Spring is around the corner and it is the time of year when I start to see a lot of discussion about growing those old, heavily sprouted potatoes that you find at the bottom of your pantry or about purchasing potatoes to plant from the grocery store because seed potatoes are so expensive. Like so many gardening topics, this one has lots of coverage online, but articles tend to be polarized, taking one position or the other, rather than clearly explaining the pros and cons. So, this post will be a detailed exploration of the topic, giving you the information necessary to make your own decision.
The main problem with growing potatoes from tubers is the possibility of introducing diseases from those tubers, but there are also sometimes problems with sprout inhibiting chemicals, increased tuber age, and the possibility of inadvertently growing a GMO potato variety.
Potato tubers can carry a lot of diseases. Diseases present a more challenging problem in asexually reproduced crops like the potato, which are rarely grown from seed. Sexual reproduction is a barrier to many plant pathogens, so seeds tend to spread less disease. Tubers do not go through this generational break, so they accumulate diseases indefinitely. Disease has long been the great vulnerability of potatoes. Before the advent of modern agriculture, potato varieties typically “ran out” and were abandoned after they became critically burdened with diseases. You cannot tell if a potato is diseased by looking at it. A tuber may appear to be perfectly healthy, only later revealing disease once you have introduced it to your garden. Potatoes with known disease exposure are often allowed to be sold as food, under the assumption that they will not be used for planting. While farmers are not going to get their seed potatoes from the grocery store, home gardeners do so frequently.
Viruses are common in ware potatoes (those intended for sale as food) and are even present to a much smaller extent in certified seed potatoes. Viral diseases of potatoes tend to be hard for casual potato growers to recognize, because they rarely kill the plants outright. The primary symptom is a reduction in yield and is often blamed on growing conditions. Some viruses grow worse from one crop to the next, eventually leaving you with a very poor crop, while others can persist in the potatoes for many generations without much change in yield. The important thing to understand about viruses is that they persist in the potatoes and spread by insect action and physical contact. So, once you introduce virus infected potatoes to your garden, it is just a matter of time until all of your potatoes are infected and, for the casual grower, there is no way to remove the viruses. To get back to a clean crop, you need to eliminate all potatoes from your garden before starting again.
Bacterial and Fungal Diseases
There are a lot of common bacterial and fungal diseases of potato. Some, like common scab and black scurf, are widely distributed in soil, and others, like early and late blight, spread on the wind. There is little that you can do to avoid these in the long run. Other diseases are generally absent unless you introduce them on tubers. One of the worst of these is powdery scab, a disease that has been spreading rapidly through the potato growing regions of North America in recent years. This is a different organism from common scab and it persists long term in the soil once you introduce it. Even if you stop growing potatoes entirely, it may take many years to die out. On Prince Edward Island, they are currently having an outbreak of potato wart, a disease that was largely conquered in North America a century ago. The organism that causes wart leaves spores in the soil that can survive for forty years. The point here is that there are are potato diseases that you will never be rid of once you introduce them. With these pathogens, cure may be impossible, so prevention is your only protection.
Ware potatoes are sometimes treated with chlorpropham or other sprout inhibitors. These are intended to keep the potatoes from sprouting while warehoused. Not all potatoes are treated and inhibitors are not allowed on Organic potatoes. Varieties respond differently to inhibitors and it appears that the application conditions also vary, so that some batches of potatoes are very difficult to sprout, while others will sprout after an extended period of dormancy. Thoroughly washing and scrubbing the tubers with soap can remove about 90% of the chlorpropham, but it isn’t clear whether that is enough. Even if they do sprout, the quality of plants grown from tubers that were treated is usually not good, because the sprout inhibitor causes problems with mitosis in the sprouts, which have already absorbed the chemical even if you wash them. The inhibitors also typically conceal increased physiological age…
The physiological age of a potato tuber is the true age of the tuber combined with the conditions under which it has been stored. Less than ideal storage conditions accelerate the aging of the tuber and traveling around on trucks before sitting on loading docks is not ideal. As potatoes increase in physiological age, the quality of the crop that they produce is reduced. The plants tend to sprout more heavily, mature more quickly, and produce larger numbers of smaller tubers, with a reduced yield.
Many people seek to avoid genetically modified foods, for a variety of reasons. Genetically modified potatoes have recently been introduced in the USA and there is no labeling requirement, so you cannot easily determine whether or not a potato purchased at the grocery store is GMO, unless it is Organic, as GMOs are not allowed under the Organic label.
Potato Tuber Sources
You have three common options when you plan to grow potatoes from tubers: you can buy certified seed potatoes, you can buy non-certified potatoes, or you can grow tubers that you saved in a previous year. Each has a different risk profile.
Certified Seed Potatoes
Certified seed is the gold standard if you are a farmer and it is a good idea even if you are not. These potatoes have the lowest risk profile among the options that are typically available to consumers. The down-side is that they are expensive, particularly if they are not available locally and you need to have them shipped. Certified tubers are the end product of a process that begins with disease free tissue culture plantlets in a laboratory. The plantlets are grown in insect-proof facilities to produce a foundation crop of small tubers. These are then bulked up for a few generations in fields that are carefully managed for disease. Both the plants in the field and the tubers are inspected and tested and the tolerance for common potato diseases is low. While the risk is not zero, since that is simply not an achievable goal for field grown potatoes, it is low enough that you would expect to see only the occasional plant with a viral disease and very rarely anything else.
It is important to understand that certified seed is not disease free, such that you can expect to buy once and never again. The certified seed system is designed with farmers in mind and the expectation that they will buy new certified seed every year. By planting new certified tubers every year, there is little opportunity for disease to accumulate in your crop. When you start to save them to replant, the risk profile changes…
Your Own Saved Tubers
The next best option to certified seed is saving your own tubers. This is, however, a much riskier option in most cases. While you avoid the other problems of ware potatoes, you still have to contend with disease. Even if your original potatoes were absolutely disease free (which isn’t very likely), there will be diseases present locally that infect your plants over time. Viruses are often spread by aphids, which can fly for miles. Fungal diseases spread by spores over even greater distances. In most cases, once a potato plant is infected, the tubers continue to be infected forever, so the trend is always toward increasing disease and decreasing yield over time. The reasons are not always apparent and many casual potato growers report gradual and mysterious declines in yield. Disease, particularly viral disease, is the most common reason for this. On the other hand, there is no chance of introducing a soil borne disease, like powdery scab or wart, to your garden if you don’t import new tubers, so this is one area in which your own tubers are even safer than certified seed tubers.
Grocery store potatoes are the worst option you can choose for growing potatoes. They have all of the problems identified above. They are often treated with sprout inhibitors, although you can avoid this by choosing Organic potatoes. They often have increased physiological age, since the grocery store is the end of a long journey that often involved a stay in a storage facility. They are much more likely to carry disease because they are more generations down the line from clean sources, they are grown in fields that do not have the stringent standards used for certified lots, they are not tested for disease, and they are stored in conditions that allow disease to spread more easily.
Assessing Your Risk Tolerance
Why do you grow potatoes? If you are a casual grower of one or two varieties and it isn’t a big deal to you if you introduce a disease that permanently reduces the quality of your potatoes, then you can probably save some money growing potatoes from the grocery store. Most people who do this get away with it. There are plenty of people out there who have been growing potatoes that they bought at the grocery store for a decade. If you are a more serious grower and potatoes are a big part of your harvest that you depend on or you have a bunch of different varieties with different levels of disease resistance, then you will probably want to be more careful. An annual or at least biennial purchase of certified seed is probably a good investment for you. If you are a collector or breeder with rare or irreplaceable potatoes, then even certified seed might not be safe enough for you, or at least you might need to partition what you grow into different plots or fields to try to reduce exposure between varieties.
Reducing the Risk
You can do a few things to modestly reduce the risk of growing potatoes from tubers. First, give the potatoes a close inspection and look for any signs of disease, like dark spots, sunken spots, corky protrusions, etc. Don’t use those for planting. Wash the remaining tubers with dish detergent and then soak them in a 10% bleach solution for 15 minutes. This will help to remove soil stuck to the tubers and to kill bacteria and fungal spores on the skin. This won’t help you if the tubers are already infected, but it will help to get rid of pathogens that are waiting for the right conditions before infecting the tuber. If you intend to cut the tubers into seed pieces, it is important to do this before you cut them. When cutting seed pieces, never use the same knife to cut different tubers without cleaning it. A dip in rubbing alcohol between cuts will help to reduce the odds of spreading a virus from one tuber to the next. There are several contact transmitted viruses like PVX and PVS that are widespread due to the practice of cutting potatoes directly into the field without sterilizing the blade.
Another option to reduce risk is to detach sprouts from the tuber to grow them separately. Once a sprout is about two inches long, you can pull it off and plant it in a pot. This removes the tuber as a source of possible contamination. This is, again, a modest risk reduction, since most diseases are present within the tissues of the potato and cannot be escaped by means as simple as this.
One alternative for producing a maximally disease free crop is to grow from true potato seeds instead of tubers. There are very few diseases that spread in potato seeds compared to potato tubers, so you can often grow your seed-grown potatoes for more generations before they become burdened by disease.