Improving sub tropical citrus production in Sikkim and Australia

• Sikkim Development Foundation, India
• Department of Agriculture, Fisheries and Forestry, Australia
• Queensland Herbarium (BRI), Australia

The once thriving citrus industry in the Indian Himalayan state of Sikkim had suffered a decline over several decades, bringing yields down to less than 10% of those in Australia. The decline was thought to be partly due to disease (especially the bacterial disease known as huanglongbing) and the very narrow genetic base of the industry. Attempts to increase productivity through improved nutrition, disease eradication, vector control and the provision of clean planting material were unsuccessful. Although large-scale orchards can control disease through vector control and the use of clean planting materials, these methods are too expensive and difficult for small farmers.
Germplasm evaluation and improvement are a fundamental component of modern citriculture, but have received limited attention in the Indian Himalayas. Extensive genetic diversity exists within Citrus and related genera, and this could be exploited to select disease-resistant genotypes suitable for local conditions that would help to increase mandarin yields and associated profitability for small farmers. Despite existing problems, citrus remains the most important horticultural crop for Sikkim, and there is a large and lucrative market that could be captured if productivity increased.

This project aimed to improve subtropical citrus productivity in the state of Sikkim by introducing improved germplasm for hardier, more vigorous plants, with greater disease resistance than those already under cultivation.

The project had five experimental components. Researchers first conducted a survey of farmers in Sikkim to learn the characteristics and constraints of the citrus industry there. Others used tissue culture and a technique known as 'nucellar embryony' to propagate improved germplasm from the collections of the Queensland Department of Primary Industries and Fisheries. The research team then evaluated the performance of rootstock and scion cultivars in local conditions in Sikkim, comparing tree growth, survival and health, and the incidence of disease symptoms. The team followed this with a field survey, to determine the distribution, disease status and usefulness of native members of citrus (part of Family Rutaceae). Finally, the researchers propagated material of different disease status onto two rootstocks and evaluated their performance at different sites in Sikkim, using molecular diagnostic techniques.
Selected agricultural officers and others associated with the citrus industry in Sikkim undertook a training course and study tour in Australia to improve their knowledge of citrus nursery and diagnostic technology techniques.
In Australia, scientists screened citrus relatives for susceptibility to canker, a bacterial disease that causes great damage in the citrus industry when it takes hold.

The project obtained proof that disease was the major impediment to the citrus industry in Sikkim, and the project team used the evidence generated to build consensus from initially disparate views. This recognition of a single major factor obstructing citrus productivity in the region is fundamental to improving the current poor state of commercial orchards, and ensuring the limited resources (both R&D, and farm-level inputs) are not further wasted on activities that offer no prospect of success.
Leaf samples collected at all survey sites in all three years, prepared within Sikkim and then transferred (under quarantine approval) for molecular analysis in Australia consistently showed a high incidence of the bacterial disease huanglongbing (HLB). At a socio-economic level, the survey established that farmers had a strong desire to grow citrus as it was their most profitable crop with substantial market demand.
The survey material also proved valuable in the development of a diagnostic protocol for HLB testing in Australia. The disease and its vector (the Asiatic citrus psyllid) are currently exotic to Australia and preparedness for incursion of either is a major concern for the Australian citrus industry. A strong diagnostic protocol is one important component of the incursion management strategy, and this project provided the material to assist in its development.
The project also provided new information regarding the susceptibility of Australian citrus relatives to another important exotic disease, citrus canker, at a time when it was dealing with an outbreak near Emerald in central Queensland. Knowledge of whether native plants can be hosts is essential to disease management, and particularly disease eradication programs. Work from this project identified a number of new hosts of citrus canker including species that were assumed resistant (on the basis of their close relatedness to other non-hosts). The results demonstrate the hazard of using taxonomic proximity to predict disease status and provided the Emerald canker eradication program with concise information on the species that could host citrus canker. This had important practical implications for the eradication program - even if the knowledge of a new host plant was not always welcome news.
Improved germplasm has been fundamental in overcoming destructive citrus diseases in the past, and germplasm evaluation activities formed a major component of this project. This project successfully introduced over 100 citrus genotypes to Sikkim, all of which are now growing successfully and awaiting evaluation. This represents an important resource for citrus farmers of the Himalayan region and has the potential to generate practical improvements to their farming system well into the future. It also represents an important scientific resource as the disease reaction of this diverse germplasm, particularly to HLB, has never been evaluated.
For the first time rootstock evaluation experiments were propagated and established in Sikkim using the locally preferred scion variety. Further impacts and practical benefits of this project activity will be evident as these trees commence fruit production.
Within Australia, the project established extensive rootstock experiments with both Eureka lemon and Imperial mandarin - until then rootstock research in temperate conditions had been very limited. The lemon experiment has already demonstrated a significant impact of rootstock on skin texture - one of the major quality attributes limiting Australian lemons on the international market. The work also demonstrated the relative poor quality of one of the rootstocks commonly used commercially, and identified two new rootstocks that offer improved nursery performance, yield and fruit quality.
Australian/PNG native citrus species had often been touted as potential rootstocks for citriculture but the research field work had never been done. This project finally put these species to the test. Unfortunately, all species proved highly susceptible to citrus tristeza virus (CTV) and would need to be hybridised to overcome this constraint.
The Imperial mandarin rootstock experiment generated by this project has also produced useful outcomes, despite its young age. This experiment included non-conventional germplasm as well as promising new rootstock genotypes from breeding programs in different parts of the world. It also included the Chinese germplasm introduced to Australia under a previous ACIAR project - the first time it had been evaluated under subtropical conditions. One rootstock variety that had performed well in research in NZ was found highly sensitive to Australian strains of CTV - trees on this rootstock declined and died shortly after field planting. Such information has significant practical implications in preventing commercial investment in this 'promising' rootstock, and it also emphasises the need for germplasm evaluation under local conditions prior to commercial planting.
Good nursery practice is essential to a viable citrus industry. This is particularly the case where graft-transmissible diseases, such as HLB, are endemic. For this reason the project conducted budding workshops for citrus farmers in different parts of Sikkim. Prior to the project all trees were grown from seed which meant that it was at least seven years before any fruit was produced. Survey results also demonstrated that these seedling trees became infected with HLB during the nursery phase - meaning that farmers received trees that not only took many years to fruit but already carried HLB.
The nursery workshops focused on practical means of reducing disease incidence, and by using superior budlines (identified and tested free of HLB by the project team) offered the prospect of commercial quantities of fruit within a few years of planting. Nursery facilities established by the project represent a significant improvement over previous production systems and are already providing farmers with improved planting material.
The project has had an important impact in improving the status of farmers. Prior to the project, farmers carried much of the blame for the poor state of the citrus industry in Sikkim. The project provided proof that the decline of orchards was disease-related, and not due to poor management. The efforts of the project team helped to build consensus around this discovery, and brought a new-found respect for citrus farmers and for the enormous obstacle they faced in managing HLB.