Cotton blue disease (CBD) is the viral disease which poses the greatest threat to cotton in Brazil. One efficient way of controlling this disease is by using resistant cultivars. However, the recent emergence of an atypical form of CBD (ACBD), caused by a new virus genotype capable of overcoming these resistant cultivars, is causing concern. Thus, the aims of this study were to evaluate the distribution of ACBD in the states of Mato Grosso (MT) and Goiás (GO), to determine the relationship between vector infestation level, disease incidence and yield, and to check the reaction of cotton cultivars to two viral isolates. In both cotton production areas, 1128 plots were surveyed and 6.5% showed plants with the virus, 97.3% and 2.7% with ACBD and CBD, respectively. In cultivars susceptible to ACBD, a positive linear relationship between changes in the levels of aphid infestation and incidence of viral infection was indentified, and a negative linear relationship between infestation level and yield. The maximum acceptable level of aphids up to 80 days after sowing for susceptible cultivars was approximately 15%. Although 83% of the cultivars were shown to be resistant to CBD, only 19.2% were resistant to ACBD. There was also a number of cultivars with considerable resistance to both isolates.

The factors that influence the ability of cotton to minimize yield loss despite parasitism by Rotylenchulus reniformis (i.e., tolerance) were evaluated for 12 cotton genotypes. Reproduction of R. reniformis and total length of the root system were measured under greenhouse conditions, and the relationship of those variables to yield loss caused by R. reniformis in infested fields was evaluated. Values for nematodes per gram of root and root length were standardized by setting the genotype with greatest value as 100% and then calculating a percentage for each genotype. There was significant variability among genotypes in yield loss, resistance, and root length. Average yield loss for the genotypes ranged from 10.4% for IAC 26RMD to 43.2% for IMACD 5675B2RF. The least nematode reproduction was on IAC 26RMD, which had 49.6% of the reproduction on the susceptible check, Deltapine 16. The genotype with the shortest total root length was 34% less than the genotype with the greatest length. There was a significant linear relationship between percentage yield loss caused by R. reniformis and root length and nematodes per gram of root, both expressed as a percentage of the maximum, represented by the following equation: Yield loss (%) = 16.1258 – 0.1918*(% maximum root length) + 0.3728*(% maximum eggs + vermiform/g of roots). We conclude that tolerance to R. reniformis in cotton is influenced by the size of the root system and the parasitic load on the plant (nematodes per gram of root). Management approaches that increase root growth may lower the parasitic load, thereby reducing losses in cotton to R. reniformis.

IMA 5801B2RF is a medium to short season cultivar with high fiber yield potential. The genes Bt cry1Ac and cry2Ab confer resistance to Lepidoptera and cp4-epsps confers tolerance to the herbicide glyphosate. The key feature of this new cultivar is resistance to ramularia leaf spot and root-knot nematode.

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