Seleção recorrente de Urochloa ruziziensis (R. Germ. e C.M. Evrard) visando a resistência às cigarrinhas-das-pastagens

Abstract

The forage Urochloa ruziziensis (R. Germ. and C.M. Evrard) Crins (synonym Brachiaria ruziziensis) has great potential to be adopted as pasture due to its good nutritional quality, among other advantages. However, this forage is susceptible to spittlebugs, the main insect pest of pastures in Brazil. Thus, the aim of this study was to select U. ruziziensis genotypes resistant to the spittlebug over 16 cycles of recurrent selection and to validate resistance in the population derived from the selected genotypes. Chapter 1 consists of the article published in the journal “Agronomy.” In this work, 13,114 U. ruziziensis plants were evaluated for resistance to spittlebugs in experiments conducted between 2008 and 2023. Each plant was infested with six eggs of the insect pest at the last embryonic stage (S4) and kept in a greenhouse. After 35 to 40 days, the surviving nymphs were counted. The original population of U. ruziziensis (POP01-2008) provided nymphal survival of 63%, regardless of the spittlebug species, while for the improved population (POP36-2023) the average nymphal survival was 32.8%. After 16 selection cycles, it was found that approximately 63% of the genotypes, or 716 plants, exhibited nymphal survival equal to or less than 33%, indicating their resistance to the nymphs of D. schach or M. spectabilis. Next, in Chapter 2, we investigated the validation of resistance in the progeny obtained from U. ruziziensis progenies from sixteen cycles of recurrent selection. In addition to the selected genotypes, nymphal survival was assessed on commercial U. ruziziensis, cv Kennedy (treatment control). Each plant was infested with six eggs of the insect pest at the S4 stage and kept in a greenhouse. Following this period, we evaluated the nymphs’ survival and calculated the nymphal survival from this data. For the D. schach species, average nymphal survival in the different progenies ranged from 19 to 64%, with an overall average of 36.3%. For M. spectabilis, the overall average nymphal survival was 25.4%, with a range from 0 to 57%. We classified 21 progenies as resistant to D. schach and 73 progenies as resistant to M. spectabilis. In addition, we identified 19 progenies as simultaneously resistant to both insect pest species. Spittlebug nymphs had a much lower chance of survival in the improved U. ruziziensis progenies than in the commercial U. ruziziensis, demonstrating the effectiveness of recurrent selection in obtaining forage plants resistant to these cercopids. The population obtained by the forage improvement program shows genetic variability and is getting closer to obtaining U. ruziziensis cultivars resistant to D. schach and M. spectabilis nymphs.