Disease Resistance

Plants may not have an active immune system, but that doesn’t mean they passively roll over and take whatever hits them. 

When pathogens strike, plants can respond in two ways to prevent infection: they use pre-existing structures and chemicals and activate responses triggered by the presence of a pathogen.

Pre-existing defenses

Just as our skin blocks many pathogens from entry, a plant’s skin, or epidermis, does the same thing. That’s why insect feeding and mechanical damage can increase the chance of a disease taking hold - something has already breached the plant’s first line of defense. Plant cell walls also block viruses, bacteria, and fungi. Unlike our human immune system, which uses white blood cells to hunt down and destroy invaders, plant cells have antimicrobial defenses built right in. These defenses take the form of saponins, glucosides, and antimicrobial proteins. Enzyme inhibitors can also stop some pathogens from feeding on the plant. Plants also have chemicals that can neutralize toxins created by a pathogen. Finally, receptors can recognize a pathogen and alert the plant to take further action.

Inducible plant defenses

Once a pathogen is recognized, plants reinforce their cell walls and produce defensive chemicals such as hydrogen peroxide, antimicrobial enzymes, and proteins. In some cases, plants have evolved a ‘hypersensitive response’ that kills the healthy tissue surrounding an infection to block further infection to neighboring cells.

Disease resistant varieties

Rather than treating a disease after it occurs, you can grow plants that defend themselves. Installing resistant plants reduces the need for pesticides and fungicides. When shopping for plants, look at the plant label to see if that particular plant is resistant to diseases that tend to appear in your garden. Plant labels use the following codes to designate specific disease resistances:

• A Alternaria stem canker
• F Fusarium wilt
• N Nematodes
• T Tobacco mosaic virus (TMV)
• St Stemphylium gray leaf spot
• V Verticillium wilt

For example, a plant label with VFN displayed tells you that the plant is resistant to Verticillium wilt, Fusarium wilt, and nematodes. While not a guarantee against disease, it does mean the chances are significantly lower.

You can help your plants fight disease by selecting resistant varieties, spacing plants properly, employing crop rotation, and providing adequate water and nutrients. Also, be sure to sanitize your tools regularly, to halt the spread of disease from one plant to another.