusing on inhibition of virulence genes; (2) activation of citrus host defense genes and metabolites of HLB-tolerant citrus triggered byCLas, and by agents; and (3) we also review the role of citrus microbiome in combattingCLas. Finally, we discuss novel strategies to continue studying mechanisms againstCLas and the relationship of above aspects.

Phytophthora-induced foot rot, also known as gummosis, is an important disease affecting citrus production worldwide. In Texas, the third-largest citrus-producing state in the United States, limited information is available on the etiology and epidemiology of foot rot in commercial orchards. This study comprises a survey of foot rot incidence and severity in Texas and the characterization of Phytophthora isolates associated with the disease. Surveys in 2015 and 2017 of 30 orchards in the Lower Rio Grande Valley (LRGV) region where commercial citrus production is concentrated in the state revealed that foot rot occurred in 97% of the orchards assessed. Overall, foot rot symptoms were observed on 33.7% of the trees evaluated and the disease severity index in the region was rated at 14.2 and 16.5% in 2015 and 2017, respectively. Lesions were mostly present on the scion, while the rootstock (sour orange) was not affected. Phytophthora nicotianae was the only Phytophthora sp. isolated from the surveyed orchards and from five additional residential sites on the Texas Coastal Bend (TCB). Sporangia and chlamydospores from 34 representative LRGV isolates of P. nicotianae were larger than those of TCB isolates. In both LRGV and TCB, A1 and A2 mating types were present in the same location, albeit the A2 mating type was more prevalent. All isolates were sensitive to mefenoxam (50% inhibition in the presence of mefenoxam [EC₅₀] < 0.5 µg/ml), except for one TCB isolate (EC₅₀ = 143.6 µg/ml). Our research indicates that treatment for Phytophthora foot rot in the region is necessary and, although mefenoxam is still useful, alternating chemistries for resistance management are required.

Root endophytes have been shown to influence plant growth and plant responses to pathogens. Phytophthora spp. are important pathogens of citrus that cause foot rot and fibrous root decline. We hypothesize that foot rot infection by Phytophthora spp. can result in changes of the endophytic microbial community in citrus roots. Therefore, the endophytic (including bacterial and fungal) microbiome of roots from Phytophthora foot-rot-affected and healthy citrus trees were analyzed by Illumina sequencing of 16S ribosomal RNA and internal transcribed spacer amplicons. Results indicate that the composition and structure of the endophytic bacterial and fungal communities were changed in roots of foot-rot-affected citrus trees. The populations of bacterial genera Asteroleplasma, Pseudomonas, and Streptomyces were decreased in foot-rot-affected trees. Also, the relative abundance of Exophiala pisciphila and Glomeraceae spp. was significantly decreased in citrus roots. Pseudomonas and Streptomyces spp. are considered beneficial bacteria and E. pisciphila and Glomeraceae spp. are dark septate endophytes and arbuscular mycorrhizal fungi, respectively, which are involved in citrus health and growth. This study provides a baseline to continue investigating the interactions between the citrus host, Phytophthora spp., and beneficial microbes.

Citrus huanglongbing (HLB) is a devastating disease for the citrus industry. The previous studies demonstrated that oxytetracycline and penicillin are effective antibiotics against Candidatus Liberibacter asiaticus (CLas). However, since CLas is uncultured, the mechanisms of action of antibiotics against CLas are still unclear. It was recently reported that the endophytic microbial communities are associated with the progression of citrus HLB after oxytetracycline and penicillin treatment. Therefore, we hypothesize that penicillin has greater antibacterial activity against CLas than oxytetracycline, which may be associated with the alteration of the structure and function of endophytic microbial communities in HLB-affected citrus in response to these antibiotics. To test this hypothesis, the microbiome of HLB-affected citrus leaves treated with these two antibiotics was analyzed using a metagenomic method. Our results indicate that the microbial structure and function in HLB-affected citrus were altered by these two antibiotics. The relative abundance of beneficial bacterial species, including Streptomyces avermitilis and Bradyrhizobium, was higher in penicillin-treated plants compared to those treated with oxytetracycline, and the relative abundance of the bacterial species (such as Propionibacterium acnes and Synechocystis sp PCC 6803) associated with CLas survival was lower for penicillin-treated plants compared to oxytetracycline-treated plants. These results indicate that penicillin has greater antibacterial activity against CLas. Based on the metagenomic analysis, this study elucidated the mechanism for the observed increase in antibacterial activity of penicillin against CLas. The data presented here are not only invaluable for developing eco-friendly and effective biocontrol strategies to combat citrus HLB, but also provide a method for revealing mechanism of antimicrobial against uncultured bacteria in host.