Calico

You may love calico cats (I do!), or have fond memories of calico dresses from a certain 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 alfalfa, lentils, potatoes, tomatoes, peas, tobacco, and 600 or so 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 causes reduced levels of important 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. Clearly visible in 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, or dry, corky areas inside your potato harvest.


If your potato plant looks more like a pale yellow Christmas tree, it is probably potato psyllid feeding.


How calico is spread

Calico is spread by several species of aphid, but potato aphids and green peach aphids are the usual culprits. Infection is normally spread when aphids move from alfalfa, clover, or wheat to potato plants. Infected seeds and pollen can also carry this viral disease, as can 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.