Potato blight is caused by a fungus, of which there are two different types – Alternaria solani in Early potato blight and Phytophthora infestans in Late potato blight.
For a gardener, there is very little that is more heartbreaking than pouring time, energy and money into a crop only for it to fail. Some pests and diseases can decimate an entire harvest in just days; for the potato farmer, the most devastating of these is blight.
How to grow potatoes
If you are interested in learning more about how to grow your own potatoes please read my article How to grow potatoes at home. This covers everything you need to know from planting, preparing seed, growing, harvesting and storage and more about this great vegetable.
What is blight?
In fact, though two types of blight can infect potatoes, the term more commonly refers to late potato blight.
This disease can affect any member of the nightshade family but is seen most often in tomatoes and potatoes; in fact, late blight was the direct cause of the Irish potato famine in the 1840s. It is quite common in warm, wet weather.
Tubers infected by late blight are particularly susceptible to soft rot; if some tubers in your crop are infected, the remaining crop should be stored in cool, dry conditions to prevent the disease from spreading to the healthy tubers.
Symptoms of late blight
The initial symptom of late blight is small dark spots with green or yellow borders on the foliage, which can develop into large brown or purplish black patches and lesions; in moist weather, a white powdery growth appears on the dead foliage and the underside of the leaves. Eventually, blight causes total rotting of the foliage. As the disease progresses, slightly depressed areas of brown and purplish skin appear on the tubers; when cut open, tan-brown, dry, granular rot extends 1-2 cm into the tuber.
More typically found in North American, early blight is a fungal disease that is not common in European gardens. It is frequently confused with magnesium deficiency. It affects plants early in the growing season but will avoid new, vigorous growth, preferring to establish itself on older leaves.
Symptoms of early blight
Spots appear first on the older, lower leaves; initially small, dark, dry, papery flecks, they will grow to become brownish-black circular or oval patches with veiny borders surrounded by a yellowish or greenish ring.
The tuber will also be affected; dark, usually circular spots with raised purple or metallic grey edges appear on the surface and when sliced open, the flesh is brown and dry, with a leathery or corky texture.
What causes potato blight
Both early and late blight are caused by fungal infections. As their names indicate, theses disease occur in the early and late growing season, respectively; they can, however, be present at the same time.
A. Solani is present in most potato production regions in North American, but doesn’t significantly affect the crop yield unless frequent wetting of the foliage allows the fungal system to develop. The disease severity is highest when the potato plants are already vulnerable such as from other illnesses, stress or lack of proper nutrition.
The life cycle of A. solani starts with the fungus overwintering in either wild member of the nightshade family or in crop residues; in the spring, spores are produced that can be spread by wind or water onto nearby uninfected plants. Once the infection has taken hold of the plant, it will produce more spores and continue spreading; diseased tubers can also spread the fungus after harvesting either in storage or if left behind in the soil.
Late blight is caused by the fungus-like pathogen Phytophthora infestans which seems to have originated in Mexico, spreading first to the United States, then into Canada and Europe by the 1840s – in subsequent decades, P. infestans spread farther, reaching worldwide distribution by the beginning of the 20th century.
The pathogen prefers moist, temperate environments and can spread rapidly in the right climates.
Though it survives poorly in nature without a nightshade plant host, it has the potential to cause devastating disease wherever potatoes are grown.
Entire fields of previously healthy plants can be destroyed in a matter of days.
Late blight can also infect seed potatoes which is why it’s recommended to buy certified seed every year rather than planting old potatoes; even diseased potatoes from previous crops can, if dumped or composted, spread P. infestans.
Typically overwintering in diseased tubers left either in storage or in the soil, P. infestans will produce sporangia that usually disperse by air over several kilometres and can survive up to an hour in dry, sunny conditions or much longer under cloudy conditions.
Upon contact with water, the spores are released onto the plant surface and penetrate into the leaf tissues.
Once established on a host, the pathogen can produce between 100,000 and 300,000 sporangia per day – from a single lesion on the host plant.
Each of these can initiate a new infection that will be visible within three to four days and that can produce its own spores in less than a week.
If an infected tuber makes its way to the storage bin, little can be done to prevent the disease from spreading throughout the bin except closely observing and checking the harvest and removing the infected potatoes as you find them.
New blight strains
The genetic populations of the fungi are continually evolving and new findings show that previously blight-resistant varieties of potato are not as effective against newer strains of A. solani or P. infestans.
Some newer strains have the potential of reproducing sexually when they meet which means that the pathogen also has the potential to produce resting spores (called oospores) in the affected plant tissues; these are released to contaminate the soil as the tissues decompose and may survive underground for several year.
If the resting spore develops and infects a plant, it may generate a new strain with characteristics of both ‘parent’ strains. Though scientists continue to investigate fungal pathogens and acquire more information about newly evolved or developing strains, we do not yet know how long oospores can survive the harsh winter climate, or whether all – or any of them will be able to develop and find a host, thus spreading blight.
There are many potato blight sprays commercially available, although these can be expensive if you only have a few drills of potatoes planted.
There is another way though- I have written an article about making your own homemade potato blight spray which shows you how to make your own blight spray when it is needed.