As temperatures dip close to freezing and the growing season draws to a close, potato tops start to die down, and potato harvest really gets underway.
Rob Spencer, horticulturist, says, "Often, it is as potatoes are being taken out of the ground and put into storage that people start to notice some problems with their potatoes.”
Unfortunately, most of the problems that manifest themselves at or after harvest cannot be fixed or managed at that point in time. However, Spencer says, by recognizing the problems when you see them, you have the chance to try and manage the problems next season.
One of the most common problems that growers of potatoes encounter is a disease called Common Scab. This disease is typically introduced to the soil on the seed tubers however it could also arrive on soil, manure (from cows fed scabby potatoes), or in potato debris.
Spencer says, "Once scab is present in the soil, it tends to stick around for an extended period, lasting well over a decade, even without potatoes present.”
Scab is aptly named, with the pathogen causing round, irregular brown lesions on the potato skin, which resemble scabs (hence the name). The severity of the disease can vary quite a lot, depending on the variety and the specific conditions in the growing location. Small amounts of scab can be more of a nuisance than a major problem, as the scab does not spread in storage, but can be a pain in peeling. As the severity increases, moisture loss can increase.
Spencer suggests that there are also a couple of insect pests that are only really noticed due to the holes that are discovered when examining the tubers or during peeling.
The tunneling action of wireworms looks like something has made a random hole in the side of the potato. The width of the hole tends to be about 5 mm. The depth of the hole varies but can be up to 25mm.
"The tissues on the sides of the holes tend to be healed over by the time that you find it, and the pest is nowhere to be seen”, says Spencer.
Tunneling by the threadlike tuber flea beetle larvae results in very fine, shallow tunnels right under the surface of the skin. Spencer says that when you are peeling the potatoes, these tunnels might look like brown spots or light brown streaks, and a couple of layers of tissue might need to be removed to get past them. The discolouration that you see is wound tissue, not any remnant of the larvae.
Some of the other problems that may be noticed at or after harvest include various physiological issues, including types of bruising, hollow heart, or other tuber diseases.
When it comes to managing scab, wireworm, and tuber flea beetle, it is not always simple and straightforward.
There are many suggestions for managing scab, including altering soil pH, using clean seed, and rotating between crops, but Spencer says that there are only two that are practical and effective in almost every growing situation.
The first strategy is to choose a variety that is resistant to the scab pathogen. Historically, most varieties were considered at least somewhat susceptible to scab, but there are more varieties available now that have some good resistance to it. Spencer suggests that you do a little bit of research on the type(s) of potato that you want to grow and see if you can find a variety that checks all the boxes.
The second strategy is to ensure that moisture levels are not allowed to fluctuate. It is particularly important to ensure that there is good moisture around the time that tubers are first starting to form. Spencer says that this is often signaled by flowering, typically 4-6 weeks after planting.
Managing wireworms and tuber flea beetles can be challenging, partly because the damage occurs long before you notice it and there are limited controls.
With wireworms, avoid planting in ground that had grasses/cereals growing within the recent history. You can try to lure wireworms to bait stations (pieces of cut carrots or something similar) but it can be hit and miss.
Feeding by the small, brown/black adult beetles (which jump when disturbed) during the growing season results in shot holes in the leaves. You might try and control the adults when you see them, or perhaps cover the crop in extreme cases, to prevent adults from getting onto the plants and laying eggs. Rotating to other crops can help knock things back a bit, but it is not a guaranteed solution.
The overall message is to watch your potatoes and note when things are not entirely right, then work the next season to try and correct things. Spencer says, "In the end, while having a few holes and scabs can be frustrating, most of the damage is superficial and can be worked past when peeling.”
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