Phytophthora Infestans: How To Control Late Blight

Phytophthora infestans is a pathogen that causes late blight in potatoes, tomatoes, and more. We provide strategies for managing this disease!

Tomato leaves with Phytophthora lesions

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Phytophthora infestans is one of the most destructive plant pathogens in the world. Also known as late blight, this pathogen is the notorious culprit behind the Potato Famine of the 1840s in Ireland, and many other severe epidemics since. 

Scientists have conducted intensive studies of the plant pathology and late blight management over the past 150 years due to the economical significance of disease outbreaks. Solanaceae crops such as potatoes (Solanum tuberosum) and tomatoes (Solanum lycopersicum) and ornamental plants such as petunias are particularly vulnerable. 

Late blight can result in severe symptoms that are visible on different parts of the host plant including stems, leaves, fruits, and tubers. In cold and wet conditions, late blight can wipe out entire fields in a matter of days. Infected plants take on a brown and wilted appearance as if being damaged by frost. 

Systemic fungicides are the most common treatment to combat this plant pathogen, but with many drawbacks. Fungicides can be expensive, most are prohibited in organic production, they pose risks to the environment, and may lead to the development of resistant strains.  Pathogen resistance is one reason why even in the present time, late blight continues to cause over $6 billion in damages every year to tomato and potato crops. 

What Is Phytophthora Infestans?

Tomato leaves with Phytophthora lesions
Tomato leaves with Phytophthora lesions. Source: zonenklaus

Phytophthora infestans is also commonly known as late blight, potato late blight, or tomato late blight. Similar to many other plant diseases, this pathogen changes its host’s physiology and immune system. However, P.Infestans is not a virus, fungus, or bacterium; it is classified as an oomycete or water mold. 

There are several other known oomycete pathogens that cause damping off, seedling blights, and downy mildew, to name a few. Of all the phytophthora species, this one is the most well-studied and is one of the most devastating phytophthora diseases worldwide. P. infestans originates from the Americas, although there is ongoing academic debate on whether it is from South America (Andes) or North America (Mexico).  

Life Cycle of Phytophthora Infestans

Late blight is a host-specific pathogen that requires plant tissue to grow. This oomycete can produce both sexually and asexually. Asexual reproduction happens rapidly and can widely disperse the pathogen throughout the growing season and sexual reproduction helps with the genetic diversity in order to develop resistance genes to combat fungicides. 

The majority of the reproduction happens asexually during the disease cycle. P. infestans develops sporangiospores or specialized branched hyphae that leaves a host plant through the stomata of stems and leaves. These sporangiospores produce sporangia or zoosporangia which are often released in the mornings when there is a drop in relative humidity and the environment is warmer.  

Zoosporangia are airborne and can spread by wind to neighboring plants where they might germinate and directly infect plant cells. When the weather is cool and there is a lack of nutrients, zoosporangia will develop biflagellates which enable them to be mobile. In this process of indirect infection, the zoosporangia will encyst, develop germ tubes, and use them to penetrate plant tissues. 

In the sexual disease cycle, P. infestans uses the A1 and A2 mating types. Male and female reproductive organs meet and produce oospores. This type of reproduction is more common in stems than in leaves. Oospores are very robust with thick walls and can survive for years in the soil. They are tolerant of cold temperatures but more sensitive to heat. Oospores also form germ tube structures and produce sporangia. 

The sporangia can then release zoosporangia and infect hosts through indirect or direct germination. Oospores are less common because they require both mating types to be found together. In the Northeastern U.S., for example, only one mating type is present. 

Symptoms Of Phytophthora Infestans

Late blight infecting tomato
Late blight infecting tomato. Source: NYSIPM

Plants with late blight disease will first exhibit small, brown, water-soaked spots on their lower leaves. Under cold and wet conditions, these lesions will spread rapidly from the leaf tips or the edges to the whole leaf. Over time in late blight development, the lesions may turn a greasy black and ringed by a yellow chlorotic halo. 

Leaves, petioles, and stems can all become blighted in just a few days. When this oomycete actively sporulates, it creates a ring of white mildew around each lesion. An infected potato plant will exhibit symptoms both above and below ground. Instead of showing healthy dark green leaves, a diseased potato leaf will have lesions and chlorosis. Infected potato tubers will show light red or brown granular rot. Diseased tissue will be streaked with brown rot, extending into healthy tissue. In tomatoes, stems are particularly identifiable from other diseases as they will develop patches of brown, almost woody-looking blight amidst the healthier green tissues.

Late blight is often accompanied by secondary tuber infection caused by pink rot or soft rot bacteria. A plant with open tuber infections will become more vulnerable to all sorts of pests and diseases. As the disease severity progresses, the outbreak will cause the cultivated area to take on a distinct odor. 

What Plants Does Phytophthora Infestans Effect?

Phytophthora infestans in potato leaves
Phytophthora infestans in potato leaves. Source: ScotGovSASA

Phytophthora infestans populations primarily impact plants in the Solanaceae or nightshade family. This family includes crops like potatoes and tomatoes and ornamental plants such as Petunia spp. and Calibrachoa spp. The pathogen also affects closely related wild species such as bittersweet nightshade (Solanum dulcamara) and hairy nightshade (Solanum sarrachoides). 

There are many different strains that are more adapted to infect different species of hosts. Some researchers also hypothesize that the pathogen spreads by overwintering in wild Solanaceae and then infecting crops in the following season. 

Controlling Phytophthora Infestans

Phytophthora infestans sporulation on underside of leaf
Phytophthora infestans sporulation on underside of leaf. Source: katzlab

P. infestans favors a cool and wet climate with day time temperatures between 60-70 F and night time between 50-60 F. Rain, dew, or overhead irrigation can all foster the moisture level and relative humidity needed for the disease to proliferate. One of the main ways to control late blight is to create an unfavorable environment for the pathogen. In general, good IPM (integrated pest management) methodologies around cleanliness and removing plant debris is also important.   

If late blight is present in your garden, there is a chance that it might spread to nearby gardens and neighboring agricultural fields. It’s important to monitor your garden to decrease this community risk. If you find late blight, reach out to your local extension office and their plant health instructor to verify and mitigate any infection risks. 

Start your growing season by inspecting tomato plants to make sure that the seedlings are disease free. Likewise, for potatoes, purchase certified seed tubers before planting. If you have cultivated potato crops in the previous season, try to remove neighboring wild varieties of Solanaceae and in-ground potato tubers so they can’t become the initial inoculum for P. infestans

Avoid planting in areas that tend to remain wet for a prolonged period of time. Use drip irrigation and do not water during the night. Tomato plants are susceptible to other fungal and foliar diseases. Good cultural practices like mulching to prevent splash backs, removing the lower leaves and suckers to encourage ventilation, and staking the plants can all help with general disease prevention. If you practice crop rotation techniques, make sure not to plant tomatoes in areas that previously grew potatoes.  

Preventing Phytophthora Infestans

Bad Phytophthora infestans infection
Bad Phytophthora infestans infection. Source: Agroscope

Fungicides are most commonly used to prevent infection even if this pathogen is not a fungal disease. Carefully read the fungicide packaging and follow application instructions. Non-organic gardeners can consider preventative fungicides that include chlorothalonil and Mancozeb. Fixed copper fungicide products may be used for organic crops.

Spray fungicides at the beginning of the flower stage. In case of high risks of late blight, spray fungicides right after transplanting and continue throughout the growing season. 

There are drawbacks to using fungicides, including environmental risks. Furthermore, P. infestans adapts quickly and could develop fungicide resistance. There is an ongoing arms race between plant researchers who are trying to develop new disease-resistant cultivars and the pathogen itself. 

Because P. infestans is a rapidly evolving pathogen with wide genetic diversity, there are many clonal lineages of the diseases. Crop scientists have developed several hybrid tomato varieties that have late blight resistance such as Mountain Magic, Jasper, and Matt’s Wild Cherry. 

Geneticists have also been working to produce a new potato variety (AWN86514-2) that is resistant to late blight, although new potato cultivars are a few years away from entering the commercial market. 

Frequently Asked Questions

Blighted stem on tomato
Blighted stem on tomato. Source: Tony Austin

Q: Which disease is caused by Phytophthora infestans?

A: P. infestans causes late blight in plants. This disease is also known as the potato late blight or the tomato late blight. This disease should not be confused with another potato blight, early blight, which is caused by the fungal pathogen Alternaria solani

Q: How is Phytophthora infestans spread?

A: P. infestans is most commonly spread by infected seed potatoes which include unharvested infected tubers left in the soil. As these tubers grow into future potato plants, they become hosts that produce airborne spores and transmit the disease to neighboring plants. 

Q: Why is Phytophthora infestans famous?

A: Phytophthora infestans is the plant disease responsible for the potato famine which led to mass starvation in Ireland during the 1840s. It has been widely studied by botanists and is considered one of the first diseases to be studied under the modern scientific field of plant pathology. 

Potato late blight epidemics can wipe out entire potato crops in just a few days. Likewise, tomato fruits are also susceptible depending on disease severity values. Often, harvested tomatoes will continue to develop lesions after they’ve been picked as a part of the ongoing disease development.  

Q: Is Phytophthora a fungus or bacteria?

A: The Irish Potato Famine fungus Phytophthora infestans is not a fungus, virus, or bacteria. It is actually an oomycete or water mold.

Q: How do you get rid of Phytophthora in soil?

A: P. infestans may produce oospores that can persist for several years in soil. However, in most parts of the world that only experience asexually produced P. infestans, the zoospores need living host plant tissue to survive. The most common way for this disease to persist in soil is from unharvested potato tubers infected with the disease. 

After harvest, make sure that all potatoes have been dug up. If there are any signs of late blight, dispose of the tubers. Remove all plant debris and potato or tomato foliage from the area and remove wild nightshade plants that help the pathogen overwinter. 

Q: Is Phytophthora harmful to humans?

A: Phytophthora infestans is not harmful to humans. However, it is not recommended to eat tomato fruit or potato that shows signs of late blight lesions. It is safe to eat unblemished tomatoes that are harvested from vines that show symptoms of late blight, but do not can or freeze these tomatoes. 

Fruits harvested from blighted vines have a tendency to develop lesions post-harvest which lowers their acidity. Other harmful microorganisms might develop in a low-acid environment. 

Q: Can Phytophthora be cured?

A: No, once a plant is infected with Phytophthora infestans, it cannot be cured of it. The best course of action is to remove the infected plant to prevent the further spread of the disease. There are several hybrid tomatoes that have a resistance gene to the disease and a new potato cultivar is currently being developed. These effector genes aggressively develop to deter incoming threats.

There is also some promise involved with potato and Solanum demissum, which is a wild Mexican potato that shows resistance to the disease. 

Q: Is Phytophthora airborne?

A: Yes, Phytophthora infestans produces millions of spores that can become airborne and dispersed by wind. 

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