August is often the make or break month for potatoes. No doubt, if you’ve planted a few rows (or a lot) of potatoes, you’ve already dug a few plants for new potatoes which are usually ready two weeks or so after the plants blossom. But if you’re waiting until the first frost so you’ll have big tasty tubers for winter storage, now’s the time to be on alert.
Warm days with high consistent humidity encourage disease, as does wet weather. The problem with potato blight (Phytophthora infestans) is that once it starts, it’s nearly impossible to make it disappear completely. Still there are things you can do to prevent and impede potato disease. The ultimate goal is to keep them from the tubers.
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If you notice dark blemishes on mature leaves, often with target-like rings, your potatoes are probably suffering from one of the most common diseases, early blight. If left untreated, this fungus will result in collar rot, essentially strangling the plant at the soil line. Luckily, there is a treatment that will slow or even stop the fungus that causes potato blight, if applied early enough. A good copper-based fungicide applied every week or so should give your spuds time to develop.
Copper-based fungicides are also good in retarding the spread of late blight, which shows up as dark spots on the underside of plant leaves. Copper fungicides are listed on the National Organic Program list but like any product used around food sources, should be applied as the directions dictate. If you live in an area where disease is a problem — or if you’ve had blight on your potatoes in previous years — it’s a good idea to apply a preventive strike of copper fungicide ahead of any visible problems.
As with many garden disease problems, prevention is the best tool a gardener has. Remove all dead or dying plants and dig up the potatoes. Discard them far away from your garden. Fall cleanup is also important. Even without signs of disease — and especially if there is — remove all potatoes and plants from your garden. Blights overwinter in potatoes left in the ground. Do not add infected plants to your compost. And pull all volunteer potato plants wherever they may come up. You never know where the enemy is hiding.
If you do live in an area with blight problems or have had them yourself, you might want to consider a potato that is resistant like Kennebec or Norgold russet. Disease resistance is not a perfect thing — plants may still develop the disease but not be as badly affected. As usual, the answer is already available in nature. And, of course, there is a concerted effort on to develop a GMO potato. Like we said, you never know where the enemy is hiding.