Phenotypic variability among isolates of Ramularia areola from Brazilian cotton

Phenotypic variability among isolates of Ramularia areola from Brazilian cotton

Larissa F. Pezenti^I; Juliana Barbosa^I; Mariane A. Vieira^I; Mariana S. Marangoni^I; Jessica Volponi^I; Wilson P. Almeida^I; Rafael Galbieri^II; Yeshwant R. Mehta^I

^IInstituto Agronômico do Paraná (IAPAR), Rod. Celso Garcia Cid, km 375, Bairro Três Marcos, 86047-902, Londrina, PR, Brazil

^IIInstituto Mato-Grossense do Algodão (IMA), Cx. Postal 149, 78850-000, Primavera do Leste, MT, Brazil

ABSTRACT

There is a lack of information about the level of resistance of cotton genotypes to a wider range of Ramularia areola isolates occurring across the cotton growing areas of Brazil. For this purpose, firstly it is necessary to know the existence or not of genotypic and phenotypic variability among the R. areola isolates from different geographical origins. The objective of the present investigation was to verify the existence of phenotypic variability among 23 R. areola isolates collected from six cotton growing states of Brazil. Two resistant genotypes, FMT 02102996 and CNPA BA 2003-2059, and the susceptible genotype FMT 701 were individually inoculated with 23 R. areola isolates under glasshouse conditions and the severity of infection was evaluated 30 days after inoculation. Genotypes CNPA BA 2003-2059 and FMT 02102996 were susceptible to three isolates and resistant to the rest of the isolates. Genotype FMT 701 was susceptible to all the isolates except the isolates 22.3 and 42.7. Results indicate the existence of variability among R. areola isolates and that the three genotypes are useful in distinguishing phenotypic variability within isolates of this pathogen.

Key words: Gossypium hirsutum, Ramularia leaf spot, varietal resistance.

Ramularia leaf spot of cotton caused by Ramularia areola can be controlled through the use of resistant cultivars provided sources of resistance to this pathogen are made available. However, there is a lack of information about the level of resistance of cotton genotypes to a wider range of Brazilian R. areola isolates. For this purpose, firstly it is necessary to understand about the existence or not of genotypic and phenotypic variability between the isolates occurring across the cotton growing areas of Brazil.

Although occurrence of the teleomorph of R. areola in nature is not known, there is an indication of existence of "field strains" of this pathogen in Brazil. Frequently inconsistent reaction under field conditions of some resistant cultivars is observed, possibly because of the existence of such field strains, resulting in frustration for breeders as well as for cotton farmers (Cia et al., 2010, 2011). Nonetheless, this situation can be altered provided knowledge about some of the fundamental aspects, like phenotyping and genotyping of R. areola isolates, is made available. Phenotyping and genotyping are two important steps for genetic breeding that help in developing a differential set of cultivars capable of distinguishing "field strains" of R. areola originated from different geographical regions and/or from different Gossypium species. This consequently leads to the identification of sources having a wide spectrumofresistance. Once such sources of resistance are identified they can be used in national breeding programs aimed at pyramiding resistance genes in commercially desirable cultivars.

In earlier studies, Novaes et al. (2011) and Zandoná et al. (2012) identified genotypes CNPA BA 2003-2059 and FMT 02102996 as resistant to the isolate 44 of R. areola, collected from Ipameri, State of Goiás, Brazil, from the culture collection of Instituto Agronômico do Paraná (IAPAR). Zandoná et al. (2012) also reported that each one of the two resistant genotypes has a different dominant gene. Some other resistant genotypes to isolate 44 were also identified by Barbosa et al. (2011). However, their resistance is not found to be stable under variable field conditions (Wilson P. Almeida, personal communication). The objective of the present investigation was to verify the phenotypic variability among 23 R. areola isolates of different geographical origins to aid in the identification of sources with a wider spectrum of resistance.

Seeds of three genotypes, two resistant (CNPA BA 2003-2059 and FMT 02102996) and one susceptible (FMT 701) to the isolate 44 of R. areola were provided by Camilo de Lelis Morello (Embrapa Algodão, Campina Grande, PB, Brazil) and Paulo H. Aguiar (Fundação MT, Rondonópolis, MT, Brazil). Plants of both resistant genotypes were segregating for resistance to R. areola under glasshouse inoculations. For this reason, plants of each genotype were inoculated during two successive generations to purify and to confirm their resistant and susceptible reactions. Seeds derived from second Phenotypic variability among isolates of Ramularia areola from Brazilian cotton generation by selfing a single plant of each one of the genotypes were further multiplied and used to make sure the homozygosity of the genotypes (Zandoná et al., 2012).

Leaf samples showing typical disease symptoms were collected from different cotton growing areas of Brazil by FRAC (Fungicide Resistance Assessment Committee) technicians between 2009 and 2010, and were received in the laboratory for monopustuler isolations. Twenty-three R. areola isolates representing different geographic areas of Brazil were stored in V8agar for further studies.

Fungal isolates were multiplied for 7-10 days on V8juice agar, the conidia were scraped from the sporulating cultures, conidial suspension of the inoculum was adjusted to 10⁴ /mL, and a few drops of Tween 20 were added to the inoculum. Twenty-five days old plants were inoculated using a hand sprayer and an approximately equal amount of inoculum per plant was sprayed. Twelve plants of each genotype were inoculated separately by each one of the 23 isolates of R. areola. Soon after inoculation the plants were incubated in a growth chamber at 21ºC in the dark and to near saturated humidity for 48 h. After this period the plants were transferred to the glasshouse bench and randomized. The glasshouse temperature varied between 20 and 30ºC. Disease severity on the leaves was assessed 30 days after inoculation using a visual scale between 0 and 100% of the leaf area infected (LAI).

The genotypes CNPA BA-2003-2059 and FMT 02102996 identified in earlier studies as resistant to R. areola isolate 44 (Novaes et al., 2011) showed variation to some isolates originated from different geographical origins. These genotypes showed a susceptible reaction to three isolates (13.2, 17.5 and 58.4) originated from the states of Paraná (PR), São Paulo (SP) and Mato Grosso do Sul (MS), respectively (Figure 1, Table 1), but a resistant reaction to the rest of the isolates. With the exception of isolates 22.3 and 42.7, genotype FMT 701 was susceptible to all isolates of R. areola.

As stated earlier, isolates of R. areola used in the present investigation were collected from different geographical origins in six Brazilian states which are representatives of the major cotton growing regions of Brazil. The results indicate the existence of variability among R. areola isolates and show that the three cotton genotypes used in the present investigation are useful in distinguishing phenotypic variability in the pathogen. While these genotypes are capable of distinguishing a few isolates, further research on the identification of other genotypes suitable to distinguish differences among the isolates will be necessary in order to develop a set of differential cultivars.

Phenotyping and genotyping form basic pillars for breeding resistant cultivars and for this reason due emphasis needs to be given in the future to this kind of investigation. Such studies would assist in establishing a set of differential cultivars for the "field strains" of R. areola occurring in different geographical regions, and consequently would help towards a more efficient breeding.

The results also indicate that the resistance of genotypes CNPA BA 2003-2059 and FMT 02102996, although governed by a single dominant gene, may not be stable and may be isolate dependent. These results would create stimulus for further work on phenotypic and genotypic variability among the Brazilian R. areola isolates so as to improve our knowledge of the cotton - R. areola interaction and to develop a broader set of differential cotton cultivars.

Considering the fact that the inheritance of resistance is simple, pyramidation of resistance genes seems feasible. The use of resistant cultivars would avoid the constant use of fungicides and would lower the cost of cultivation.

ACKNOWLEDGEMENTS

Thanks are due to the technical assistance provided by Bonnie Vieira and Tanara Novaes (IAPAR).

Received 19 October 2012

Accepted 4 April 2013

Author for correspondence: Y.R. Mehta, e-mail: [e-mail]

TPP 2012-0113

Section Editor: Rosana Rodrigues

• Barbosa J, Séleri A, Novaes TG, Almeida WP, Fonseca N, Mehta YR (2011) Resposta diferencial dos genótipos do algodoeiro brasileiro a Ramularia areola In: VIII Congresso Brasileiro de Algodão, Resumos... Campina Grande PB. Embrapa Algodão. pp. 447-453.
• Cia E, Galbieri R, Fuzatto MG, Kondo JI, Oliveira AB (2010) Indicativo de variabilidade de Ramularia areola em algodoeiro no Brasil. Summa Phytopathologica 36 (CD-Rom) (Abstract)
• Cia E, Fuzatto MG, Kondo JI, Ohl GA, Galbieri R (2011) Severidade da mancha da Ramularia em genótipos de algodoeiro em duas regiões produtoras do Brasil. In: VIII Congresso Brasileiro de Algodão, Resumos... Campina Grande PB. Embrapa Algodão. pp. 454-459.
• Novaes NT, Almeida WP, Schuster I, Aguiar P, Mehta YR (2011) Herança de resistência do algodoeiro a Ramularia areola Summa Phytopathologica 37:150-152.
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Publication Dates

• Publication in this collection
  02 Aug 2013
• Date of issue
  Aug 2013

History

• Received
  19 Oct 2012
• Accepted
  04 Apr 2013