Submitted to: Canadian Journal of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 10, 2011
Publication Date: October 17, 2011
Citation: Liu, J., Bell, A.A., Wheeler, M.H., Stipanovic, R.D., Puckhaber, L.S. 2011. Phylogeny and pathogenicity of Fusarium oxysporum isolates from cottonseed imported from Australia into California for dairy cattle feed. Canadian Journal of Microbiology. 57:874-886. Interpretive Summary: A unique wilt causing fungus called Fusarium oxysporum found in Australia in 1993 is favored by neutral or alkaline heavy soils, and does not require microscopic worms called nematodes to cause disease, making it a new threat to 4-6 million acres of USA Upland cotton. In 2001-2002, several shiploads of live cottonseed were imported into California for dairy cattle feed. We isolated seventeen Fusarium oxysporum isolates from the imported seeds that were able to cause disease in cotton. These isolates were compared to the Australian and other known Fusarium oxysporum isolates. These isolates were categorized into several groups. One of the isolates was as pathogenic as the Australian isolates and it was able to form structures with the Australian isolates that may allow them to exchange genetic material. Unlike USA isolates, all the seed isolates and the Australian isolates were not able to cause disease when the isolates were introduced into the stem. Both USA and seed isolates as well as, Australian isolates were able to cause disease when the roots of cotton plants were soaked in the fungus solution. This difference enables us to distinguish USA isolates from those other isolates. The seed isolate that is as pathogenic as the Australian isolate, and the Australian isolates, were able to survive in the dead plant root residue in the soil for up to 4 months in a greenhouse chamber and kill the cotton plants subsequently planted. This isolate or related isolates could cause a severe Fusarium wilt problem in USA cotton fields if they become established.
Technical Abstract: A unique biotype of the Fusarium wilt pathogen found in Australia in 1993 is favored by neutral or alkaline heavy soils and does not require nematodes to cause disease, making it a new threat to 4-6 million acres of USA Upland cotton (Gossypium hirsutum L.). In 2001-2002, several shiploads of live cottonseed were imported into California for dairy cattle feed. Thirteen Fusarium oxysporum Schlecht. f. sp. vasinfectum (Atk) Sny. & Hans. isolates and four isolates of a Fusarium sp. that resembled F. oxysporum were isolated from the imported cottonseed. The isolates, designated by an AuSeed prefix, formed four vegetative compatibility groups (VCG) all of which were incompatible with tester isolates for 18 VCGs found in the USA. Isolate AuSeed14 was vegetatively compatible with the four reference isolates of Australian biotype VCG01111. Phylogenetic analyses based on EF-1 alpha, PHO, BT, Mat1-1, and Mat1-2 gene sequences separated the 17 seed isolates into 3 lineages (race A, race 3, and Fusarium sp.) with AuSeed14 clustering into race 3 lineage or race A lineage depending on the genes analyzed. Evidence of recombination also was detected between race 3 lineage isolates and race A lineage isolates, and between Fusarium sp. isolates and race, A lineage isolates. Indel analysis of the EF-1 alpha gene sequences revealed a close evolutionary relationship among AuSeed14, Australian biotype reference isolates, and the four Fusarium sp. isolates. The Australian seed isolates and the four Australian biotype reference isolates caused disease with root-dip inoculation, but not with stem-puncture inoculation. Thus, they were a vascular incompetent pathotype. In contrast, USA race A lineage isolates readily colonized vascular tissue and formed a vascular competent pathotype. The AuSeed14 isolate was as pathogenic as the Australian biotype and it or related isolates could cause a severe Fusarium wilt problem in USA cotton fields if they become established.