Variabilidade genética e resistência de fungicidas em populações de Corynespora cassiicola provenientes de soja
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2018-12-11
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Universidade Federal de Goiás
Resumo
The fungus Corynespora cassiicola (Berk. & M. A. Curtis) Wei belongs to the phylum
Ascomycota and has been reported as a pathogen in more than 402 plant species. Among the
hosts of C. cassiicola is the soybean of which is the causal agent of the target spot and root
rot of Corynespora. The intensive use of fungicides to control diseases in soybean (mainly
Asian rust) impacts populations of pathogens, and monitoring is necessary to detect the
emergence of isolates resistant to the main fungicides. The efficient control of diseases
depends on several factors, among them the correct identification and the knowledge about
the genetic variability of the phytopathogen. The objective of this study was to: i) identify the
isolates using molecular techniques and evaluate the genetic diversity of 203 C. cassiicola
isolates obtained from soybean plants using molecular markers ISSR; ii) to determine the
EC50 and EC98 of seven fungicides used in the control of Asian rust against "wild" isolates of
C. cassiicola; iii) to evaluate (from EC50 and EC98) the sensitivity of C. cassiicola isolates
collected from areas where systematic applications of fungicides for the control of Asian rust
have been carried out; iv) to study the possible differences between C. cassiicola isolates from
leaves and roots of soybean through the morphological and molecular characterization,
genetic variability from ISSR molecular markers and cross pathogenicity between both groups
of isolates; v) determine the EC50 and EC98 for the isolates from leaves and roots of
soybean. All 203 leaf isolates with symptoms of target spot were identified as C. cassiicola.
The genetic diversity of C. cassiicola was analyzed with nine ISSR primers, with 100%
polymorphism. The Mantel test showed a positive correlation (r = 0.91) between the
geographic distance and the genetic distance. Through AMOVA, it was possible to observe a
medium structure for the state and high for the municipality of origin, suggesting gene flow
among the populations of C. cassiicola from soybean crops. For the determination of the EC's
50 and 98, there was variation in sensitivity among the "wild" isolates of C. cassiicola,
indicating an existing variability in this fungal species. The pathogen showed variability in
sensitivity to the fungicidal groups triazoles (DMI) and strobilurins (QoI), which shows a high
risk of selection for resistance. In the studies related to the differentiation between isolates
from leaves and soybean root (target spot and Corynespora root rot), it was possible to
observe great differentiation between both groups of isolates. In the molecular
characterization there was a difference between the results of the primers specific for C.
cassiicola (CCF-1 / CCR-1 and GA4F / GA4R) and also in the sequencing for the two genes
(ITS and β-tubulin). The genetic diversity with ISSR primers obtained a percentage of
polymorphism of 97.2%. The genetic dissimilarity values, calculated according to the
complement of Dice index ranged from 0 to 0.797, reflecting a high genetic variability among
the groups of C. cassiicola isolates. Through AMOVA, it was possible to observe a high
structure between the two groups of isolates, referring to the plant tissue from which they
come (root and leaf). The cross-pathogenicity test demonstrated the ability of the two groups
of isolates to infect both plant tissues (leaf and root), but presenting differences in the speed
and intensity of symptoms. In the evaluation of the sensitivity of the isolates to fungicides, it
was possible to observe that the root sensitivity to fungicides, and the EC50 and EC98 doses
estimated for this group were also lower than those determined for leaf isolates. From the
results obtained in this study, it is possible to say that the two groups of isolates from
soybean (leaf and root) do not belong to the same fungal species.
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Citação
JESUS, J. M. I. Variabilidade genética e resistência de fungicidas em populações de Corynespora cassiicola provenientes de soja. 2018. 179 f. Dissertação (Mestrado em Agronomia) - Universidade Federal de Goiás, Goiânia, 2018.