This page will be a repository for unpublished (in peer-review journals) data and analyses resulting from my previous work as a research scientist.
1) Rhexocercosporidium panacis. This fungus is a pathogen of ginseng (Panax quinquefolius). My lab discovered and named this fungus and we were the first to demonstrate its role as a pathogen of ginseng. The disease ‘rusted root’ (also known as ‘the rust’ or ‘rusty root’) was first described by Hildebrand in Ontario in the 1930s and was widespread throughout ginseng-growing regions in North America at that time. Symptoms and some historical notes can be found in the first citation below. Several different fungi have been identified as possible causes of this disease, but none (until the discovery of R. panacis) of them were able to adequately reproduce the symptoms. See here for some background information on this disease.
Rusted root disease is characterized by slightly raised reddish brown to black root lesions of varying size. Small blister-like lesions expand and coalesce to form large lesions that possess a diffuse margin. The peridermal tissue often becomes very dark red, almost black, in color as the disease progresses. The lesions, regardless of size, remain superficial; however, peridermal tissue is ruptured and sloughed off, giving the root a pitted or scabbed appearance. Fragments of the blackened and ruptured periderm may remain attached to the root at harvest. When roots have been dried, lesions often appear light brown in color. Except where symptoms are severe, the disease appears to have little effect on yield; however, the discolored, scabby, and often deformed appearance of diseased roots reduces their value. The term “rusty root” also is applied to this disease; however, Hildebrand’s use of “rusted root” has historical precedence and may be preferable, in part due to the use of “rusty root” to refer to other unrelated problems that occasionally are observed on roots.
Note: Click on the above image to view more clearly in a separate page.
After pathogenicity assays with various fungal genera isolated from diseased roots using agar-based culture media (yielding mainly Fusarium and Cyndrocarpon isolates) failed to reproduce observed disease symptoms, my lab began using DNA extraction techniques in 2004 and 2005. Our first set of extractions from diseased-tissue samples (amplified using universal fungal primers) yielded a wide variety of species when submitted to GenBlast but, consistently, we found DNA identified as belonging to a Rhexocercosporidium species. Healthy ginseng root tissue did not contain this DNA sequence. Comparison of this DNA to DNA extracted from isolates of Rhexocercosporidium carotae (the only other known species in this genus) suggested that the Rhexocercosporidium from ginseng was a previously undescribed species. When we used some alternative agar media (which suppressed competing Fusarium and Cynlindrocarpon species adequately), we were able to isolate this fungus consistently from diseased ginseng root tissue. Using ginseng roots (grown in greenhouses to avoid field-related latent infections), we were able to reproduce symptoms of the disease.
The isolates of Rhexocercosporidium from ginseng were sufficiently different from R. carotoae that we have described a new species, R. panacis, to accommodate these isolates.
Our published scientific articles relevant to this topic are:
Reeleder, RD, Hoke, SMT, and Yun Zhang. 2006. Rusted root of ginseng (Panax quinquefolius) is caused by a species of Rhexocercosporidium. Phytopathology 96: 1243-1254 doi 10.1094/PHYTO-96-1243. (note: an editing error resulted in incorrect primer sequence information in the manuscript. These were corrected in a Note in the journal and correct sequences for ONBCU3 and ONBCL2 primers are given in the poster linked to below.)
The following reports, which add supplementary data, have not been published in peer-review journals (except as abstracts):
- Poster for CPS-SWO Regional Meeting 2007 Rhexocercsoporidium in ginseng gardens This poster describes some subsequent work (up to early 2007) that shows that stratified seed is the most likely method of introduction of R. panacis into ginseng plantations.
- Abstract for above poster May 2007
- Managing rusty root disease update August 2007 (prepared for ginseng industry)
- Tables for above update (data showing recovery from various seed, soil, and straw sources)
- Table Invitro Rhexo fungicides report (Invitro response of R. panacis to fungicides: carbendazim was most inhibitory and may be a possible seed treatment)
Suggested management options for this disease:
- Evaluate use of carbendazim, thiophanate-methyl, and fenhexamid as possible seed treatments, pre or post-stratification.
- Avoid use of soil/sand contaminated with R. panacis for seed stratification. Consider above-ground controlled stratification procedures. The fact that some lots of stratified seed are free of R. panacis suggests that some growers are successful in reducing the incidence of contamination/infection.
- Use detection methods such as qPCR to avoid planting into R. panacis – infested soil.
2) Cylindrocarpon destructans – detection in soil using qPCR
Cylindrocarpon destructans f.sp. panacis causes disappearing root rot of ginseng and is responsible for a replant decline that limits repeated use of land for ginseng production (Kernaghan, Reeleder, and Hoke 2007). Using qPCR methods, we have been able to detect C. destructans in field soils and demonstrate a relationship between qPCR signal and likely disease severity. Using these methods we were able to map populations of C. destructans in naturally-infested field soils and detect the pathogen in soils many years after ginseng production. The method may be useful in avoiding the use of infested soil in ginseng plantings.