Citrus Greening Disease: The silent killer of citrus groves

Ngangbam Punita *

Citrus is one of the most commonly consumed fruits in the world with commercial cultivation spanning over hundred Nations. In India, citrus ranks as the third most significant fruit crop, trailing bananas and mangoes.

Notably, the North Eastern Hill region stands out as a treasure trove of citrus genetic diversity, boasting an impressive array of 23 species, one subspecies, and 68 varieties - a true haven for citrus enthusiasts and researchers alike.

Beyond its juicy appeal, citrus fruits have an impressive resume of health benefits, earning them a spot among the world’s top superfoods. Rich in vitamin C, antioxidants, and anti-inflammatory compounds, citrus fruits have been shown to combat cancer, diabetes, and obesity. But despite their miraculous properties, these fruits are fragile, falling prey to a plethora of fungal, bacterial, and viral diseases that threaten global production.

Among the diseases affecting citrus, Huanglongbing (HLB) or citrus greening has ravaged global citrus production, resulting in the loss of over 60 million trees worldwide. Tracing its origins to southern China, Huanglongbing’s (HLB) global footprint expands far beyond its initial outbreak.

Intriguingly, India’s encounter with HLB was attributed to Citrus die back in the 1700s, resulting in a hypothesis that the disease was already established in India before spreading to China. In India, the disease has been reported in major citrus growing States such as Manipur, Meghalaya, Assam, Maharashtra, Punjab, etc.

Initially, the disease was suspected as viral due to its graft transmissibility. The 1967 discovery of MLOs further led to strengthening the belief that the causal agent of the disease to be an MLO.

However, advancement in electron microscope and cytochemistry techniques exposed a gram negative bacterial culprit. HLB is associated with a phloem limited unculturable bacteria, Candidatus Liberi- bacter spp. Based on the geographic locations of their origin they have been classified as Candidatus Liberibacter : asiaticus, africanus and americanus.

Notably, Candidatus Liberi-bacterasiaticus is the most destructive and widespread variant, responsible for unprecedented economic losses in global citrus production. Its remarkable divergence and adaptability have made it a formidable foe, challenging citrus industries worldwide.

Huanglongbing’s insidious spread is facilitated by two primary means: grafting and citrus psyllids, the Asian citrus psyllid (ACP) and African citrus psyllid (AfCP) that serves as unwitting vectors. Characterized by its remarkable persistence, the Asian citrus psyllid (ACP) effectively transmits Huanglongbing’s causative bacterium to uninfected plants within 30 minutes.

This vector demonstrates heat resilience up to 45°C but succumbs to humidity levels exceeding 90%. Conversely, the African citrus psyllid (AfCP) favors cooler, more humid conditions yet is compromised by temperatures surpassing 32°C. Notably, HLB transmission is solely attributed to grafting and the ACP and AfCP vectors, and is not dessiminated through wind, rain, or contact with contaminated field workers.

Citrus greening affects nearly every citrus species, with younger trees being disproportionately vulnerable. Once infected, the bacteria severely clog the phloem tissues in roots and leaves, disrupting the flow of essential nutrients and sugars. Identifying HLB symptoms is complicated due to its varied and subtle manifestations.

The disease’s symptoms often masquerade as other common citrus problems, including Citrus tristeza virus infections and nutritional deficiencies, making accurate identification a daunting task. The disease is dubbed the “silent killer” because it sneaks up on unsuspecting growers, often showing no visible symptoms until it’s too late.

It infects trees without immediate noticeable effects, allowing it to spread undetected and masquerade as nutrient deficiencies, making diagnosis challenging. Infected trees are gradually weakened, reducing its production and quality before eventually killing the tree. Unlike other diseases, it lacks dramatic visual cues, making detection difficult. Additionally, HLB can remain dormant for months or even years, striking when least expected.

The hallmark symptom of HLB is the “yellow shoots” phenomenon, characterized by leaves displaying a mosaic of yellow, green, and intermediate shades, with no distinct boundaries between colours, known as “blotchy mottle.” Initially, yellowing along leaf veins and blotchy patterns emerge, later resembling zinc and iron deficiency symptoms.

As the disease progresses, defoliation and dieback occur, while the root system remains underdeveloped due to nutrient starvation. Additionally, infected fruits exhibit abnormalities, including: stunted growth, irregular shapes, poor coloration, and bitter taste.

The symptoms are more pronounced during cooler months compared to warmer periods. Interestingly, infected plants can remain symptom-free for months or even years, harbouring large quantities of bacteria, which poses a significant risk of transmission to uninfected plants.

While symptoms can manifest throughout the year, they are most visible between September and March, with a notable surge in symptomatic expression during this period. Symptoms are easily seen in the shade or overcast days, as well as in almost all parts of the plants, including the canopy, leaves, twigs, and fruit where infection occurs; the entire tree decreases rapidly as the disease progresses.

Despite ongoing research, a cure for citrus greening remains elusive, making it challenging to halt the disease’s spread to new citrus-producing regions. However, an integrated management approach can help mitigate its impact and slow down the disease’s progression. Available management strategies include:

o Control of psyllid population chemically or biologically
o Removal of infected trees
o Planting of disease-free nursery trees.
o Antibiotic application (although this approach has a risk for antibiotic residue contamination in fruits and risk for development of resistant bacterial strains)
o Foliar phosphate fertilizers application
o Harnessing the potential of citrus endophytes
o Utilization of tolerant cultivars as rootstocks

In conclusion, citrus greening poses a significant threat to the citrus industries globally, requiring innovative research and commu- nity based initiatives on providing awareness which will offer a beacon of light in the fight against the disease.