Potato blight diseases are the bane of gardeners everywhere. These fungal diseases wreak havoc in vegetable gardens throughout the growing season, causing significant above ground damage to potato plants and rendering tubers useless. The most common potato blights are named for the part of the season when they’re common– early blight and late blight. Blight control in potatoes is difficult, but armed with some knowledge you can break the disease cycle.
Both types of blight are common in American gardens and pose some risk to other closely related plants like tomatoes and eggplants. Symptoms of potato blight are distinct when the timing of their appearance is taken into account, making blight easy to diagnose.
Potato early blight is caused by the fungus Alternaria solani and attacks older leaves first. Fungal spores overwinter in plant debris and tubers that were left behind after harvest, but waits to activate until the humidity is high and daytime temperatures first reach 75 degrees F. (24 C.). Alternaria solani penetrates the leaf tissues quickly under these conditions, causing visible infection in two or three days.
Lesions start as small, dark, dry flecks that soon spread into dark circular or oval areas. Early blight lesions may have a bull’s eye appearance, with alternating rings of raised and depressed tissues. Sometimes these ring groupings are surrounded by a green-yellow ring. As these lesions spread, leaves may die but remain attached to the plant. Tubers are covered in spots similar to leaves, but the flesh below the spots is usually brown, dry, leathery, or corky when potatoes are cut open.
Potato late blight is one of the most serious diseases of potatoes, caused by the fungus Phytophthora infestans, and the disease that single-handedly caused the Irish Potato Famine of the 1840’s. Late blight spores germinate at humidity levels above 90 percent and temperatures between 50 and 78 degrees F. (10-26 C.), but grows explosively at the cooler end of the range. This disease is often seen in early fall, toward the end of the growing season.
Lesions start out small, but soon expand into large brown to purple-black areas of dead or dying leaf tissue. When humidity is high, a distinctive white cottony sporulation appears on the undersides of leaves and along stems and petioles. Late blight-infested plants may put off an unpleasant odor that smells like decay. Tubers frequently become infected, filling with rot and allowing access to secondary pathogens. Brown to purple skin may be the only visible sign on a tuber of internal disease.
When blight is present in your garden it can be difficult or impossible to kill entirely. However, if you increase the circulation around your plants and carefully water only when needed and only at the base of your plants, you may be able to slow the infection significantly. Pick off any diseased leaves carefully and provide additional nitrogen and low levels of phosphorus to help potato plants recover.
Fungicides can be used if the disease is severe, but azoxystrobin, chlorothalonil, mancozeb, and pyraclostrobin may require multiple applications to destroy the fungus completely. Most of these chemicals must be discontinued two weeks before harvest, but pyraclostrobin can safely be used up to three days before harvest begins.
Prevent future outbreaks of blight by practicing a two to four year crop rotation, removing volunteer plants that may carry disease, and avoiding overhead watering. When you’re ready to dig your tubers, take great care not to injure them in the process. Wounds can allow post-harvest infections to take hold, ruining your stored crop.
Hot, humid summers create ideal breeding ground for pests and diseases. One of the worst is potato blight (Phytophthora infestans). This fungal disease can obliterate crops in no time flat, but there are ways to prevent and treat it. Read on to find out how.
One it has established itself, potato blight is almost impossible to get rid of. That said, there are a number of steps you can take to protect your crop. The following guide will help you to identify this issues, as well as tell you what to do should the disease ever strike. We also offer practical suggestions for protecting not only your potatoes, but also the rest of your garden.
Weather conditions over the next 10 days will favour the spread of potato and tomato blight – you should take action now to prevent damage to your crop.
So, how do we prevent and treat potato blight? Here, our garden expert, Paraic Horkan, shares all of his expert information and knowledge on the prevention and treatment of potato blight.
Potato Blight can be spread through the air and in water. Wind carries spores of the disease from plant to plant. Water can wash the spores into the soil, where it can infect young tubers.
Potato Blight is a destructive disease. It can wipe out a full crop of potatoes overnight. In a sack or a crate, it will travel from potato to potato, infecting them all. The fungus is generally killed by cold weather, but there are some rare resistant strains that survive over the winter.
Bayer Garden Blight Control is effective against the early stages of Phytophthora Infestans. You simply spray your plants to protect them from the blight fungus spores. It is rain-fast within 1 hour, so should be applied immediately. For maximum protection from potato blight, crops should be sprayed four times a year, with 10 day intervals. This will protect the leaves, stalks and also the tubers from the risk of late blight infection after harvest. Each 100ml bottle of Bayer Garden Blight Control will treat up to 625sqm.
Do you have questions about potato blight? If so, just contact your local store where one of our helpful and knowledgeable staff will be happy to help and answer any questions you may have. If you’d like to learn more about growing potatoes, just read our blog here.
The Potato Famine changed Ireland and the United States, too. The picture shows a Dublin monument to everyone who was forced to leave the country because of the famine. Photo: Thinkstock SHOW MORE
In 1845, parts of Europe had become dependent on a plant imported from the New World a few centuries before. The Irish especially based their diet largely on potatoes that grew in the barren soil.
As a result, the consequences were devastating when the potato blight infected European crops. The first known outbreak took place on the east coast of the United States in the early 1840s, and from there the blight spread to Belgium and much of Western Europe.
Among the Irish, one million people died and at least as many Irishmen emigrated to the United States and other places. Tens of thousands of Scots left the Highlands. The rest of Europe had more diverse crops, and were not hit as hard, but Belgium lost around 45,000 people and Prussia over 40,000.
A blight outbreak in a weakened and partly isolated Germany contributed to 700,000 people dying of starvation during World War I. The disease is still wreaking havoc.
@parmnparsley- Applying mulch is a good idea. it made a big difference for my nightshades. I also bake eggshells, and pulverize them into a powder that I spread on the soil around my plants as a and potato blight treatment. I have read that calcium is a good organic treatment that kills the fungus (I think). I have also resorted to using drip irrigation on my nightshades. My garden supply store manager told me that the light contaminates plants from water splashing the spores onto the lower stems and leaves. Good luck with your blight treatment. parmnparsley April 18, 2011
@chicada- Blight is one of the worst diseases a gardener can encounter. I have had both tomato and potato blight, and it is hard to get rid of. The best course of action is to pull your plants at the first sign of blight and burn or dispose of them properly. Keep the infected plants away from other plants, and do not mulch them in your compost.
When you plant your potatoes next season, plant them in a different area (if you can). Finally, when you grow potatoes you should mulch them with a thick layer so that the soil does not splash onto the plants when they are watered or it rains. The fungus or algae that cause the two types of blight live in the soil, often being transferred to the plants during watering. I would also raise the issue with your garden supply store about contaminated soil. chicada April 16, 2011
I think I have potatoe blight. I am not exactly sure because I have never had potatoe blight before, but it is starting to look like early symptoms of blight. What bothers me the most is I bought the soil and the plants from a national chain home improvement and garden supply store. I bought a few yards of bagged organic gardening soil and a few early girl tomatoes among other things.
My tiny garden was new this year and all of the soil that was there before I dug out to almost two feet deep. Within a month of planting, I had an infestation of mushrooms (They looked similar to the straw mushrooms one would find in a Chinese dish). Now my potatoes are spotting on the lower leaves and a couple of the top leaves, although there are no black blighted spots yet.
My soil pH is good, and my nutrients are good. I have no idea what is going on with my plants. Does anyone have any recommendations on what I should do?
The fungus responsible, Phytophthora infestans, is the most devastating infection of potatoes and tomatoes. It is largely a problem of outdoor tomatoes as those grown in greenhouses are generally sheltered from the fungal spores by which the infection is spread. The spores are carried by rain splash or on air currents and so pretty well any susceptible plant can be attacked. I find that early crops can often be lifted before the infection is around, whereas main crop potatoes are far more likely to suffer from blight infections. With tomatoes, the best solution is to try to grow your plants in a greenhouse, frame or similar structure.
The blight fungus is very weather-dependant and for the infection to occur there needs to be at least two consecutive 24hr periods each of which have a minimum temperature of 10C (50F) and where there is a relative humidity of at least 89% for a minimum of eleven hours. Sadly these conditions are surprisingly easy to achieve, especially towards the middle to back end of summer when blight usually hits. I’m a great fan of the BBC weather app – it acts as my early warning system!
They are difficult to diagnose but the key symptoms are yellowing of the leaves, general poor growth and small tubers. The only way to confirm this very common pest is to dig up affected potatoes and examine the roots with a magnifying glass or microscope. Potato eelworm do NOT eat into potato tubers, they feed on the roots alone. If you have holes in your potatoes suspect slug or cutworm or wireworm (see below) damage. Click here for our detailed article on potato eelworm.