Plant Pandemics

We have all become more familiar with pandemics than any of us expected, thanks to Covid.

​Plants have pandemics and epidemics, too, and they are becoming more frequent and more devastating. Plant pandemics destroy more than half of our food crops globally, costing an estimated $220 billion annually.

While over 7 million people have died from Covid-19 as of October 4, 2021, more than ten times that number died at the same time from hunger-related fatalities.

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Global agriculture or local gardens

Whether we are talking about pandemics in the global food supply or epidemics in neighborhood home gardens, the spread of disease affects us all.

Finding cures and treating sick plants takes time, costs money, and reduces crop size. Farmers and home gardeners can work to prevent these problems (easier) or deal with the consequences (harder). Plant pandemics are increasing in frequency and effect because of rising temperatures, global trade, pathogen spillover, and evolution.

Rising temperatures

Temperatures are rising around the world. Many pests that act as disease vectors used to die in winter. Warmer winters mean more disease-carrying pests are attacking plants, and pathogens, such as bacteria and viruses, are more active. And they are expanding their territories.

Changing storm patterns, drought occurrence, and other conditions also weaken plants and favor diseases. Monitoring plants regularly for signs of trouble can help nip those problems in the bud.

Global trade

Travel along the Silk Road 2,000 years ago brought untold wealth and riches to the far reaches of the globe. It also furthered the development of science, literature, and medicine. And it increased the spread of diseases, such as intestinal parasites, the Black Death (bubonic plague), smallpox, and STDs (sexually transmitted diseases). Modern global trade has had similar effects on our gardens.

Silk Road routes (Wikiality) CC BY-SA 3.0

A single pest, virus, bacteria, or other plant problem can catch a ride on a shipment of produce, furniture, or dollar store doo-dad bound for your neighborhood. Before you know it, you’re up against something new. Some of those new problems can be devastating. And many countries are forced, through poverty, to ignore trade restrictions regarding pests, diseases, and chemical use. As a result, low-priced produce often comes at a higher long-term cost.

Pathogen spillover

In some cases, the carriers of diseases develop a taste for something new. Expanding food preferences are becoming more common as we craft new varieties of plants that tolerate non-traditional temperature ranges. As crops are grown in new areas, pests and diseases that favor those crops follow. Once established, those pests and diseases often develop a taste for the local flora, spilling over into these local crops. Also, where one plant disease may be tolerable, multiple diseases are often devastating.

Disease evolution

One thing I learned while studying for the Master Gardener’s exam was that pathogens push plants to develop better defenses. It’s one of Nietzsche’s “What doesn’t kill you makes you stronger” situations. Plants that die are removed from the gene pool, leaving more resistant specimens to reproduce. Of course, we don’t want anything to kill our herbs, tomatoes, or fruit trees. We want them to grow and thrive and produce. 

Like everything else, diseases evolve. A simple mutation can change everything. Insects and microorganisms evolve much faster than plants (or us). Sometimes a plant disease can have a mild effect for several years and suddenly mutate and become devastating.

Common plant epidemics

Monoculture and the increasing demand for food have led to the rise of several plant pandemics. These are some of the most common plant pandemics that you probably never heard about in the news:

• African cassava mosaic (Cassava mosaic begomovirus) – is spread by whiteflies and results in 25 million tons of food loss annually.
• Barley yellow dwarf--spread by more than 20 different types of aphids and is the most economically significant disease of cereals, causing up to 64% of crop losses in barley, corn, oats, rice, triticale, and wheat. 
• Cacao swollen shoot virus (CSSV)—killed over 200 million cocoa trees, carried by mealybugs.
• Citrus tristeza virus (CTV)—also known as citrus sadness, is responsible for the death of millions of citrus trees worldwide and is spread by brown citrus aphids.
• Maize lethal necrosis (MLN)—spread by aphids, mites, thrips, and more, causing up to 40% crop loss.
• Papaya ringspot—nearly caused the extinction of papaya in Hawaii, and it also infects cucurbits, such as squash and watermelon.
• Plum pox--spread by aphids; estimated to cost $10 billion annually.
• Potato leafroll--spread by aphids and causes the loss of 20 million tons of potatoes annually.

These are only a few of the plant pandemics currently happening. Like Covid-19, many of the carriers of these diseases can be asymptomatic, which means they look perfectly healthy. But adding them to your landscape can cause years of headaches and work.

Disease vectors

You can reduce the risk of plant diseases causing problems in your garden by monitoring insect movements in your region. Many insects carry diseases. Knowing which insects are coming your way can help you be better prepared. The Big Bug Hunt is a good tool I’ve used to see what’s coming. I’m sure there are others, but the Big Bug Hunt has the added benefit of providing photos and identification tools.

These other tips can help break the chain of plant epidemics and pandemics:

• Wash your hands and sanitize your garden tools frequently.
• Invest in certified pest- and disease-free seeds, seedlings, and plants.
• Look for disease-resistant plants.
• Quarantine all new plants.
• Time plantings to ensure favorable growing conditions.
• Encourage beneficial insects to call your landscape home.
• Avoid using broad-spectrum pesticides.
• Employ crop rotation.
• Get your soil tested every 3 to 5 years and fertilize plants based on science, not propaganda or guesswork.
• Remove and dispose of infected plants.
• Think twice about buying international products and only buying from reputable companies dedicated to preventing the spread of disease.

Finally, learn more about the relationships between the plants you are growing and the pests that threaten them.

Did you know that green peach aphids hide and spend their winters on stone fruit trees and can then carry potato leafroll to your potatoes in spring? Now you know.

Remember, the more you know, the better equipped you are to halt the spread of these and other plant diseases. We may not be able to vaccinate our garden plants to protect them (yet), but there are other things we can do.