Evidence for Seed Transmission of Xylella fastidiosa in Pecan (Carya illinoinensis)

Abstract

Pecan bacterial leaf scorch, caused by Xylella fastidiosa subsp. multiplex, is an economically significant disease of pecan with known detrimental effects on the yield of susceptible cultivars. In this study, endosperm was harvested from developing pecan seeds, and direct qPCR and sequencing were used to detect and confirm the presence of X. fastidiosa. DNA was isolated from mature seeds originating from seven trees, revealing a positivity rate up to 90%, and transmission of X. fastidiosa from infected seed to the germinated seedlings was found to be over 80%. Further epidemiological analyses were performed to determine where X. fastidiosa localizes in mature seed and seedlings. The highest concentrations of X. fastidiosa DNA were found in the hilum and outer integument of the seeds and the petioles, respectively. High-, medium-, and low-density seeds were harvested to determine the impact of the bacterium on seed density and seedling growth rate. The growth rate of seedlings originating from low-density seeds was significantly reduced compared to the medium- and high-density seeds. Despite the increased growth and germination rates, the high-density seed group had a greater proportion of samples that tested positive for the presence of X. fastidiosa by qPCR. The results demonstrate the ability of X. fastidiosa to colonize developing seeds and be efficiently transmitted from well-developed seeds to germinated seedlings. Continued research is needed to understand the plant-microbe interactions involved in the colonization of pecan seeds by X. fastidiosa and to develop effective phytosanitary approaches to reduce the risks posed by seed transmission.

Influence of Geographical Orchard Location on the Microbiome from the Progeny of a Pecan Controlled Cross

“…Fungicide treatment of pecan trees in orchards is a common practice where diseases such as scab negatively impact crop production. Seed-borne pathogens in other crops [ 30 ] are a common occurrence that negatively impact crop production. Since antibiotics are known to shift microbial communities in pecan [ 31 ], the effects that fungicide treatments may have on the microbial composition of maternal trees, and thus seed-to-seedlings, should be considered.…”

“…Research on wheat ( Triticum aestivum C. Linnaeus) leaves and common bean ( Phaseolus vulgaris C. Linnaeus) leaves revealed a shift in relative fungal abundance of epiphytic and endophytic fungi after fungicide treatments [ 32 , 33 , 34 ]. As reported in the literature, transmission of microbial organisms from seed to seedling is a natural occurrence [ 30 , 35 , 36 , 37 ]. Thus, if a shift in microbial composition of the maternal plant occurs, the microbiome of the resulting seeds may in turn be altered due to the influence that maternal plants have in shaping seed microbiomes [ 38 ].…”

“…Wells, the causative agent of pecan bacterial leaf scorch belongs to the family Xanthomonadaceae . Xylella fastidiosa has been shown to be endemic in GA orchards [ 45 ] and most recently seed to seedling transmitted in pecan [ 30 ]. Functional inference analysis of sequences from the families referenced in this study ( Supplementary Table S1 ) identified EC numbers that may be involved in the disease suppression of plants.…”

“…Studies on Brassica napus C. Linnaeus seeds have identified environment to have a stronger effect on the microbiome composition than the host genotype [ 27 ]. The microbiota present in different plant organs (i.e., leaves, stems, roots), however, do differ and are shaped by various factors [ 28 , 29 , 30 ].…”

The core microbiome of Carya illinoinensis (pecan) seedlings of different maternal pecan cultivars from the same orchard

“…Once in the seed coat, Xanthomonas travels through the intercellular spaces and spreads throughout the tissue of the seed (Zaumeyer, 1930). Seed to seedling transmission of microorganisms has also been reported in pecans with Xylella fastidiosa (Cervantes et al, 2022). A study by Brinkerhoff and Hunter (1963), found Xanthomonas malvacearum to be present in the embryos and seed coats of cotton.…”