Impacts of HLB Disease on Citrus Fruit Preharvest Drop

Huanglongbing (HLB) disease is devastating the Florida citrus industry.  HLB was first confirmed in Florida in 2005, by 2010 HLB had been confirmed in every citrus production region in Florida; current estimates suggest that greater than 75% of all citrus trees Florida are infected with Candidatus Liberibacter asiaticus (CLas), presumed causal agent of the disease.  The disease has also been confirmed present in commercial and dooryard citrus in Texas, and a single dooryard tree in California.  Following initial CLas infection there is a lengthy latent phase prior to expression of disease symptoms, which eventually result in loss of productivity, poor fruit and juice quality and eventually tree death.  As disease symptoms escalate there has been a concomitant increase in pre-harvest fruit drop which translates into significant loss of yield.  We have initiated experiments to determine how HLB impacts citrus fruit detachment force (FDF) (abscission), the ultimate mechanism which leads to fruit drop.  We compared fruit detachment force for HLB-asymptomatic (AS) and HLB-symptomatic (S) fruit.  Fruit detachment force was determined for ‘Hamlin’ (early season variety) and ‘Valencia’ (late season variety) sweet oranges (Citrus sinensis).  Fruit detachment force was measured with a force gauge using either a straight pull method (to emulate fruit drop) or with a twisting method (to emulate hand harvesting).  Our results indicate that FDF differed was significantly lower for S than AS fruit regardless of pull method.  Rind plugging was significantly greater for AS fruit than for S fruit when the straight pull method was used.  Although ‘Hamlin’ fruit were sampled late in the harvest season when the greatest difference in FDF was expected, initial results with ‘Valencia’ indicate a greater difference in FDF than for ‘Hamlin’.  Our data confirm that FDF is reduced as a consequence of HLB and that the effect can be seen very early in fruit development.