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Potato rots, St Helena

February 2015. Potato are imported into St. Helena from South Africa. A recent shipment had diseases. What are they likely to be? It was said that the potatoes were intercepted in imported potatoes from South Africa and so it is a biosecurity issue. As the pathology didn’t match our quarantine pests of Synchytrium endobioticum wart disease and Ralstonia solanacearum race 3 brown rot we allowed them in and are worried that we are letting in an important new pest to the island’s potato crop. St Helena is a small, isolated island, population 4500, five days voyage by ship from Cape Town, with no airport yet. So destroying a consignment can mean no potatoes at all on the island for 3 weeks, until the next voyage of the ship, if local production is low at that time. Our island potatoes are happily relatively disease free which is good, but this of course limits our experience in identifying diseases and their symptoms.

Because of our size we don’t have the facilities or skills to isolate or culture bacteria, unfortunately, but shall look into the diagnostic kits, these could be invaluable tools for us.

There were a number of questions/suggestions:

  • Perhaps the potatoes were kept at too low a temperature and were damaged so, later, when they were returned to ambient temperatures rots occurred.
  • Where there any small black specks which could be the remains of pyncnidia. And is there a fouls smell? If there is, it is likely to be Erwinia carotovora. It is unmistakeable. (Erwinia carotovora is now called Pectobacterium carotovorum).
  • Poor handling and infection by Erwinia species are possibilities. An APS leaflet was sent which described the symptoms of one of the possibilities, Erwinia carotovora subsp. atroseptica (synonym: Pectobacterium atrosepticum). It explains the disease and how rotting occurs in storage. However, there are other Erwinia species, for instance, E. chrysanthemi and other subspecies of carotovora, also occur in storage if the tubers are not handled carefully and/or the storage conditions are not ideal. These may not cause a disease in the field, but just contaminate the tubers and take advantage of damage that occurs after harvest.
  • It was suggested to use diagnostic kits. There are several for potatoes – see e.g. And and article here:
  • The best way forward is to make isolations for fungi and bacteria onto PDA or, if it is possible, to extract DNA and send for analysis using Erwinia specific primers.
  • A bacteriologist wrote: Looks like typical soft rot. Usually a distinct smell develops at later stages of infection. Rotting tissue is usually odorless in the early stages of decay but develops a foul odor …. as secondary organisms invade. (See Compendium of potato diseases. APS Press.).
  • Another bacteriologist (DSMZ) said: If the tuber is solid and there is browning then it is more likely to be Pectobacteria, but it could also be Dickeya. If you wash potato – like those were washed – and do not properly dry, then theses Pectobacteria can cause rots. They are all over the place so nothing too special. A fact sheet was sent:

It was confirmed that there was no putrid smell, just a musky one, and that the ooze appeared uniform in consistency, and also gelatinous, it formed strings. Does that make it less likely to be Erwinia and more likely to be a storage condition?

Later, other symptoms were found in consignments of potatoes from South Africa.The rots that are present seem to be early stages of bacterial soft rot (Pectobacterium spp.) as described in the various emails and attachments. However, there is another symptoms (see images below). The brown tissue is dry and corky, without a bad smell, and there is no sign of bacterial ooze. In some potatoes it is the vascular tissue affected, in others the heart.

Fortunately, a member had a copy of the potato compendium, and was able to suggest that the brown, corky rot could be Fusarium dry rot (top photos). Quoting the Compendium “Internal necrotic areas are shades of brown, from tan to dark chocolate brown. The advancing margin is distinct if the tissue is a darker shade of brown, but is not distinct for the lighter shades. Rotted tissue is dry and often develops cavities lined with mycelial and spores of various colours. When relative humidity in storage is high or tuber surfaces are wet, bacterial pathogens (Erwinia spp. or Clostridium spp.) frequently invade through dry rot lesions to cause soft rot decay.”

The causal organism is most commonly Fusarium sambucinum in USA. The rots are also caused by F. avenaceum, F. culmorum, and F. oxysporum.

The vascular staining looks more like brown rot – or possibly Fusarium wilt? Fusarium wilt causes tan/brown staining. Fusarium wilt is caused by F. avenaceum, F. oxysporum or F. solani f.sp. eumartii.

Brown rot is a bacterial disease caused by Ralstonia solanacearum. The vascular tissue of infected tubers is usually a distinct grayish brown. A distinctive symptom is glistening beads of bacterial slime that ooze freely from vascular tissue of freshly cut infected tubers, usually without squeezing. This did not match the images sent. Eyes with Ralstonia turn grayish brown. A sticky exudate may form at eyes or at the point where the stolon is attached to the tuber.

So, without any bacterial ooze, it is more likely to be a fungal cause (Fusarium).

This diagnosis was confirmed by a member of SARDI, the South AustralianResearch and Development Institute. The first rot (left) looks like Fusarium Dry rot. The second one (right) is more difficult. If you squeeze the tuber, does anything come out of the blackened areas? The blackened areas are aligned with the vascular system of the tuber. There are several diseases that could cause this. Apart from physiological blackening, there is also bacterial wilt (Ralstonia) which will cause cream ooze to come out of the vascular system when squeezed (and if fresh), Vertcillium (but that usually is not as large in area). Too difficult without seeing the tuber!

The one with the central rot (below right), it seems there is white fungal growth. Look under a microscope, for banana shaped Fusarium spores. If it is Fusarium they will be there.

Quarantine aspects: A member wrote that one of the concerns he has with the development of biosecurity services is that restrictions are put in place for risks that have been readily accepted for many years, based on a new capacity to detect pests. In the case of potatoes from South Africa, there has been trade for many years and the only current risk is a changed status of new pests in South Africa.

The CABI Crop Protection Compendium lists 156 pests on potato in South Africa of which 31 are fungi and 14 are bacteria. As an example, 3 of these are Erwinias (Pectobacterium spp). If these were new South African detections then there would be concern that they pose a new threat to you. However, the three bacteria have been recorded in South Africa in 1964, 1986 and 1991. With your regular commercial trade in potatoes you have been accepting the risk in the pathway for many years and the imposition of a restriction would not be valid at this time.

I hate to speculate, but I suggest that the pathogen has already come into the country and either not established because of your crop/environment conditions, or has established but does not have a detectable economic impact on your potato/solanaceous crops.


Title Owner Date Size
Bacterial rots of potato tubers Grahame Jackson 5/17/2015 287.57 KB