Piramiding Resistance to Main Cotton Diseases in Brazil

Since the 1990s, the cotton growing area increased drastically in the Brazilian savannah (cerrado), and, currently, more than 95% of cotton fiber is produced in this environment. The production system is based on extensive areas with few cultivars, aggravating the impact of diseases, mainly those without curative control, as bacterial blight - BB [caused by Xanthomonas citri subsp. malvacearum], cotton blue disease – CBD [caused by Cotton leafroll dwarf Virus - CLRDV] and the root-knot nematode – RKN [Meloidogyne incognita]. Segregating population was generated from a triple cross [(BRS 368RF x M-315) x BRS 430B2RF], where BRS 368RF and BRS 430B2RF are sources of resistance to CDB, and BB, and M-315 to RKN. Population was advanced in bulk for three generations. F4 plants were selected and genotyped to CBD, BB, and RKN using SSR molecular marker DC20027, CIR246, CIR316 and BNL3661 labeled with fluorochrome. These primers are linked to resistance genes that confer resistance to the three diseases. All plants that amplified the homozygous band pattern associated with all resistance genes were selected to progeny row tests. In 2016-2017 season 32 progenies with good agronomic traits were selected. In 2017-2018 season three independent field trials were conducted to evaluate incidence of CBD and severity of BB and RKN, as well agronomic traits, two in areas of non-occurrence of nematodes, and one in an area with high RKN populations. In greenhouse it was evaluated the gall index (GI) and egg mass index (EMI) in two independent trials. From the 32 lines, 18 were also tested to RKN reproduction factor (RF). All 32 lines were completely resistant to BB and CBD. GI, EMI and RF did not differ from the resistant control (M-315). These lines can contribute to the sustainable management of cotton diseases in a tropical environment.