No abstract
Wheat blast (WB), caused by Magnaporthe oryzae Triticum pathotype, recently emerged as a destructive disease that threatens global wheat production. Since few sources of genetic resistance have been identified in wheat, genetic transformation of wheat with rice blast resistance genes could expand resistance to WB. We evaluated the presence/absence of homologs of rice blast effector genes in Triticum isolates with the aim of identifying avirulence genes in field populations whose cognate rice resistance genes could potentially confer resistance to WB. We also assessed presence of the wheat pathogen AVR-Rmg8 gene, and identified new alleles. A total of 102 isolates collected in Brazil, Bolivia and Paraguay from 1986 to 2018 were evaluated by PCR using 21 pairs of gene-specific primers. Effector gene composition was highly variable, with homologs to AvrPiz-t, AVR-Pi9, AVR-Pi54 and ACE1 showing the highest amplification frequencies (>94%). We identified Triticum isolates with a functional AvrPiz-t homolog that triggers Piz-t-mediated resistance in the rice pathosystem, and produced transgenic wheat plants expressing the rice Piz-t gene. Seedlings and heads of the transgenic lines were challenged with isolate T25 carrying functional AvrPiz-t. Although slight decreases in the percentage of diseased spikelets and leaf area infected were observed in two transgenic lines, our results indicated that Piz-t did not confer useful WB resistance. Monitoring of avirulence genes in populations is fundamental to identifying effective resistance genes for incorporation into wheat by conventional breeding or transgenesis. Based on avirulence gene distributions, rice resistance genes Pi9 and Pi54 might be candidates for future studies.
Laurel wilt disease (LWD), caused by the ambrosia fungus Raffaelea lauricola, is a deadly vascular disease affecting numerous hosts in the Lauraceae family, including important forest species and avocado trees. The fungal pathogen is spread by beetle-vectors and through root grafts. Symptoms include leaf wilting, foliar desiccation, stem and limb dieback, and depending on the host, localized or systemic vascular damage. R. lauricola and its main vector, Xyleborus glabratus (Redbay Ambrosia Beetle), are invasive species that were introduced into the United States in 2002. This guide describes methods used to diagnose LWD in avocado. R. lauricola is isolated from infected sapwood or ambrosia beetles using the semi-selective medium CSMA (cycloheximide–streptomycin malt agar). Molecular identification is based on the amplification of the LSU (Large Subunit of the ribosomal cassette) or of the species-specific microsatellites IFW and CHK by conventional PCR. More current detection methods use DNA extracted directly from infected material and include the amplification of specific DNA regions through conventional PCR, qPCR, or LAMP. However, to date, there is no method that can detect the pathogen early in the infection process (<3 days) when the vascular tissue remains asymptomatic. Besides detailed protocols on how to diagnose the disease and confirm the presence of the causal agent, we include protocols on pathogen preservation and inoculation. Although this guide focuses on avocado, the techniques included here can be implemented to diagnose LWD in other susceptible species.
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