Methodologies for selecting cassava with resistance to dry and black root rot under controlled conditions.
2018
SANTIAGO, C, H. | SANTANA, M. P. | CAIRO JUNIOR, L. R. | OLIVEIRA, S. A. S. de | OLIVEIRA, E. J. de
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.
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