Rusty Root and Rust Spot of Ginseng – What is the Cause?
Rusty-Root Causes
Rust Spot Causes
Diagnosing the Cause of Rusty-Root and Rust Spot of Ginseng Rust
Controls

Rusty Root and Rust Spot of Ginseng – What is the Cause?

Ginseng fields will be prepared for digging over the next few weeks. At harvest many rusty root and rust spot symptoms will be noted on roots even if the crop looked healthy from above. Growers often ask about the cause of rusty symptoms and how to prevent it in the future. Unfortunately, the cause cannot be easily determined in many cases. However, there are some things you can look out for that can provide some clues to the potential cause.

The disease often called “rust” on ginseng is not a rust at all. Rust diseases of many field crops are caused by completely different fungi and occur on above ground tissues. Pest control strategies that control rust in field crops will not have any effect on the root disease in ginseng. To avoid confusion, we use the term “rusty root” to refer to mostly superficial rusty symptoms caused by fungi (Figure 1), and “rust spot” to refer to the superficial rusty spots caused by abiotic factors such as nutrition or temperature (Figure 2). The symptoms of both diseases/disorders often consist of superficial, flaky, rust-coloured damage than can be scraped off with a fingernail leaving healthy tissue beneath (Figure 3). Sometimes symptoms can also be deeper into the root with a corky texture below the surface. The symptoms can be located anywhere on the root, but more often they are located closer to the crown. Rarely are there any visual symptoms of injury to the tops, and often the damage does not interfere with the normal growth of the plant. The main problem with these symptoms is the reduction in marketability of the roots.

Figure 1. Rusty root on ginseng is caused by fungal infection. These fungi often cause superficial symptoms but can cause a deeper, dry crumbly rot.

Figure 2. Rust spot on ginseng is often much more superficial than rusty root and will have little impact on the growth of the plant but a large impact on marketability. 

Figure 3. Most rusty root and rust spot symptoms can be scraped off leaving healthy tissue beneath.

Once rust-like symptoms develop on ginseng roots, there is usually nothing that can be done to fix the problem. Management of the disorder requires an adjustment of production practices to avoid roots developing symptoms in the first place. One of the biggest challenges when dealing with the problem is identifying the cause. Multiple factors (both disease and abiotic) can cause very similar symptoms. Proper diagnosis is essential before production practices can be modified to prevent problems in future gardens. 

Rusty-Root Causes

Rust-coloured symptoms have been associated with several different fungi including Cylindrocarpon, Fusarium and Rhizoctonia. These fungi also cause deep rots such as the “disappearing root rot” caused by Cylindrocarpon. Rusty root may be caused by the same fungi in less ideal conditions, weaker strains of the same fungus, or in the case of Fusarium, possibly a different species. It has been suggested that minor damage due to the root-lesion nematode could cause rusty-root symptoms or could lead to fungal infection and eventual development of rusty-root symptoms. However, a link between nematode damage and root disease has not been conclusively proven. 

Proper identification of the fungal causes can only be done by someone trained in pest diagnostics. In most cases it is impossible to determine the cause of rusty-root based on symptoms alone. Occasionally, the root can have a crumbly or soft texture below the surface that penetrates deeper into the root than the superficial symptoms associated with abiotic disorders (rust spot). However, deeper symptoms do not necessarily always appear. Proper diagnosis can even be challenging for trained diagnosticians because it is often difficult to determine if a fungus that is found in the root is the primary cause of the problem or if the fungus is only secondary and came in after the root was already damaged. 

Rust Spot Causes

Rust spot consists of scaly, rust-coloured areas, usually on the taproot, often near a fibre or branch, that may or may not penetrate to the root interior. It is possible that these areas can be sloughed off without further root rot. In rust spot, there is no crumbly layer underneath the surface of rusty lesions as would occur in the presence of Cylindrocarpon. 

Research from British Columbia has indicated that rust spot can be caused by adverse environmental conditions. Low temperatures where roots are exposed to –6°C or lower can cause rust spotting. Fluctuating temperatures appear to be more damaging to the roots than prolonged low temperatures. 

Other research has indicated that rust spotting is associated with high manure levels, excess nitrogen, the use of the ammonium forms of N, or the reduced form of iron that occurs in wet soils. In the laboratory, rust spotting has been generated by exposing roots to ethylene. Similar symptoms can also be caused by boron toxicity.

Diagnosing the Cause of Rusty-Root and Rust Spot of Ginseng

When trying to decide what is the most likely cause of rusty root, here are some things to consider:

Location – where is the disease located? Is it in high or low areas? Is it associated with one lot of seed or one batch of straw? If it only occurs on one seed lot, it may just be a seed-borne fungus or the genetics of that seed lot predispose the crop to damage. If the damage is in a low area of the field, it could be a fungus or it could be cold temperature damage during one of the winters. 

1. Pattern in the field – Is every root affect? Is it scattered throughout the field or localized in larger patches? Is it on the hills or in the trenches? Damage concentrated in the trenches could indicate areas with more moisture, suggesting a soil-borne pathogen. Random and patchy disease could suggest a seed-borne pathogen.        
2. Seed source – where did you get your seed? It has been suggested that seed lots originating under certain environmental conditions, such as hot temperatures, may be genetically predisposed to rusty root and rust spot damage due to an adaptation of the ginseng population to those conditions over time.        
3. Crop history. It is a good idea to keep track of the previous crops grown on the land planted to ginseng, because over time you may be able to associate diseases and disorder of ginseng to a previous crop grown on that land.        
4. Fertility. Keep track of your fertility program and also collect soil samples and have them analysed for all macro- and micro-nutrients. Over time you may be able to associate damage to a nutrient deficiency or toxicity or differences in your fertility practices.         
5. Proper diagnostics. It is very important to submit rusty root and rust spot affected roots for diagnostics by a trained plant diagnostician. Fungal causes can often be determined in this way. It is a lot easier to correct a problem when you know the cause.

Controls

Until you know the cause of the problem, it is difficult to decide on an effective solution. More research is required to link symptoms with the potential causes and to find effective solutions. Quadris is registered and effective for control of Rhizoctonia, but there are no chemical controls that are registered for use on the other more common rusty root causes, Cylindrocarpon and Fusarium. These fungi may be controlled using proper fumigation strategies. Many problems can be reduced by proper site selection, balanced fertilization, local and proven seed sources, disease free seed, and proper and continued mulch application. 

If you note rusty root or rust spot problems in your field and would like to discuss the potential causes and solutions, please give me a call at the number below.  

Questions about ginseng can be directed to Sean Westerveld, OMAFRA’s Ginseng and Medicinal Herbs Specialist at 519-426-4323; E-mail: sean.westerveld@ontario.ca.