Methodologies for selecting cassava with resistance to dry and black root rot under controlled conditions

Camila Hohenfeld Santiago,Luiz Rodriguez Cairo Junior,Mariana Pereira Santana,Saulo Alves Santos De Oliveira,Eder Jorge De Oliveira

doi:10.1590/1678-4499.2017298

Camila Hohenfeld Santiago, Luiz Rodriguez Cairo Junior + Show 3 more

Open Access

https://doi.org/10.1590/1678-4499.2017298

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Abstract

ABSTRACT Cassava root rot diseases such as dry and black root rot are listed among the major threats, since its affect the main product (tuberous roots), causing a progressive decline in yield and affecting subsequent crop cycles, being the use of resistant varieties the most reliable control measure on field level. The objective of this study was to identify inoculation methods for the early evaluation of genotypes, considering the level of resistance to dry (DRR) and black (BRR) root rot diseases. Different methodologies and plant tissues were evaluated, based on the immersion of cassava tissues (roots and stem cuttings), soil infestation, and inoculation of detached tissues (leaves, tuberous roots and stem cuttings). The following parameters were evaluated for inoculations based on tissue immersion: disease index (ω); aerial part weight (g); fresh weight of the roots (g); and volume (cm3). For the inoculations on detached tissues, the percentage of lesioned area was determined. Immersion methods for roots and lesioned stems did not show typical symptoms of DRR and BRR during the two-month evaluation period. The soil infestation method did not differ from the stem immersion method as to the reduction of aerial part weight and the disease index, whereas both can be recommended for resistance selection trials. There was a positive correlation between the BRR and DRR data for the stem inoculation (r = 0.94, p = 0.001) and for DRR in the peel and root pulp (r = 0.73, p = 0.05). Therefore, the resistance within each tissue is apparently independent and should be compared with the behavior of the genotypes in the field.

Highlights

MATERIAL AND METHODSDry (DRR) and black (BRR) root rot are two of the most destructive pathologies in cassava (Manihot esculenta Crantz), whose economic importance is increasing in major producing countries (Onyeka et al 2005a; Vilas Boas et al 2017)
The other methodologies developed symptoms similar to what was previously reported for each pathogen, enabling the re-isolation thereof and in such, completing Koch’s postulates (Oliveira et al 2013), and were effective in determining varieties of cassava that are resistant to black root rot (BRR) and Dry root rot (DRR) diseases under controlled conditions
In the methodology based on the immersion of cassava stem cuttings in spore suspension of dry (IM-DRR) and black (IM-BRR) root rot pathogens, except for the control, all treatments showed typical symptoms of the disease, such as wilting and yellowing of leaves, in an average period of 45 days after inoculation (DAI), culminating in the death of the plant at 60 DAI

Summary

Introduction

MATERIAL AND METHODSDry (DRR) and black (BRR) root rot are two of the most destructive pathologies in cassava (Manihot esculenta Crantz), whose economic importance is increasing in major producing countries (Onyeka et al 2005a; Vilas Boas et al 2017). This study aimed to identify inoculation methods for different plant tissues, for evaluating the levels of dry and black root rot resistance in cassava genotypes.

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Results

Conclusion