Impact of HLB on the physiological quality of citrus rootstock seeds and non-vertical ‘Candidatus Liberibacter asiaticus’ transmission

Abstract

Huanglongbing (HLB) is the main disease of citriculture worldwide, being mainly caused by the bacterium known as ‘Candidatus Liberibacter asiaticus’ (CLas), limited to the phloem and naturally transmitted by vector insects known as psyllids. For the production of citrus seedlings, it is necessary to use rootstocks that are propagated by seeds from fruits produced by mother plants grown in field, however being submitted to natural HLB transmission. Although there is no evidence of CLas transmission from seeds to seedlings, little is known about the effects of this bacterium on the physiological quality of seeds obtained from CLas-infected plants. Therefore, the aim of this work was to evaluate the physiological quality of seeds from ‘Rangpur’ lime and ‘Swingle’ citrumelo rootstocks, from CLas-infected plants, with symptomatic and asymptomatic branches. The parameters evaluated, in addition to fruit development in mother plants, were germination rate, tetrazolium test, electrical conductivity, seedling emergence in greenhouse, and vegetative development and molecular analysis via qPCR for the detection of bacteria (leaf petiole, fruit peduncles, roots, seed coats, and seed endosperm). The presence of CLas in plants of both rootstocks has negative impact on fruit development, as well as on the number of normal seeds. In addition, there is reduced germination and seed emergence. Although CLas was detected in different tissues (leaf petiole, fruit peduncles and seed coat, and seed embryo), no vertical bacteria transmission was detected. Results have shown that seeds from CLas-infected branches have impaired physiological quality, but without bacterial transmission from seeds to seedlings.