Genetics of resistance to cotton leaf curl disease in Gossypium hirsutum

Twenty-two cotton varieties were screened for resistance to cotton leaf curl disease (CLCuD), a disease of viral origin, using three procedures: field evaluation, whitefly transmission assay and graft inoculation. Viral infection of cotton varieties was determined by visual symptom assessment as well as dot-blot and multiplex PCR diagnostic techniques. Crosses were made between the most susceptible variety (S-12) and highly resistant varieties (CP-15/2, LRA-5166 and CIM-443). All F-1 plants of these crosses were resistant, showing dominant expression of the resistance as well as the absence of extrachromosomal inheritance. The F-2 plants of the crosses CP-15/2 x S12, LRA-5166 x S-12 and CIM-443 x S12 exhibited a ratio of 13 resistant (symptomless) to three susceptible (with symptoms). Screening of the F-2 generation for virus infection by multiplex PCR further subdivided the resistant class into those exhibiting a high level of resistance (HR; PCR-negative) and those exhibiting resistance (R; symptomless, yet showing virus replication by PCR analysis). Hence, the final ratio was 3:10:3 (HR:resistant:susceptible). The F-3 progeny of susceptible F-2 plants segregated for resistance, indicating the probable presence of a suppressor gene (S). These findings are consistent with three genes being involved in G. hirsutum resistance to CLCuD, two for resistance (R-1CLCuDhir and R-2CLCuDhir) and a suppressor of resistance (S-CLCuDhir).