EFFICACY OF INTERPLANTING CITRUS WITH GUAVA AS A CONTROL STRATEGY FOR HUANGLONGBING

This research is related to inhouse objective 3: Verify if Murraya paniculata is, or is not, a preferred reservoir of huanglongbing (HLB) pathogens, whether it can be used as trap plants for the control of citrus HLB. This project was designed to examine the potential disease control of citrus HLB by interplanting citrus with guava. In Vietnam guava has been shown to be an effective deterrent to HLB, slowing the disease and keeping plantings alive for up to 15 years that normally succumb in 2-3 year. For all plots and experiments, Guava trees, (Vietnamese white cultivar) were propagated and grown to appropriate size requiring ~1 year. Both nursery and field citrus trees are assayed for HLB every 60 days, and have been assayed multiple times. Psyllid populations are also being monitored continuously every 2 weeks within interplanted plots to document any repulsion of the vector due to guava. Guava vs no guava nurseries: Two nursery sites, a guava protected citrus nursery versus an unprotected nursery, have were established with disease free, PCR-negative citrus trees (2 sweet orange and 1 grapefruit cultivars) in June 2009 and were located in the protected and unprotected plots. The guava trees were grown to appropriate size as indicated in Vietnam prior to outplanintg. To date HLB appears to be progressing more slowly in nursery plots interplanted with guava that non interplanted plots. Citrus/guava interplantings: 3 commercial plantings with multiple replications were established. This required 1 year to grow the guava transplants and a second year to become established in the field before interplanting with citrus per Vietnam protocols. One trial was established in a commercial orchard with collaborators in Southern Gardens Citrus, but two years of freezes and replanting prompted the grower/collaborator to abandon the planting. A second was established in grove in Martin county but the plantation was sold and removed before data could be collected. A third trial planting was established at the USHRL Picos Farm in Fort Pierce. The Picos plot was interplanted with citrus in August 2009. Severe frosts during 2008/2009 and again during Dec 2010 winters continue to affect the USHRL plots and have caused a delay in the experiment. To date, several HLB+ (PCR+) plants have been identified in the USHRL plots after multiple assays. The majority of these positives have occurred as the result of an edge effect. That is, treatment plots closest to an existing HLB+ planting of Valencia were affected. There continues to be no significant difference among the treatments thus far, however this data is insufficient to draw any conclusions at this point. Research using a Y-tube olfactometer to investigate guava volatiles as repellents of the psyllid has been challenging and resulted in no clear psyllid attraction or repulsion. The best response was 1-wk-old psyllids with C. macrophylla shoots, although psyllid attraction to this odor source was weak. Research with white guava (germplasm from Vietnam) did not lead to any improvement. We will continue to investigate repellency of guava alone or in combination with citrus and refined methods to achieve more sensitivity.