Synergism of abiotic stresses and Hop Stunt Viroid infections causing citrus decline in Jahrom Orchards, Iran

Abstract

Citrus is an economically important crop in subtropical regions including Jahrom, Iran. Productivity of these fruit crops can severely decline due to biotic and abiotic stresses. Interactions were investigated between Hop Stunt Viroid (HSVd) infections and abiotic stress factors (drought and soil salinity) for effects on orchard decline. HSVd infection, a key biotic factor causing citrus cachexia, along with abiotic stresses of drought, high temperatures, and poor orchard management, compromise tree health by disrupting water balance and causing stunting and bark damage. Combined effects of HSVd infection and environmental stresses on citrus decline were assessed. Experimental treatments of healthy (experimental control), HSVd infected, abiotic decline- affected, and HSVd + abiotic decline was assessed in three citrus orchards. Compared with controls, trees from the other three treatments had reduced yields, plant heights, crown sizes, and fruit diameters. Plant heights varied among orchards, while fruit diameters did not. Healthy (control) plants grew and yielded best, while yields from plants infected with HSVd and subjected to climate change yielded least. Trees under climate-induced decline had increased fruit diameters, likely due to reduced fruit set under drought stress and redirected assimilates to reduced numbers of developing fruits. Similar responses have been reported in citrus under water stress conditions. Molecular diagnostics (RT-PCR and dot-blot hybridization) confirmed HSVd presence exclusively in symptomatic trees (samples of 10 trees per orchard), while assays for citrus Tristeza Virus (CTV) were negative. These results are consistent with HSVd being the primary biotic agent associated with the observed citrus decline, although causal confirmation requires controlled inoculation studies. Stresses negatively impacted citrus productivity in 85 orchards in Jahrom. Given the observed associations between HSVd presence, abiotic decline indicators, and reduced productivity, integrated citrus crop management is recommended. This should include reductions of drought (through optimized irrigation scheduling), and soil salinity (mitigation of sodium adsorption ratio), and appropriate phytosanitary measures. Phytosanitation should include use of certified viroid-free planting material, and targeted molecular surveillance for viroid infections. Implementation of these strategies should be guided by follow-up studies that verify absence/presence of other graft-transmissible pathogens, quantify rootstock effects, and test mitigation strategies in controlled and replicated field trials.