Potato Mop Top

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

I immediately pictured the Carrot Top crossed with Mr. Potato Head. But it’s nothing like that. In fact, plants infected with potato mop top often look perfectly healthy. What’s strange about this  viral infection is that it is spread by a different disease.

The powdery scab connection

You know how you are more likely to get a cold sore when you catch a cold? Well, plants infected with powdery scab become more vulnerable to the potato mop top virus (PMTV). Both diseases are caused by a Cercozoa [think amoebae] called Spongospora subterranea f. sp. subterranea.

Potato with wig (StickPNG) Public Domain

Powdery scab is a slime mold that occurs in areas with poor drainage. When powdery scab is present, your potato plants are at higher risk of catching mop top. They can still be infected with PMVT without powdery scab, but the disease is less likely. Those 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 to become active is some moist weather, cool temperatures, and 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 sugary 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 become infected 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, and others may have V-shaped yellow patterns.

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

Cut into a potato infected with mop top, and it becomes clear that 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 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%. 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. 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.

These good sanitation and vector controls are your only options when it comes to managing potato mop top:

• Install certified disease-free seed potatoes and nothing from the grocery store.
• Improve drainage by mulching unplanted areas with a thick layer of free arborist chips, which improves soil structure.
• Avoid excessive applications of fertilizer that contain nitrate or ammonium nitrogen. These stimulate root growth, providing tender new victims for both diseases. 
• Allow the soil to dry out between waterings, especially early in your potatoes’ development. 
• Wait until the soil is warmer to plant in the first place. This helps your potato plants become stronger and healthier from the start. 
• As always, be sure to disinfect your garden tools regularly.

As spring approaches, watch your potato patch begin to wake up from its winter sleep. As new shoots appear, monitor for signs of this and other diseases so you can nip them in the proverbial bud.