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Búsquedas previas al 2023, Núm. 3. En la sección Volúmenes 30 - 41 (2012 - 2023).
Biological and molecular characterization of nectrial species associated with an avocado decline syndrome
By Jeny Michua Cedillo, Gustavo Mora Aguilera*, Gerardo Acevedo Sánchez
* Corresponding Author. Email: / Institution:
Received: 30/July/2024 – Published: 31/December/2024 – DOI: https://doi.org/10.18781/R.MEX.FIT.2406-7
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Abstract Background/Objective. Nectriaceae members increased regional occurrence in Michoacán since 2019. However, root species identity, geographic distribution, and association to other families are unknown. The objective was to characterize biological and molecularly species of Nectria associated with Persea americana.
Materials and Methods. Seventy samples of wilt trees from 13 municipalities in Michoacán were processed. Thirty isolates selected based on epidemiological criteria were cultured in malt-agar, PDA, and oat-agar extracts to determine cultural and morphological characterization. Five morphotypes of Nectria with varying radial growth and brown coloration were obtained. From mycelial DNA, TEF 1-a and RPB2 genes were amplified, sequences were cleaned and aligned with SeqAssem and MAFFT, respectively. Bayesian inference and maximum parsimony phylogenetic algorithms were performed using PAUP 4.0 and MrBayes 3.2 complemented with 66 and 65 sequences from GenBank for TEF 1-a and RPB2, respectively. S. chartarum was used as the external species and four other Hypocreales.
Results. Bayesian inference revealed greater phylogenetic consistency. Three genera and three species were identified with TEF 1-a (>94 % homology) and three genera and five species with RPB2 (>97 % homology) belonging to Ilyonectria (56 %), Dactylonectria (33 %), Mariannaea (6 %), and Thelonectria (3 %). Associations of Nectria were observed mainly to Armillaria (97.1 %), Fusarium (92.9 %), Paecilomyces (56.4 %), and Morthierella (47.3 %).
Conclusion. A decline syndrome in avocado trees associated with a fungal complex characterized by descending defoliation, wilt, reduced fruit size, and root necrosis is postulated. This is the first report of Nectria associated fungi in avocado trees in Mexico.
Keywords: Hypocreales, Persea americana, Ilyonectria, Dactylonectria
Figure 1. A. Healthy tree and longitudinal and transversal section of 1.5 cm diameter root and association of Ilyonectria liriodendri; B. Apical defoliation, longitudinal and transverse root section with restricted necrosis associated to I. liriodendri; C. Fruit size reduction, longitudinal and transverse root cutting showing invasive necrosis associated to I. liriodendri; D. Defoliation and small leaves associated with Armillaria + I. liriodendri and root with medullary necrosis; E. Moderate wilt associated with Phytophthora + I. liriodendri and longitudinal root cutting showing restricted necrosis; F. Yellow canopy, small leaves and canopy reduction associated to Fusarium + Dactylonectria macrodidyma and root with medullary invasive necrosis. G. Progressive wilt and root necrosis, and invasive mycelia in root tissue associated with Armillaria sp.; H. Wilt, yellowing of canopy and root with subcortical necrosis associated with Phytophthora cinnamomi; I. Total wilt, necrotic foliage adhering to branches, root with lines of necrosis in vascular tissue associated to Verticillium sp.
Figure 2. Nominal scale of six classes of vigor and damage in secondary roots of avocado trees associated with four genera and five species of Nectria. 0. Healthy tree with 100% vigor and root without lesions; 1. Apical defoliation in upper branches, root with reddish lesions <1 cm in central tissue (80 % canopy tree); 2. Progressive apical defoliation in upper branches, yellowing in lower stratum of leaves, root with reddish lines in medullary parenchyma (75 %); 3. Defoliation, wilt, and root with invasive necrosis in subcortical and medullary tissue (50 %); 4. Partial defoliation, small leaves and root with necrosis in xylem and primary and secondary phloem (35 %); 5. Defoliated tree and root with invasive necrosis (15 %).
Figure 3. Cultural morphotypes of avocado tree’s root isolates with Nectria-associated symptoms molecularly identified with 97 and 64% homologies for RPB2 y TEF 1-α genes. A-D. Dactylonectria macrodydima; F-I. Dactylonectria novozelandica; J-M. Ilyonectria liriodendri; N-Q. Mariannaea samuelsii; R-T. Thelonectria lucida.
Figure 4. Cultural morphotypes in EMA at 2 % (A, B) and PDA (C, D) isolated from symptomatic avocado tree roots associated with Nectria and other organisms and identified morphologically. A. Armillaria sp; B. Armillaria gallica; C. Phytophthora cinnamomi; D. Fusarium sp.
Figure 5. Phylogenetic tree obtained by Bayesian inference analysis of sequences associated with RPB2 gene of Nectria isolates collected in commercial avocado trees. Nectria species from this study are indicated with the MICH prefix. The tree had S. chartarum (KM231994.1) as external reference. The scale-bar represents the expected number of nucleotide changes per site.
Figure 6. Phylogenetic tree obtained by Bayesian inference analysis of sequences associated with TEF 1-α gene of Nectria isolates collected in commercial avocado trees. Nectria species from this study are indicated with the MICH prefix. The tree had S. chartarum (KM231994.1) as external reference. The scale-bar represents the expected number of nucleotide changes per site.
Figure 7. Distribution and regional prevalence of soil and root communities of organisms putatively associated with avocado tree decline and wilt syndrome in 13 municipalities of Michoacán. A. Phylogeographic distribution of four Nectria genera. B. Phylogeographic distribution for six Nectria species. Armillaria spp. and Fusarium spp. are included as species of high regional prevalence (comparative purposes). Green-Yellow-Brown shows the epidemic intensity (low, moderate, and high) assessed by severity scale per tree sampled
Figure 8. Associativity plots of organisms isolated from avocado tree roots with symptoms of decline syndrome. A. Pearson's correlation (r) showing association between Nectria and other soil-root organisms; y B. Cross-correlation in pairs and ordered by level of associativity for the top 25 significant correlations.
Table 1. Characteristics of orchards from Nectria isolates selected for DNA extraction and amplification.
Table 2. Physicochemical characteristics of orchards sampled for Nectria isolates in avocado trees.
