Location:2010 Annual Report
1a. Objectives (from AD-416)
Develop control strategies that will minimize yield and fiber quality losses caused by nematodes and microbial pathogens that are emerging as significant impediments to sustained profitability by cotton producers. Of particular concern is the spread of the reniform nematode in the southern U.S. and south Texas, the emergence of a new disease called South Carolina seed rot in the southeast, the introduction of a particularly virulent isolate of Fusarium oxysporum f. sp. vasinfectum that was inadvertently imported from Australia on cottonseed and that could adversely affect 50% of U.S. cotton production, and the identification of race 4 of F.o.v. for the first time in the U.S. To address these emerging diseases we will: 1) Complete the introgression of reniform nematode (Rotylenchulus reniformis) resistance from Gossypium longicalyx and G. barbadense into Upland cotton (G. hirsutum) and develop markers for the trait suitable for marker-assisted selection; 2) Describe mechanisms of pathogenesis and identify virulence genes in the bacterial pathogen(s) causing the disease known as South Carolina seed and boll rot; and develop detection methods for the pathogen(s); and 3) Determine whether fusaric acid contributes to differences in virulence among races and biotypes of Fusarium oxysporum f. sp. vasinfectum to provide a basis for developing strategies for controlling Fusarium wilt of cotton.
1b. Approach (from AD-416)
1) Introgression of Reniform Nematode Resistance: Standard procedures for cotton flower emasculation and pollen transfer will be followed for backcross breeding. After each cross, progeny will be bioassayed for resistance to the reniform nematode, and the most highly resistant progeny will be retained for subsequent crossing, self seed generation, and DNA extraction for marker development. Plants will be grown under field conditions and evaluated for agronomic performance. Standard techniques and commercially available technology will be used for cotton DNA extraction. PCR amplification, electrophoresis, and fragment size detection will be utilized to identify molecular markers. 2) South Carolina Seed and Boll Rot: A mutagenesis system will be used to identify genes involved in production, regulation, and/or secretion of factors that cause boll rot. Based on these results, a set of predicted gene sequences associated with pathogenicity will be used to develop a PCR based method for detecting seed and boll rotting bacteria in field samples. Bolls from greenhouse grown plants will be used in initial testing to determine the efficacy of the developed amplification system. 3) Relation of Fusaric Acid to Virulence of Fusarium oxysporum f. sp. vasinfectum (F.o.v.): Biotypes of F.o.v. will be monitored for their ability to produce high levels of phytotoxins, and virulence of biotypes will be determined. The biosynthesis of those phytotoxins that correlate with virulence will be determined by feeding labeled substrates to the pathogens. Genes involved in the biosynthesis of these phytotoxins will be identified. Knock-out mutants will be generated to assess the role of specific phytotoxins in virulence and pathogenicity.
3. Progress Report
Work under this project during FY 2010 resulted in significant progress on all three objectives. In work on the reniform nematode, we discovered that susceptibility to the fungal root rot pathogens Thielaviopsis basicola, Pythium spp. and Rhizoctonia solani in the presence of the nematode was causing stunting in the field in cotton lines carrying the Renlon gene for immunity to the reniform nematode. Under the seed/boll rot objective, we determined that developing cotton bolls become immune to both insect damage and bacterial infections three weeks after flower fertilization. In addition, our results in greenhouse studies on transmission of boll rotting organisms by piercing/sucking insects was substantiated in field studies in cooperation with scientists in Georgia. Under the Fusarium oxysporum f. sp. vasinfectum (F.o.v.) objective, we developed a tomato seed germination bioassay to test the pathogenicity of the mutants generated in our laboratory that lack the ability to produce fusaric acid. The tested mutants were significantly less virulent than the wild type from which they originated; this is the first direct proof that fusaric acid is directly associated with the pathogenicity of F.o.v. to certain plants including cotton.
1. Second source of resistance to reniform nematode identified. In 2009, U.S. cotton producers lost an estimated $60 million to the reniform nematode, a microscopic worm that attacks the cotton root system and greatly weakens the plant. Previous ARS research identified a source of resistance on chromosome 11 from the wild cotton Gossypium longicalyx. Now a second source has been found by ARS researchers at College Station, Texas, and Starkville, Mississippi. This source was discovered in Gossypium barbadense accession #713. Two of the resistant genes are located on chromosome 21, and the strongest of these is linked to the DNA marker BNL3279-105. Advanced cotton lines carrying this marker are being evaluated in the field, and there is a high probability that new germplasm will be released to cotton breeders within a year. Lines derived from this germplasm have the potential to significantly increase cotton yields and fiber quality in reniform-infested cotton fields.
Medrano, E.G., Esquivel, J.F., Bell, A.A., Greene, J., Roberts, P., Bachelor, J., Marois, J.J., Wright, D.L., Nichols, R.L., Lopez, J. 2009. Potential for Nezara virdula (Hemiptera: Pentatomidae) to transmit bacterial and fungal pathogens into cotton bolls. Current Microbiology. 59:405-412.