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

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.

摘要：木薯根腐病（Cassava root rot）是威胁木薯生产的主要病害之一，其中干腐病（Dry Root Rot, DRR）与黑腐病（Black Root Rot, BRR）为主要亚型。该类病害侵染木薯的核心经济产物——块根（tuberous roots），会导致产量逐步下降并影响后续种植周期，而种植抗病品种是田间最可靠的防控措施。本研究旨在针对干腐病（DRR）与黑腐病（BRR）的抗病性水平，筛选可用于基因型早期鉴定的接种方法。研究基于木薯组织（块根、茎切段（stem cuttings））浸渍法、土壤接种法（soil infestation）以及离体组织（detached tissues）接种法（涵盖叶片、块根、茎切段），对不同试验方案与植物组织材料展开评估。针对组织浸渍接种法，本研究测定了以下评价参数：病情指数（disease index, ω）、地上部鲜重（aerial part weight，单位：g）、块根鲜重（单位：g）以及块根体积（单位：cm³）；对于离体组织接种法，则通过测定病斑面积占比（percentage of lesioned area）开展抗性评估。在为期两个月的评估周期内，块根浸渍法与染病茎段浸渍法均未显现DRR与BRR的典型病症。在地上部鲜重降幅与病情指数两项指标上，土壤接种法与茎段浸渍法无显著差异，二者均可推荐用于抗病性筛选试验。茎段接种法的BRR与DRR抗性数据间呈显著正相关（r=0.94，p=0.001），块根表皮与髓部的DRR抗性数据同样呈正相关（r=0.73，p=0.05）。由此可见，不同组织的抗病性看似相互独立，其评价结果需结合基因型的田间表现进行综合比对。