Location:2012 Annual Report
1a. Objectives (from AD-416):
Objective 1: Develop non-chemical tactics to reduce Fusarium oxysporum f. sp. vasinfectum (FOV) race 4 inoculum levels, focusing on optimization of solarization methods, impacts of rotational crops, and development of resistant or tolerant germplasm. Subobjective A: Optimize solarization methods focusing on effects of applied moisture levels and duration of coverage on reductions in race 4 FOV inoculum. Subobjective B: Determine the effects of common rotational crops on race 4 FOV inoculum levels. Subobjective C: Develop cotton breeding populations segregating for FOV resistance and evaluate resulting lines for resistance and fiber quality traits. Objective 2: Investigate the population biology of FOV in the U.S. cotton belt. Objective 3: Identify and validate molecular marker associations with fiber quality characteristics and resistance to FOV. Objective 4: Determine collection efficiency and fidelity of population estimates to true population densities for standard lygus sampling methods. Objective 5: Define host-plant relations and cotton cultivar-specific responses to lygus injury. Subobjective A: Define relationships between lygus feeding, feeding preferences, and fruit loss rates. Subobjective B: Develop evaluation criteria and screen a cross-section of available cotton germplasm for lygus tolerance. Objective 6: Evaluate the potential of Beauveria bassiana as a non-cotton season, ecologically-based lygus control tactic. Objective 7: Define key aspects of lygus overwinter ecology and adult diapause.
1b. Approach (from AD-416):
An interdisciplinary team consisting of a plant pathologist, geneticist, and two entomologists will collaborate with Federal, State, and University investigators to solve the major problems facing sustainable production of irrigated crops in the context of environmental stewardship. Efforts to improve insect sampling methods combined with new knowledge of crop responses to insect populations will result in more effective pest management rules and decreased pesticide use. Definition of pest overwintering ecology will provide guidelines for testing and developing ecologically-based management tactics using diseases specific to pest insects. A survey of Fusarium oxysporum vasinfectum (FOV) in the U.S. cottonbelt will indicate potential risks associated with known and undescribed races. Optimization of non-chemical, environmentally benign control tactics will provide safe and effective means of limiting the spread and impact of FOV. Conventional breeding methods in combination with modern molecular techniques will lead to cotton germplasm with improved yield and fiber qualities and increased resistance to insects and diseases.
3. Progress Report:
Per the FY2012 Consolidated Appropriations Act (H.R. 2112) passed by Congress and signed by the President on November 18, 2011, the Western Integrated Cropping Systems Research Unit has been terminated. This is the final report for project 5303-21220-005-00D. New genetic markers for cotton resistance to two strains of Fusarium wilt (FOV) and the root-knot nematode were identified. Collaborations with University of California scientists showed levels of resistance to FOV and root-knot nematodes varied with environment and the parent contributing the resistance gene. These findings improve understanding of gene interactions between resistant and susceptible cottons and aid in marker-assisted breeding. Studies using solar energy to reduce populations of FOV spores (solarization) indicated temperatures of 40 degrees C in moist soil caused rapid spore death. These findings promote efforts to optimize solarization for FOV control. A survey documented new strains of FOV from the southeastern U.S. Most strains were weak pathogens, but two isolates from Alabama were highly damaging to some cotton varieties. These results accentuate the need for cottonbelt-wide monitoring for infections by novel FOV isolates. Tests to screen cotton for resistance to race 4 of FOV (a critically important problem currently unique to California) traditionally employ easily propagated conidiospores. However, field infection is usually caused by a different spore type (chlamydospores). Improved methods of culturing chlamydospores allowed their use in resistance assays. In these assays, chlamydospores produced greater disease responses compared with conidiospores, allowed detection of moderate levels of resistance, and showed that FOV reproduction increased with increasing cotton susceptibility. Results were consistent with field observations of FOV infection, and provide insights into the role of plant susceptibility in FOV management. Greenhouse studies of plant bug (lygus) feeding injury to cotton indicated young adults cause more injury than older adults, and that the oldest nymphs cause the most injury. Behavioral assays indicated lygus prefer to feed on large flower buds (squares) compared with small squares or fruit (bolls), and that small nymphs tended to feed and rest within the bracteoles of large squares whereas large nymphs and adults tended to feed on small squares in the plant terminal. These results lend insights into the dynamics of lygus injury, and provide the basis for improved assays to quantify lygus injury and to screen cotton cultivars for resistance. Field studies showed that the sweep net sampled older adult lygus more effectively than young adults in Pima cotton, but sampling effectiveness was similar between the two age classes in Upland cotton. Studies to define effects of temperature on lygus egg and nymph development, and on adult reproductive development were completed, as were studies demonstrating the incidence of reproductive dormancy in adult lygus diminishes in response to laboratory rearing. These collective results provide insights into lygus ecology that are necessary to design improved ecologically-based management strategies.
1. Improved resistance assays for Fusarium wilt of cotton. Fusarium wilt caused by race 4 is the most important disease of cotton in California and poses a severe threat to other U.S. cotton production regions. An ARS scientist in Shafter, California developed an improved method of producing Fusarium spores (chlamydospores) in the laboratory, more efficient processes of handling large numbers of plants in greenhouse assays, and methodology to quantify Fusarium reproduction in infected plants. When combined, these methods facilitate rapid, high-throughput assays that provide results that are more consistent with field observations than are provided by established assay methods. These improved assays provide public and private breeders an efficient, economical, and accurate means of rapidly screening cotton germplasm for resistance to race 4 of Fusarium wilt.
2. Lygus age-class differences in injury to cotton. Plant bugs (lygus) currently constitute the most important pest complex attacking U.S. cotton. Results of previous efforts to quantify lygus-induced injury to cotton were highly variable. ARS scientists at Shafter, California used a combination of laboratory and greenhouse studies to identify and partition sources of variation influencing lygus-plant assays. Collectively, these studies provide evidence that the lygus age class most injurious to cotton is the oldest nymphs, followed by young adults. Observed injury was also consistent with the distributions of lygus age classes within plants. These results provide insights necessary to design efficient assays for screening cotton germplasm for lygus tolerance, and for improving economic thresholds used to determine the need for insecticide treatment.
Cooper, W.R., Spurgeon, D.W. 2012. Oviposition behaviors and ontogenetic embryonic characteristics of the western tarnished plant bug, Lygus hesperus. Journal of Insect Science. 12:36.