I love growing potatoes. They’re one of those crops that are beyond versatile in the kitchen. They store well, and just a few plants provide an abundance of stock to feed family and friends.
So what on earth could ruin the harvest? Early and late potato blight, that’s for sure. These blights are a serious threat to home growers and commercial alike. Research efforts are ongoing to develop new strategies for tackling early and late potato blight.
In the meantime, we’ll just have to do our best to keep it from killing our potato harvest. To find out what you can do to join the battle, read on.
What is Potato Blight?
Potato blight is a fungal disease caused by a variety of pathogens.
It’s a devastating disease that affects potato plants and can cause significant damage to crops. Not only does it affect potatoes, but it impacts other members of the Solanaceae family, such as tomatoes and eggplants.
There are a few key differences between early and late blight, with different symptoms and spread. Knowing the difference will help you find the best approach to dealing with it.
The first stage begins with the initial infection of the plant by spores carried on the wind, water, insects, infected tools, or infected plant material.
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Early Potato Blight
Early blight is caused by the fungal pathogens Alternaria solani and A. tomatophila, both of which can live in the soil and plant matter.
The ideal temperatures for early potato blight to take hold of your crop are between 80-85°F, though it can reproduce in temperatures as low as 60°F. The pathogen must also have standing water on the foliage to germinate. They can’t reproduce on a dry leaf.
High. humidity, rain, or watering on the foliage rather than the soil level can all provide the water necessary for reproduction.
Infection occurs during the early stages of plant growth, hence the name. It’s a slow-moving disease compared to late potato blight.
Symptoms
Signs initially appear on the surface of the host plant’s leaves. These come in the form of dark, papery, dry lesions. These little spots will grow into large circular or oval patches. In early blight infections, the leaf spots will have a distinct bulls-eye pattern.
The outer ring is usually raised and the center is depressed.
Then, the spots coalesce and cover the leaf. This causes leaf death, which reduces yields.
The older leaves are usually symptomatic first. Stems are usually not involved.
Underground, the tubers will develop irregular circular spots with raised, purplish edges. If you dig up and slice open a tuber, you will usually find a corky or leathery texture. Over time, the tuber rots and turns mushy.
Late Potato Blight
This is a water mold pathogen known as an oomycete. The causual pathogen, Phytophthora infestans, prefers damp and cold conditions to infect plants.
It’s more severe and destructive than its counterpart, early potato blight. It was the disease that caused the Irish potato famine in the 1840s. Late potato potato blight is more likely to cause the complete collapse of the plant than early blight.
Late blight thrives in temperatures between 53-74°F with sufficient moisture for its zoospores to spread. As with early potato blight, there must be free water for the spores to reproduce.
Symptoms
Late blight symptoms start out as pale or dark green spots with an irregular shape. They may or may not have chlorotic halos. These spots don’t extend past the veins.
The lesions quickly grow in moist conditions. They turn purplish brown and expand over the entire leaf. The entire leaf turns black or dark brown and the discoloration may extend down the stems. When the plant is heavily infected, you might see white fungal growth.
The affected leaves may show signs of wilting and curling as the potato blight progresses through the leaf tissue towards the stem.
Infection within the stem will show similar discolored lesions as the leaves but without the yellow border. Lesions will appear as sunken areas that run around the circumference of the stem.
Foliage may die off in a matter of days.
With late blight, the rot may be accompanied by a fetid odor.
At this point, the disease restricts the flow of nutrients throughout the host plant. The plant will then weaken significantly and is at risk of complete collapse.
The tubers will also be involved, especially if the cultivar is susceptible to the disease. They will develop sunken, mushy areas and granular rot inside. The tubers will continue to rot even in storage.
Prevention
Early and late potato blight pathogens aim to multiply and spread to continue their lifecycle. In order to prevent an infection, we need to stop the spores from spreading into our garden.
Zoospores are mobile, asexual, singular cells that can trigger new growth and replicate the mature pathogen parent. Under favorable conditions, the spores can easily spread and cause widespread damage to your entire potato crop.
To protect your potato crop it is important to implement a proactive management plan from the outset.
Follow these steps to deal with both early and late potato blight effectively.
1. Keep an Eye on the Weather
Always monitor the weather conditions by keeping up to date on forecasts. This is particularly relevant in today’s more unusual climates.
Weather fronts threatening periods of high humidity, rain, wind, and variable temperatures heighten the risk of potato blight.
There are apps that can assist in long-range weather forecasts, though any forecast beyond about a week is highly unreliable. Still, the earlier the warning, the more time you have to prepare. Keep an eye on that weather!
It’s also worthwhile investing in a weather monitoring system that can provide real-time information to help to make informed decisions of disease management.
For example, this wi-fi weather station has an outdoor sensor and sends current conditions along with the forecast to a handy digital panel in your home.
2. Plant Resistant Cultivars
Plant resistant potato varieties and make sure the seed potatoes purchased are from a reliable source. Any of the following will be resistant to early blight.
- ‘Acoustic’ – high-yielding and good for all-purpose uses
- ‘Cara’ – best for mashing and baking
- ‘Nicola’ – best for boiling, baking and roasting
- ‘Sarpo Mira’ – high-yielding, good for general use
- ‘Setana’ – perfect for baking, roasting, and making fries
These are resistant to late blight:
- ‘Allegany’ – a fabulous all-purpose cultivar
- ‘Elba’ – excellent all-purpose and long keeper
- ‘Kennebec’ – ideal for frying
- ‘Rosa’ – red skinned cultivar great for mashing, roasting, and boiling
- ‘Sebago’ – great for mashing, boiling, roasting, and frying
There are no cultivars that are immune, so know that your potatoes might still be infected but they’ll be more resilient.
3. Companion Planting
Companion planting is a smart, natural way to support the health of your potato plants. When you strategically plant helpful species near your potatoes, you promote a healthier environment for both plants.
There are no known plants that will deter the pathogens that cause potato blight, but there are some that will attract them. Avoid planting any other nightshades nearby. You should also keep nightshade weeds far away from your garden.
Then, plant the following species to deter pests and foster a healthy environment.
- Marigolds
- Clover
- Alliums
- Horseradish
- Yarrow
- Peas and beans
Creating a multi-plant growing area can help create a balanced environment that improves the ecosystem and encourages beneficial predators. It also helps your plants withstand pathogens better, so don’t skip it!
4. Crop Rotation
Always practice good crop rotation by planting non-host crops in between growing potatoes or any nightshade in the same spot. Leave several years in between planting nightshades.
Break any potential disease cycle by planting such crops as these:
5. Promote Good Air Circulation
Follow recommended planting guides for good cultivation techniques. This will significantly reduce the risk of early and late potato blight, as well as numerous other impactful diseases.
Space plants properly to improve air circulation and reduce humidity build-up around the potato plants.
Avoid overhead irrigation, as wet foliage provides ideal conditions for the disease to take hold.
Use drip line irrigation or automatic watering systems to reduce the workload and risk of disease spread.
6. Mulch
Use good organic mulch and compost over the soil. This suppresses weeds and helps the ground retain water.
Mulching also helps prevent water splash, which is a common way of spreading the pathogen spores.
7. Practice Good Sanitation
Always maintain good sanitation of machinery, pets, human, and equipment. That means when you move from one part of the garden to the next, you should clean your shoes and equipment to prevent carrying the pathogens with you.
You should do this not only while tending to any of your crops, but also when harvesting and moving things around the property, fields and land.
Be considerate to neighboring potato growers. If you suspect early or late blight is in your crops, let others know so they can take preventative measures.
Treatment
There is no effective treatment for late or early blight. Once your plants are infected, it’s best to pull them.
Unfortunately leaving it in will only threaten your entire crop and other harvests as well.
But if you’ve had it before or you know its in the area, you can treat your plants preventatively with fungicides.
Copper, chlorothalonil, mancozeb, and maneb are all effective. Bonide’s Fung-Onil is a good option that contains chlorothalonil.
You should always keep a spray on hand in case signs of early or late potato blight pop up on your crop. Be sure to rotate the use of different fungicides that have varying modes of action in order to prevent the development of resistant strains of potato blight.
Always follow the instructions as specified on each of the fungicides packaging.
