LIBRO-CONGRESO-CITRUS

concerns have prompted governments to promote alternatives to the application of synthetic chemicals to
reduce risks and impacts of pesticide use. The development of novel resistance-inducing chemicals represents
an alternative to protect crops against pathogens. Over the last years we have been working on strategies
based on natural compounds and we have demonstrated that soil-drench treatment with hexanoic acid protect
citrus against Alternaria alternata. In the present work, we have analyzed the effectiveness of hexanoic acid
(Hx) in the control of ABS disease in ‘Fortune’ plants, and the physiological and metabolic changes associated
with the Hx treatment. Preliminary results show an improvement of all the physiological parameters analyzed,
including transpiration, photosynthesis or chlorophyll levels. Moreover, the metabolomics study of A. alternata
infection and hexanoic acid treatment show strong changes in the metabolic profile depending on infection
and treatment. In conclusion, these studies will shed light into the complexity underlying induced disease
resistance. Hx-IR represents an attractive tool for the molecular characterization of the priming phenomenon
and for the integrated pest management strategies with the advantage of being a natural compound.
S15P16
Strobilurin resistance of Alternaria alternata, the causal agent of Alternaria Brown Spot, in
Florida tangerine hybrid groves
Vega B., and Dewdney M.M.
Citrus Research and Education Center, University of Florida (CREC, UF), Plant Pathology, USA. mmdewdney@ufl.edu
In Florida, strobilurin fungicides are commonly used for Alternaria alternata control but are becoming less
effective. A statewide survey was initiated in 2010 to determine the sensitivity of A. alternata populations to
azoxystrobin and pyraclostrobin covering 8 counties and 46 blocks. Azoxystrobin and pyraclostrobin sensitivity
was determined via a resazurin-based microtiter assay. Resistance was widespread among counties and 69%
of isolates were highly resistant with EC50 (effective concentration to inhibit 50% growth) values greater than
5 µg/ml for azoxystrobin and 1 µg/ml for pyraclostrobin, while the mean EC50 values for sensitive isolates
were 0.1680 and 0.0216 µg/ml respectively. Cross-resistance was demonstrated by highly significant and
strong correlations (P < 0.0001). Mixed populations of sensitive and resistant isolates were in 37% blocks and
only resistant isolates in 50%. Disease severity was high in the cultivars ‘Minneola’ and ‘Dancy’ and low in
‘Sunburst’ and ‘Orlando’. Our observations showed that strobilurin resistance occurred more frequently on
susceptible cultivars with intense fungicide use (P < 0.0001). Partial sequencing of the cytochrome b gene was
used to evaluate the resistant genotypes and the typical resistance inducing amino acid substitution of G143A
was found. Two introns profiles were found in cytochrome b but did not change the isolate phenotype. Profile
II was more frequently associated with resistant isolates and sensitive isolates were often profile I.
S15P17
Evaluation of fungicide mixtures as replacement for carbendazim for Citrus Postbloom Fruit Drop control
in Brazil
Silva-Junior G.J. 1 , Spósito M.B. 2 , Marin D.R. 1 , and Amorim L. 3
1 Fundo de Defesa da Citricultura (Fundecitrus), Scientific Department, Brazil; 2 Escola Superior de Agricultura Luiz de Queiroz (Esalq),
Department of Crop Science, Brazil; and 3 Escola Superior de Agricultura Luiz de Queiroz (Esalq), Department of Plant Pathology and
Nematology, Brazil. geraldo@fundecitrus.com.br
Control of Citrus Postbloom Fruit Drop (PFD), caused by Colletotrichum spp., is based mainly on fungicide
sprays. Carbendazim was an important fungicide used for PFD control in Brazil, but recently, it was banned
in export citrus production. Thus, the search for other fungicides to control of PFD became necessary. In
this study, the effectiveness of carbendazim was compared to the mixtures trifloxystrobin/tebuconazole (Tri/
Teb) and cyprodinil/fludioxonil (Cyp/Flu). Two experiments were carried out in 2009⁄2010 season in 18 and
20–yr–old ‘Pera’ sweet orange groves in Sao Paulo State, Brazil. Four sprays of carbendazim (1000g⁄ha), Tri/
Teb (80 and 160g⁄ha) or Cyp/Flu (94 and 63g⁄ha) were performed at 7–day interval, starting at the green bud
stage. Untreated control (UTC) trees received no fungicides. The incidence of symptomatic petals, number of
persistent calyces and fruit set per branch, and yield per tree were assessed. In both experiments, only Tri/Teb
differed significantly from the UTC trees for all variables studied. While trees sprayed with Tri/Teb yielded 88
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