Calico

You may love calico cats, as I do, or you may have fond memories of calico dresses from a favorite prairie-crossing children’s series, but calico in the plant world is something else entirely.

Alfalfa mosaic virus on potato leaves (Howard F. Schwartz, Colorado State University, bugwood.org) CC BY 3.0

Calico is a viral disease that can infect lentils, potatoes, tomatoes, peas, tobacco, and 600 or more other plants. There are several strains of this virus, most of which are species-dependent.The calico virus prefers warm, sunny days and soil with a slightly alkaline pH of 7–7.5. Research has shown that plants infected with calico have difficulty absorbing critical plant nutrients such as copper, iron, manganese, and zinc.

Symptoms of calico

Calico, also known as Lucerne mosaic or alfalfa mosaic virus (AMV), is easy to spot. Visible in a dark green sea of potato plants, you will see a bright yellow patch or yellow blotching. Infected leaves may look shiny compared to their healthy neighbors. You may also see wilting or severe stunting. Closer inspection will show dead stems and tubers. The inside of your potato harvest may look dry or corky. If your potato plant looks more like a pale yellow Christmas tree, it is probably potato psyllid feeding.

How calico reaches your garden

Several aphid species can carry calico. Potato aphids and green peach aphids are the usual culprits. Infected seeds and pollen can also transmit this viral disease, as can alfalfa, clover, wheat, and parasitic dodder. Infected plants should be removed and tossed in the trash, not the compost pile.

To avoid AMV in your potato patch, plant only certified disease-free tubers, keep your potatoes away from clover and alfalfa, and sanitize your tools regularly.