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Late Blight Advice - September 22

Fall is here and the days are short and the nights are long and the dew is on the crops for many more hours than it was back in July. All of that means that late blight spores can travel further with less risk of sun or heat killing them, and germinate if they get lucky and land on a tomato or potato plant. We were lucky this summer with weather that was very unfavorable to late blight and there was very little of it around the state.

Really only a small pocket of the state was hit hard and I feel sorry for those folks, many of them are reading this email.

For the rest of us it has been a fabulous tomato harvest, but now I am getting more calls and hearing more reports about late blight finds. I think it is so late in the season that we just accept it. We frosted the night before last here on my farm. A light frost that did not kill anything, but it is a sign that the plants are going to die and probably soon.

Remember, the fungus that causes late blight is an obligate parasite that only can live on living tissue. So, when the frost kills our plants, the late blight fungus will die too. That is going to happen soon so I would not worry too much if you find late blight. The tomatoes are still fine to eat, and those that are not infected will probably ripen up just fine.

Killing and bagging or burying the plants will prevent spread and that is why that was recommended through the season.

The concern now is about next year. Since late blight only survives on living tissue, the only place it survives winter here in New England is in potato tubers. Protecting tubers from infection, managing cull potatoes so any that are infected do not sprout next year and release spores, and scouting next spring for volunteer potato plants that may sprout from culls or potatoes missed during harvest are the three key activities.

Potatoes are at risk of getting infected. Of course, the leaves are infected first. Then, especially with a heavy rain, the spores may be washed down and infect the tubers. Or, tubers may be infected when harvested through diseased foliage. It may be worth your while to top kill potatoes and wait a week so there are not living spores at the time you harvest. This practice is also better for the potatoes too because the tubers cure after the tops are dead. Some folks wait until there is a good frost to kill the tops before harvesting. Actually, most potatoes are dead already thanks to the potato leafhopper.

Cull potatoes are the single most likely source of late blight infection each year. If they were just laying on the ground that would be best because they would freeze, die and thus the late blight dies with them. But many people make piles of culls, throw them in compost piles that get warm but not hot, let them get buried in the field, i.e., all sorts of things that keeps them from freezing. Then next spring they sprout and a sprout from an infected potato will be infected and release spores to start the disease spread again.

Please do not let any late blight infected potatoes sprout next year. Scouting for volunteer potato sprouts next year is the last and important practice you have available to protect you and your neighbors from late blight. If you see any volunteer sprouting potatoes next spring, kill them.

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