Huanglongbing, also known as HLB and citrus greening, is a bacterial disease caused by the bacterium Candidatus Liberibacter asiaticus (CLas) and vectored by the Asian citrus psyllid (ACP, Diaphorina citri). Symptoms of HLB include asymmetrical chlorosis of the leaves, often referred to as “blotchy mottle.” Eventually, dieback of the shoot and major limbs can occur, and the entire tree may succumb to the disease (Bové, 2006). Fruit from affected trees often do not color properly, are of inferior quality, and can prematurely abscise. HLB affects all known commercial citrus genotypes and relatives. There is neither a cure nor a widely recognized effective management strategy (Albrecht et al., 2012).
The mobility of ACP has made managing the spread of HLB in Florida a challenge. By 2010, HLB had spread throughout the Florida industry, and as growers came to accept the widespread endemic reality of HLB, they abandoned attempts to control inoculum levels of CLas through eradication of symptomatic trees. Instead, Florida growers searched for cultural practices that would maintain production from HLB-affected trees until tolerant or resistant rootstocks and scion cultivars became available.
Pruning and applying plant nutrients through foliar sprays started two cultural practices that growers started implementing. Information on their overall effectiveness, however, has been limited. Pruning has been a recognized practice in Florida to rehabilitate freeze-damaged trees (Davies and Jackson, 2009; Fake, 2012; Wright and Kelly, 2008; Zekri and Rouse, 2013). If a freeze kills a significant portion of a tree’s canopy, the tree can be “buckhorned,” which means pruning a tree back to its scaffold limbs. The severe pruning results in the maximum stimulation and the strongest regrowth (Nesbitt, 2010; Rouse and Zekri, 2012). To our knowledge, buckhorning or pruning HLB-affected trees, however, has not been investigated before this trial. Severe root dieback occurs with HLB (Bové, 2006), and it was not known whether an affected tree could withstand the severe pruning to reestablish a canopy and produce an acceptable fruit yield.
Many Florida citrus growers have implemented foliar treatments of micro- and macronutrients as a method to satisfy a tree’s nutrient requirements after HLB has blocked nutrient flow via the phloem (Gottwald et al., 2012). The enhanced foliar nutritional treatments do not have bactericidal effects on CLas; however, they may be employed to maintain the nutritional health and production of HLB-affected trees (Rouse et al., 2010, 2012). Since 2006, some growers using foliar nutrition treatments on HLB-affected citrus trees have harvested ‘Hamlin’ and ‘Valencia’ oranges at yields that are close to those obtained before the appearance of HLB (Rouse et al., 2010, 2012).
Planting and replanting citrus were at an all-time low between 2006 and 2010. Many growers were reluctant to reset missing trees in existing groves or replant entire blocks because of concern that the young trees would rapidly succumb to HLB and never reach profitable production (Florida Agricultural Statistics Service, 2014). By 2010, the Florida citrus industry realized that there was a need to either begin replanting trees or wait for greater confidence in replanting. At the same time, there was mounting evidence that an enhanced foliar nutritional treatment was having a positive effect on the health of existing trees (Rouse et al., 2010, 2012).
The objectives of this study were 1) to evaluate the severe pruning in combination with foliar nutrition treatments on tree growth, yield and juice quality and 2) to evaluate the cost effectiveness of rehabilitating a tree through pruning and an enhanced foliar nutritional treatment.
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