By: Mary Ellen Ellis
Potatoes in your garden can fall victim to a bacterial infection called blackleg. The term blackleg is often used to describe both the true disease, which arises from infected seed potatoes, and a condition called stem rot. With the right potato blackleg information, you can prevent or control this disease for which there is no chemical treatment.
What is Potato Dickeya – Blackleg Potato Symptoms
Two groups of bacteria cause this infection: Dickeya, which is one alternate name for the disease, and Pectobacterium. Previously these groups were both classified under the name Erwinia. Blackleg caused by Dickeya is more likely in high-temperature conditions, and is, therefore, more common in warmer climates.
The symptoms of this bacterial infection begin with lesions that appear water soaked. These turn up on the base of the plant’s stem. As the infection progresses, the lesions will come together, get larger, turn darker in color, and move up the stem. When conditions are wet, these spots will be slimy. When conditions are drier, the lesions are dry and the stems desiccated.
As the lesions develop on the stem, secondary infections may begin higher up. These then progress downward, meeting the original lesions. Other symptoms may include yellowing, brown, or wilted leaves attached to the affected stems. Eventually, the entire plant may collapse and you may see rotting in the tubers.
Controlling Dickeya Blackleg of Potatoes
Potatoes with blackleg, once infected, cannot be treated with any chemical spray. This means that prevention and management through cultural practices are the best and really only ways to avoid losing a crop to the infection.
One of the most important things you can do is buy and use seed potatoes that are certified to be disease-free. Even with clean seed potatoes, infection can get in, so use those that don’t have to be cut or clean equipment well if you do have to cut seed potatoes.
If the infection is in your garden already, you can manage it with several cultural practices:
- crop rotation
- using well-drained soil
- avoid over-watering and over-fertilizing
- removing and destroying infected plants
- regularly cleaning up plant debris from the garden
Harvest your potatoes only when they are fully mature, as this ensures the skin is set and the tubers won’t bruise easily. A few weeks after the plant is dried and desiccated should ensure the potatoes are ready for harvest. Once harvested, make sure the potatoes stay dry and remain unbruised.
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Causal Organisms: Dickeya spp. and Pectobacterium spp.
Tuber soft rot, blackleg and aerial stem rot are important potato diseases caused by closely related species of bacteria. More specifically, these diseases are caused by the bacteria Dickeya spp. and Pectobacterium spp. (formerly classified in the genus Erwinia). Dickeya and Pectobacterium can infect many plant species, although they don’t appear to thrive on legumes or small grains. Dickeya and Pectobacterium produce enzymes that can break down the pectin of plant cell walls, resulting in tissue maceration or decay. These diseases occur sporadically in potato production areas worldwide. They can be particularly destructive when environmental conditions favor their development. Aerial stem rot and blackleg disease have increased in prevalence over the past decade in some European countries and Israel.
Visually, Dickeya and Pectobacterium cause similar symptoms. Therefore, lab-based diagnostics are important in specifically identifying the bacteria that cause tuber soft rot, blackleg and aerial stem rot. Symptom expression can also differ based on environmental conditions. Wet conditions are conducive to disease development, enabling these pathogens to aggressively attack tubers and stems, causing significant crop losses.
The diseases caused by Dickeya and Pectobacterium result in a reduction in plant emergence, plant health and overall plant stand. The bacteria also cause losses in storage, transit and marketing. All potato varieties are prone to infection, with some varieties being more susceptible. Certain varieties can be stored long-term with a reduced risk of tuber breakdown. Disease management strategies can be employed at planting, during the growing season, at harvest and in storage to reduce the risk of these pathogens. However, as with most potato diseases, an integrative approach is important to reduce crop losses.
Potato Blackleg Information: Tips For Treating Dickeya Blackleg Of Potatoes - garden
Scientific description of Dickeya (2020)
The authors of this content are Amy Charkowski, Kalpana Sharma, Monica L. Parker, Gary A. Secor, John Elphinstone
Blackleg and Soft Rot of Potato Caused by Dickeya
Taxonomy and Nomenclature
The genus Dickeya is a member of the ß-Proteobacteria in the family Pectobacteriaceae within the order Enterobacterales. The Pectobacteriaceae family also contains the genera Brenneria, Lonsdalea Pectobacterium, and Sodalis (Adeolu et al. 2016).
Members of the Dickeya genus originally belonged to the genus Erwinia represented by strains within species E. chrysanthemi (Burkholder et al. 1953).
Later this species was reclassified as Pectobacterium chrysanthemi (Hauben et al. 1998), until Samson et al. (2005) elevated the species to the genus Dickeya with six species.
There have since been some changes and additions to these species, which currently include D. aquatica, D. chrysanthemi, D. dadantii, D. dianthicola, D. fangzhongdai, D. paradisiaca, D. solani, and D. zeae (Brady et al. 2012 Parkinson et al. 2014 Samson et al. 2005 Tian et al. 2016).
Dickeya has a broad host range and can infect plant species in at least 12 dicot families in 10 orders and 10 monocot families in 5 orders, and include ornamentals such as chrysanthemum, carnation, dahlia, and calla lily as well as important crops including carrot, tomato, and, the most economically important, potato (Charkowski 2018 Ma et al. 2007a, b Samson et al. 2005).
While all Dickeya species, with the exception of D. paradisiaca, have been found on ornamentals in Europe, only D. dianthicola and D. solani have caused significant economic losses on potato (Toth et al. 2011). In both cases, the lack of genetic diversity between isolates on potato and ornamental hosts suggests the organisms may have spread to potato from such a host (Parkinson et al. 2009 Slawiak et al. 2009).
Only D. aquatica, which was isolated from waterways in the UK (Parkinson et al. 2014) and Maine (J. Hao, personal communication), has not yet been associated with a plant disease.
As with Pectobacterium, Dickeya species have been reported on a wide range of hosts in different countries around the world (Samson et al. 2005). While D. zeae, D. solani, and D. dianthicola have wide geographic distributions, D. paradisiaca appears to be restricted to Colombia (Samson et al. 2005 Toth et al. 2011).
D. dianthicola was the first Dickeya species to be associated with plant disease in Europe, occurring on Dianthus in the Netherlands, Denmark, and the UK and later spreading to other nations (Hellmers 1958). It was later associated with other ornamentals and crops in a number of European countries, including potato.
In some cases, D. dianthicola replaced P. atrosepticum as the dominant blackleg pathogen (Parkinson et al. 2009 Toth et al. 2011). D. solani was recognized independently as a new Dickeya pathogen on potato by several groups from 2004 through 2010 (Laurila et al. 2008 Parkinson et al. 2009 Slawiak et al. 2009).
Isolates of both D. dianthicola and D. solani show little genetic diversity compared to isolates from ornamentals, and within these species there is a high degree of genetic similarity. Therefore, it seems likely that these pathogens have independently jumped host from an ornamental onto potato (Toth et al. 2011).
Although Dickeya can cause tuber soft rot, it primarily causes blackleg on potato. Blackleg symptoms include necrosis of the potato stem, originating from the mother tuber and spreading several centimeters above ground (image below).
Plant leaves will wilt and curl as the disease develops and the plant vascular system will become necrotic. The pith of the stem is often decayed. D. dianthicola can also cause severe seed decay and lack of plant emergence in severe cases.
Infected plants produce few or no tubers and any tubers produced may decay prior to harvest. Both Dickeya and Pectobacterium may be present together in diseased plants. In the United States, P. parmentieri is the most common species found together with Dickeya.
Foliar symptoms of Dickeya dianthicola on potato. Initial symptoms are either a lack of emergence or leaf curling (a). The base of the stem turns dark brown or black and this necrosis can extend several centimeters from the soil line (b). The pith inside symptomatic stems is often decayed and the xylem are necrotic for several centimeters above the external stem necrosis and the pith decay (c). Disease symptoms may only develop on one stem of a multi-stem plant (d).
(Courtesy: Amy O. Charkowski | Colorado State University)
Initial seed potato production relies on pathogen-free micropropagated plantlets. These plantlets are grown in greenhouses or screenhouses to produce minitubers, which are used for field planting (Frost et al. 2013). Dickeya will kill micropropagated plants within a few days and is not typically found in greenhouses or screenhouses.
It appears to contaminate potatoes after they have been grown for at least one generation in the field, with the risk of contamination increasing with each generation in the field. Dickeya does not appear to survive in soil, but it can contaminate waterways and survive for long periods in surface water (Toth et al. 2011).
It may also survive in weeds (Fikowicz-Krosko and Czajkowski 2017) or volunteer potatoes and spread by insects (Rossmann et al. 2018). Like Pectobacterium, Dickeya appears to spread mainly at harvest, where it can spread from infected vines and tubers to previously uncontaminated tubers.
The bacteria are mainly found on tuber lenticels, but may also be present in the tuber stolon scar. Asymptomatic infestations are common, so it is not possible to visually assess seed potato lots for risk.
Blackleg development is highly dependent on the environment and it is unpredictable, even when a seed lot is known to be contaminated with Dickeya. Plants grown from infested seed lots planted in warm, humid areas tend to develop disease, while plants grown from the same infested seed lot planted in cooler, drier climates may remain healthy.
Temperatures above 30 °C during the growing season appear to be particularly conducive to disease development. Co-contamination with Pectobacterium and Dickeya appears to lead to disease development more frequently than when only Dickeya is present.
Pathogenicity Determinants and Resistance
Dickeya pathogenicity relies mainly on pectate lyases and other plant cell wall-degrading enzymes secreted by the bacterial cell, although several other virulence genes are known (Charkowski et al. 2012). Although both Pectobacterium and Dickeya use plant cell wall-degrading enzymes, there are some important differences in enzyme genes and gene regulation between the genera that may account for some of the differences in disease symptoms.
There are no examples of gene-for-gene resistance with Dickeya and the basis for resistance to Dickeya in wild potato species or for host range is poorly understood. There are no resistant commercial potato varieties, but varieties do differ in tolerance.
Significance and Economic Loss
The relative importance of Dickeya as a potato pathogen appears to be increasing (Toth et al. 2011). D. solani caused severe losses in the early 2000s in multiple countries and in 2015 D. dianthicola was in up to 20% of seed potato lots in some states in the US. Recent development of species-specific PCR assays for Dickeya will likely reveal that it is widespread in potato. As with Pectobacterium, farmers lose millions annually to blackleg caused by Dickeya.
Cultural practices are important for Dickeya management and the recommendations are essentially the same as for Pectobacterium (Czajkowski et al. 2011, 2013). Growers initiate potato production with micropropagated plantlets that are free of Dickeya, but tubers may become contaminated once they are planted in fields.
To reduce the risk of disease spread, growers should sanitize equipment thoroughly between seed fields, especially if blackleg is present. At planting, growers should fully suberize seed if they are using cut seed, and they should not plant seed that is too cold or into saturated ground.
During the growing season, they should irrigate with ground water if possible and not overfertilize with nitrogen. Rouging infected plants is likely to spread the pathogen if diseased plants are present. At harvest, the Dickeya may multiply on the vines as they senesce, so quickly killing potato vines may aid in reducing disease incidence the following year.
Tubers should be allowed to heal before cooling storages. Good airflow and high humidity in potato warehouses will also aid in reducing soft rot in storage. High levels of carbon dioxide in warehouses will promote soft rot development.
Seed potatoes may be tested for Dickeya prior to planting (Czajkowski et al. 2015 Humphris et al. 2015) and growers should avoid planting contaminated seed lots in areas where growing conditions are conducive to blackleg.
Potato Plant Turning Yellow
If your potato plant is turning yellow, there could be several reasons for that. Here are some.
Diagnosis – Wilt
Wilt is the most common reason why your plants are turning yellow. Of course, there are two types of wilt, and you have to figure out which one you have on potatoes to know how to fix that problem.
For one, there’s the verticillium wilt, which primarily affects the leaves. Under its effect, the leaves change into yellow color, and they become weaker. The first place where you might notice the signs is the bottom of your plant since this disease travels upward. If the problem remains unmanaged, it will cover the entire plant.
This is why it’s important to check your plants frequently. If you do, you’ll notice the problem sooner and be able to save your yield. This infection is quite stubborn, and it won’t just go away. The problem is that it can survive well in cold weather — so cold won’t help you get rid of it — but the heat helps it spread. In either case, you have a tough infection on your hands.
The biggest issue with this disease is that it will prevent your plant from using water.
The other type of wilt you might be dealing with is fusarium wilt, which is similar to the former type. However, the yellowing will appear first in the veins of your leaves, and it will move from there into the stems. The hot weather makes this type of wilt worse, and it is persistent, just like the former type of wilt. It starts at the roots, though, so noticing it will be harder.
But, you can act as soon as you notice discoloration in the leaves.
Wilt, as mentioned, is stubborn, and it won’t just go away. However, you can start the management process by identifying which wilt you’re dealing with. Most of the time, you’ll have a problem with verticillium wilt.
To avoid it in the future, plant grass or cereals at the spot where the potatoes have been to remove it. Rotate these plants every year. You might be able to fix it with fungicides, but it’s best to ride it out and then rotate plants to remove it completely.
Potato Psyllids Diagnosis
This is a bug that attacks potatoes, but it’s quite small. It’s black with white parts, and it releases a chemical that makes the leaves of the potato plant turn yellow. The leaves might curl as well.
The damage will start at the edges of your leaves and travel all over your plant. Of course, you want to prevent this. But, even a small amount of nymphs of these bugs can create a disaster for your potatoes.
Potato Psyllids Remedy
Start by identifying that you really do have a problem with these bugs. Do this with traps that you can purchase at your local gardening store. Then, once you know that you have this problem, you can move on to treatment.
The treatment includes using certain chemicals that repel and kill these bugs. Follow the guidelines on these select pesticides and apply them properly. Remember to repeat the process when necessary.
Potato Blackleg Caused By Dickeya
Symptoms of Dickeya blackleg in the photographs below were found the first week of June 2016 (starting May 31). Affected plants had black stems extending up from the plant base and rotting seed piece, typically with an earthy smell that occasionally was offensive indicating soft rot bacteria were also present. High temperatures (in the 80s) during late May provided conditions favorable for symptom development. This pathogen is difficult to manage when developing in a crop because it is inside stems moving upward. In contrast, Pectobacterium, the pathogen causing the type of blackleg occurring in the past, starts on the outside of stem tissue, infects through wounds, and then moves downward as well as upward causing stem rot that is dark brown. Affected tissue typically has an offensive odor. Certified potato seed that is free of Dickeya is key management practice for this disease.
The following photographs were taken in commercial potato crops on Long Island by Sandra Menasha, Vegetable/Potato Specialist with Cornell Cooperative Extension-Suffolk County.
Margaret McGrath, Associate Professor
Long Island Horticultural Research & Extension Center
Riverhead, NY 11901-1098
Email: [email protected]
Every summer and fall, I hire research assistants to help me conduct my research. Interested? Contact me: [email protected]
Podcast: What’s wrong with my tomatoes?
Meg McGrath talked with Margaret Roach about tomato troubles on Roach’s popular A Way to Garden podcast.