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December 2006. In Vietnam, placing guava trees in orange and grapefruit groves can protect the citrus crop from a devastating disease called greening.
Vietnam has little citrus, but some small plantings there have survived at least 15 years despite the presence of greening.
A member attended at a small international workshop on the disease that was held in Japan on 6-7 December 2006. The meeting was sponsored by JIRCAS (Japan International Research Center for Agricultural Sciences). A paper, Aspects and Insights of Australia-Asia Collaborative Research on Huanglongbing, co-authored by Paul Holford, David Mabberley, Tony Haigh, Randy Bayer and Pat Broadbent (Barkley) was presented. The following is an extract.
The impact of guava on D citri and HLB, as noted above for our ACIAR project site in southern Vietnam, appears to be real, but no replicated experiments have been undertaken. However, the impact at the site is supported by observations in orchards with and without guava interplants in the Cai Be (106° 2′ E, 10° 20′ N, 1 m asl) region of the Mekong Delta, and by two un-replicated experiments in Cai Be (Le Quoc Dien, SOFRI, pers. comm.). In addition to reported effects on the vector and the disease, it seems guava interplants also lead to lower populations of aphids, scales, mealybugs and citrus leafminer (Do Hong Tuan et al., SOFRI, pers. comm.), but higher incidences of citrus red mite, citrus canker (Xanthomonas campestris pv. citri (Hasse) syn. X axonopodis pv. citri (Hasse) Vauterin, Hoste, Kersters & Swings [Pseudomonadales: Pseudomonadaceae]) and citrus scab (Elsinoe fawcettii Bitancourt & Jenkins [Dothideales: Elsinoaceae]) occur in interplanted orchards (Do Hong Tuan et al., SOFRI, pers. comm.). A marginal increase in the use of copper-based fungicides may be required for control of citrus canker and citrus scab in such situations. However, farmer observations suggests that interplanting with guava, regular monitoring and removal of flush growth on which psyllids are observed, application of mineral oils to flush growth, and spot-spraying with oil on other occasions, is a cost-effective management program in the Mekong Delta for controlling the psyllid and for delaying significant impacts of HLB for 15 or more years (Le Quoc Dien, SOFRI, pers. comm.).
Our impression is that current interest and research on guava interplants stems from farmer observations over the past 15-20 years. We have initiated experiments with guava interplants in Indonesia and encouraged colleagues in Sarawak and China to also conduct experiments. We are interested in confirming the observations in Vietnam and evaluating the use of guava in hilly and less humid environments than in the flat humid environment of the Mekong Delta. At this point, we assume the guava interplants at Cai Be reduce numbers of adult psyllids landing on adjoining citrus trees by interfering with one or more of the following: (a) visual recognition of host foliage, (b) masking of host volatiles, or (c) avoidance of guava volatiles. Based on current knowledge of the chemical ecology of aphids and psyllids (Kristoffersen 2003), masking and avoidance would seem to be more likely than impacts on visual cues. The observations at Cai Be also suggest that adult D citri choose to avoid landing within interplanted orchards. If proven, the use of guava interplants to control HLB and D citri could have major ramifications for the citrus industries of Southeast Asia. It has the potential to provide farmers with income from guava fruit in the 3-4 years after planting before citrus trees bear, while simultaneously prolonging the viability of citrus orchards and significantly reducing expenditure on pesticides.
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