Potato Mop Top

With a name like potato mop top, you know I had to dig deeper!


I immediately pictured Carrot Top crossed with Mr. Potato Head. Ends up, it’s nothing like that. In fact, plants infected with potato mop top may look exactly like healthy plants. We’ll get to that in a minute. What’s strange about this viral disease is that it is spread by another disease.

Potato with wig (StickPNG) Public Domain

The powdery scab connection

You know how when you catch a cold, you’re more likely to get a canker sore? Well, plants infected with powdery scab become a lot more vulnerable to the potato mop top virus (PMTV). Both diseases are caused by a Cercozoa [think amoebae] called Spongospora subterranea f. sp. subterranea. Imagine having to write that return address every time you sent a letter!


Anyway, powdery scab is a type of slime mold that occurs in areas with poor drainage. When powdery scab is present, your potato plants are more likely to catch mop top. They can still catch PMVT without powdery scab, but it is not as likely, and infections that occur tend to be less damaging.


Potato mop top lifecycle

Potato mop top spores can remain viable in the soil for up to 18 years. All they need is some cool, moist weather and some potatoes infected with powdery scab to work their way into tubers and the root system. From there, they move into the xylem and travel throughout the plant, blocking the flow of water and nutrient-rich sap. And potatoes aren’t the only garden plants susceptible to this disease. Your tomatoes and groundcherries may also become infected. Other nightshade family members and plants in the amaranth family, such as beets and spinach, may also end up with potato mop top.


Potato mop top symptoms

Leaves of infected plants will show signs of yellowing and the internodes will be shorter. Internodes are the spaces along a stem between two leaves, buds, or stems. This creates a stunted, “mop top” appearance. Foliar (leaf) symptoms can vary widely. Some potato varieties may exhibit a bright yellow blotch pattern (aucuba pattern) in the lower leaves, while others may have V-shaped yellow patterns.

Leaves infected with potato mop top (William M. Brown Jr., Bugwood.org) CC BY 3.0

When you cut into a potato infected with PMTV, you know something is wrong. You’ll see dark brown lines and spots throughout the tuber. Those symptoms are known as spraing. This damage looks similar to the alfalfa mosaic virus. You may also see deep cracks and skin distortions.

Tubers infected with potato mop top (Owusu Domfeh) CC BY-SA 3.0

What’s strange about potato mop top is that plants grown from cuttings of infected plants may not show any aboveground symptoms, but they will have even more infected tubers than the parent plant.


Potato mop top management

The potato mop top virus is responsible for crop losses of up to 67% and it is a tough disease to control. There are no resistant varieties. Not yet, anyway, though russet-skinned potatoes seem to be less sensitive to PMTV. And there are no chemical treatments against viruses either.


Commercial growers apply fungicides that contain fluazinam to the soil to control the long-winded amoeba-like creatures mentioned earlier. Fluazinam products are commonly used against late blight, another potato disease.


Good sanitation and vector controls are really your only options when it comes to managing potato mop top. This means insisting on certified disease-free seed potatoes and nothing less.


Plus, you can reduce the occurrence of powdery scab by improving drainage. The easiest way to do this is by mulching with a thick layer of free arborist chips. [My concrete-like clay soil is now loose, dark, and productive, thanks to free wood chips!]


Other practices that help reduce the odds of potato mop top occurring in your garden include avoiding excessive applications of fertilizer that contain nitrate or ammonium nitrogen. These stimulate root growth, providing lots of tender new victims to both diseases. Also, allow the soil to dry out between waterings, especially early in your potatoes’ development. And wait until the soil is warmer to plant in the first place. This helps your potato plants be stronger and healthier from the start. And, as always, be sure to disinfect your garden tools regularly.


If a plant becomes infected with potato mop top, it must be removed and thrown in the trash. What’s worse, you can never grow potatoes in that soil again. Ever.


As spring approaches, I’m watching my potato patch as it starts waking up from its winter sleep. New shoots are beginning to appear and I am monitoring for signs of this and other diseases so I can nip them in the proverbial bud. (Sorry, I couldn’t resist.)