The 2012 ASHS Annual Conference

The following study was conducted to determine if soil applied fertilizer can affect movement of Candidatus Liberibacter asiaticus, the putative causal agent of citrus greening, into new growth flushes of citrus in the absence of its primary vector, Asian psyllid. A 2 cultivar x 2 Liberibacter  x 2 fertilizer experiment was conducted in an enclosed greenhouse. Liberibacter was introduced by grafting two infected buds on each tree in August, 2010.  In October, 2010, the fertilizer was terminated on one half of the trees. Two growth flushes occurred in March/April and May/June and leaves from each flush were removed for sampling. Plants with Ct values between 32 and 36 were removed from the study since in this range it is unclear if they are infected. Of the remaining plants, infected trees had infection rates ranging from 64% to 100 %. Uninfected trees had a 0% infection rate. The infection rate tended to be higher for ‘Valencia’ than ‘Hamlin’ and was not affected by fertilizer treatment.  Incidence of the disease symptoms leaf mottling, Zn island greening, veinal chlorosis, and vein corking were much higher on trees that did not receive fertilizer. There was a significant interaction between fertilizer treatment and Ct value for P and Mn with unfertilized trees having low levels of both nutrients across Ct values and fertilized trees having declining Ct values. Ca and Mg did not exhibit the same pattern as P and Mn perhaps because of the high concentrations of these nutrients in the well water used to water the trees.  Zn declined with Ct value with fertilization having little effect. The fertilization treatments in this study were extreme and yet had little effect on the incidence of Liberibacter in the two growth flushes that occurred after infection.