Use of young plants for biological indexing of graft transmissible pathogens of citrus

In budwood certification programs, routinely used, technology-based laboratory methods for detecting known citrus pathogens need to be complemented with biological indexing to confirm the absence of known and unknown graft transmissible pathogens (GTPs). We report here a procedure for biological indexing using very young indicator plants, 75-90 days old. Assay results of traditional biological indexing and mini-plant indexing conducted for 51 quarantine accessions and 29 positives infected with known pathogens were compared. The evaluation indicated similar sensitivity of detection between the two methods. Traditional bioindexing requires premium greenhouse resources and a lengthy incubation period; the use of mini-plants for indexing can provide a viable alternative since the technique can be conducted with a fraction of the resources typically needed. In southern California, we were able to index for GTPs requiring cooler temperatures during summer by placing mini-bioindicator plants near the cooling pads in an evaporatively-cooled greenhouse. Indicator plants kept under favorable conditions expressed symptoms throughout the year. Limited greenhouse space is required due to the small size of the indicator plants, and they can be used for biological indexing within 3 months as compared to 9-12 months with traditional indicator plants. Young plants require much less space with 240 plants/m2 of bench space as compared to 30 plants/m2 using the conventional approach for biological indexing. The use of mini-plant biological indexing will enable citrus 'clean stock' and certification programs to function more efficiently with limited resources and facilities.

Automated summary

This study introduced a procedure for biological indexing using very young indicator plants and compared it with traditional biological indexing. The results showed that the two methods had similar sensitivity of detection. The use of mini-plant biological indexing could provide an efficient alternative with limited resources.