ARS | Publication request: Under severe HLB and citrus canker pressure, Triumph and Jackson are more productive than flame and marsh grapefruit

Submitted to: Proceedings of Florida State Horticultural Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 18, 2012
Publication Date: N/A

Interpretive Summary: Huanglongbing (HLB) and Citrus Canker (CC) threaten the viability of Florida grapefruit production. We compared tree health and productivity of two grapefruit-like cultivars (‘Triumph’ and ‘Jackson’) to two standard grapefruit cultivars ( ‘Marsh’ and ‘Flame’) over a 3 year period in a site with widespread HLB and CC. In each year, overall tree health of the two new cultivars was significantly better than that of the two standard cultivars. Citrus canker was less severe in the two new cultivars than on the two standard cultivars. Leaf symptoms of HLB in the fall were similar among varieties and no significant differences in the incidence and severity of HLB were found among the four cultivars. However, under disease pressure, the new cultivars were generally better than the standard cultivars with respect to tree growth (larger canopies at least during some years, persistence of symptomatic leaves on trees, and increased tree height although no differences were found among the cultivars with respect to trunk diameter) and productivity (greater numbers of fruit per tree, normal fruit size/shape, less fruit drop, and consistently of higher fruit quality). These results suggest that ‘Triumph’ and ‘Jackson’ and perhaps other grapefruit-like varieties may be viable alternatives to standard grapefruit varieties in the presence of severe HLB and CC. Furthermore, it provides evidence that useful tolerance to HLB exists within conventional citrus varieties.

Technical Abstract: Huanglongbing (HLB) and Citrus Canker (CC) threaten the viability of Florida grapefruit production. ‘Triumph’ (T), reportedly a grapefruit/sweet orange hybrid, is similar to seedy white grapefruit with earlier maturity and lower bitterness. ‘Jackson’ (J) is a low-seeded budsport of ‘Triumph’. Tree health and productivity of T, J, and standard cultivars ‘Marsh’ (M) and ‘Flame’ (F) planted in a replicated field trial were assessed for 3 years, in a site with endemic HLB and CC. In each year overall tree health of T/J was significantly greater than M/F. Severity of CC was significantly less on T/J than on M/F, while foliar HLB symptoms in the fall were similar among cultivars. Titers of Liberibacter asiaticus were assessed by PCR in January 2010: there were no significant differences among cultivars in random leaf samples or most-symptomatic diagnostic samples in the 2010 analyses and in 2012. M/F developed very thin canopies while T/J had normal canopy density. T/J had extensive blotchy mottle characteristic of HLB on leaves year-round while M/F trees appeared to drop severely HLB-symptomatic leaves in the winter. Cumulative numbers of fruit/tree were greater for T/J (255/220) than for M/F (29/66). Tree height of T/J was slightly greater than in M/F. Canopy volume was greater in T/J than M/F in some years, but trunk cross sectional area (TCSA) was not different, and there were no cultivar differences in TCSA increase over the study period. Cumulative percent fruit drop was greater in M/F than T/J (F=50; M=53; T=15; J=14). Fruit quality assessments were made each cropping season. T/J fruit always met commercial maturity standards whereas M/F fruit usually did not due to low total soluble solids and low Brix/acid ratios. In 2011/2012 many M/F were small and/or misshapen while T/J fruit were of normal size and shape. These results suggest that T/J or other grapefruit-like cultivars may be viable alternatives to standard grapefruit cultivars in the presence of severe HLB and CC, with apparent tolerance to HLB in T/J. Furthermore, it provides evidence that useful tolerance to HLB exists within conventional scion genotypes.