Phytosanitary Information for Potato

Seed tubers intended for large scale agriculture are usually produced under certification programs that test for common diseases.  I don’t offer certified tubers because certification programs are not really designed for a small scale operation with lots of varieties like ours.  I still do my best to produce disease free seed tubers.  I now only offer tubers grown from plants that started as clean tissue culture plantlets in the year they were grown.  Because of this, viruses should be very rare in the tubers that I offer.

Among other diseases, I do not pass any potatoes that show signs of late blight or black leg.  I would also not pass most other bacterial or fungal conditions, but I have seen few other diseases here so far.  I often see signs of purple top phytoplasma and I destroy those plants, but I cannot test for the phytoplasma that causes purple top, so it is possible that some tubers might be infected.  I may pass potatoes with minor common scab, with minor physiological conditions like slight cracking, plants that have recovered from early blight with no signs of tuber infection.  I would not pass any tubers with indications of powdery scab, but I suspect, based on occasionally seeing root galls or scab with papery margins, that it may be present in my soil.  Powdery scab is now nearly as widespread as common scab and it is not always easy to distinguish between the two.  None of these are very common here, except for early blight.  Unlike viruses, which are diagnosed with biochemical tests, I diagnose bacterial and fungal diseases based on visible symptoms, so minor cases may escape detection.  Plantlets do not suffer from any of these conditions and so they are always a safer choice.

Virus	Method	Schedule	Rationale
Pospiviroid genus	PCR	Once at introduction	This is primarily a test for Potato Spindle Tuber Viroid, which is a rare but very bad potato pathogen that could quickly destroy a collection.  This will also catch Chrysanthemum Stunt Viroid. In 2021, the US potato genebank announced that some accessions that previously tested negative for PSTVd were actually positive and put into question the PSTVd status of the entire collection.  I have grown a large number of accessions of both domesticated and wild potatoes from the genebank and I do not test for PSTVd in tubers grown from genebank seed.  I couldn’t afford to even if I wanted to.  So there is some very small, but non-zero probability that PSTVd could lurk somewhere in my collection.  I have never seen symptoms consistent with PSTVd in any potato, but it does not always present clear symptoms.
Potato Leafroll Virus	ELISA	Annually	This is a very common virus and has been present in our field grown plants for years, although it is now rare to get a positive result.  I got no positives for the first time in 2019.
Potato Mop Top Virus	ELISA	Once at introduction	PMTV can result in long term field infection and I already have the powdery scab organism in which it can persist.  I don’t want it.
Potyvirus genus	Immunostrip & ELISA	Annually	The Potyvirus genus includes some common and damaging potato pathogens like Potato Virus A and Potato Virus Y, both of which have been detected in our field in the past.
Potato Virus M	ELISA	Once at introduction	This is a fairly common virus, but I have never seen a positive in our field.
Potato Virus S	Immunostrip & ELISA	Annually	Potato Virus S is a common potato virus that has been present in our field grown plants in the past.  I have not had a positive result from the field in two years.
Potato Virus X	Immunostrip & ELISA	Annually	Potato Virus X is a common potato virus that has been present in our field grown plants in the past.  We got no positive results in 2019.
Potato Virus Y	Immunostrip & ELISA	Annually	Probably the most widespread and damaging potato virus.  It has been present in our field for years, but I think we may finally be free of it.
Tobacco Mosaic Virus	Immunostrip	Once at introduction	Not common in potato, but TMV is a real pain to get rid of once you have it.
Tobacco Streak Virus	Immunostrip	Once at introduction	We have had previous positive tests for TSV in dahlia, but not potato.
Tomato Spotted Wilt Virus	Immunostrip & ELISA	Annually	I have seen this virus quite a few times in potatoes from private collections and it also infects other Andean root and tuber crops.