Cultivars and IPM Systems For Organic Cotton Production: Field Mapping Populations For Thrips Resistance.

Early-season thrips (Thysanoptera: Thripidae) pest management is an important issue in cotton (Gossypium hirsutum L.) production on the Texas High Plains, especially in certified organic systems where synthetic pesticide use is prohibited.  Use of resistant cultivars has the greatest potential for reducing the economic effects of these pests, but there are few options available for commercial production.  Identifying and utilizing molecular markers for thrips resistance could significantly accelerate the cultivar development process and subsequent availability of cultivars for commercial organic production.  Field trials were conducted in 2012 and 2013 to evaluate a population for potential genetic mapping for thrips resistance.  An interspecific cross was made between a G. hirsutum line (‘07-7-1407CT’) and a thrips-resistant, day-neutral G. barbadense cultivar (‘Cobalt’).  The 2012 trial included both parents, 40 F₁ individuals, 247 F₂ progeny, and resistant (‘TX 110’) and susceptible (‘Atlas’) checks.  Thrips injury was assessed using a visual damage rating, on a 1 to 9 scale (1 = necrosis; 9 = undamaged).  The 2013 trial included the parents and 204 F₃ families.  Visual ratings of the F₃ generation (2013) were conducted on both individual family members and for the entire family (progeny rows).  Ambient thrips pressure was greater at the 2013 location than in 2012.  In 2012, Cobalt and TX 110 sustained less injury than Atlas (P ≤ 0.05).  In 2013, however, TX 110 appeared severely damaged by thrips.  This was likely due to inadequate stand establishment and subsequent leaf development, as a result of poor seed quality for the accession that year.  Damage ratings were not different among the other parents and susceptible control (P > 0.05).  A continuous distribution of phenotypes was observed among the F₂ and F₃ individuals, alluding to a resistance mechanism that is multigenic and thereby a possible candidate for QTL analysis to accelerate the breeding process.