“Disease-resistant” plants

By Steven Jakobi, Master Gardener Volunteer Cornell Cooperative Extension Allegany County

It’s late January and the garden catalogues are beginning to show up in the mailbox. By this time of the year, we are getting tired of the dark skies and the cold, snowy weather and we yearn for spring, for sunshine, mild breezes, and for new life emerging from winter’s icy grip. The awesome pictures of mouth-watering, perfect fruits and flowers spur us on to start making lists (even of things we hadn’t planned on growing) and open our wallets to buy more and more seeds.

Along with the colorful photos in these slick publications, you are likely to see basic information, such as the name of the variety or cultivar, number of days it takes for crop maturity, size of the plant or its fruit, what you can use the product for, and other useful tidbits. For many types of plants, you will also find codes, like “N,” “V,” “CMV,” or words like “resistant to Tomato Etch Virus,” or “resistant to powdery mildew,” etc. These codes refer to the ability of the crop variety to withstand or weather microorganisms (viruses, fungi, bacteria, nematodes, etc.) that would otherwise harm or kill the plant.

New cultivars or varieties of crops are constantly produced by plant breeders to keep disease-causing organisms, called pathogens, in check. But you may ask yourself: Why breed plants to resist bacteria, fungi or other pathogenic organisms, instead of making the crop immune to the disease? The question is a perfectly good one, but one for which the agronomist has a ready answer.

Scientists know from bitter experience that “immunity” is an elusive prize in crop science. Whenever a breeder stumbles upon a way to stop microorganisms, the effect is short-lived. On average, when a new “resistant” cultivar is created, microbes will overcome that block in about 10 years. The battlefield is littered with failures to control not only microbes, but insects, slugs and other invertebrates, as well as parasitic plants. It is evolution in action, right in front of our eyes.

In any population of pests or pathogens, there will be a few individuals or groups that already possess the inherent capacity of overcoming the roadblock thrown in front of their anatomical or biochemical capabilities. Trying to breed “immune” crops only hastens the process and will likely fail even faster because the pest population will be under greater pressure to develop means of survival. (This is similar to what we see when we talk about antibiotic-resistant bacteria in human medicine, or the need to get a flu shot every year for a different strain of the influenza virus or, most recently, the virus that causes COVID).

The best approach for the geneticist is to try to incorporate several partial blocks (termed “horizontal resistance” genes) against the population of the pathogen. These genes then “code” for anatomical or chemical defenses that the plant can produce. By lessening or slowing the progress of the disease-causing organism in an infected plant, the host may be tolerant and be able to produce the desired crop and, thereby, extend the usefulness of the new cultivar before the pathogen figures out a new strategy for its survival.

Here are a few examples of plants and their pathogens that breeders are always trying to keep in check.

Tomato: Verticillium and Fusarium are two fungi that can plug the water-transporting pipes of the plant, as well as produce toxic chemicals that wilt and ultimately kill the affected plant. Hybrid tomato varieties, such as ‘Better Boy’, ‘Big Beef’, ‘Kellogg’s Breakfast’ and many others are available to combat these fungi. In most seed catalogues, the varietal name will be followed by the letters “F” (or F1/F2/F3 to indicate different races of the pathogen) and “V” to denote the resistance capability of the plant to Verticillium.

Other symbols:

N= root knot nematode (microscopic worm that saps the roots’ capacity to absorb water/ nutrients)

T= Tobacco Mosaic Virus (affects photosynthesis and many metabolic functions; loss of fruit follows)

A= Alternaria ( a fungus that causes “blight” of leaves, stems and fruit and loss of the crop)

AN= anthracnose (another fungal disease of mature tomato fruit; renders the product inedible)

S= Septoria leaf spot ( fungal cells and spores reduce yield)

SWV= Spotted Wilt Virus (affects both foliage and fruit; production is reduced, fruit ruined)

Since tomato and pepper are closely related, many of the above also cause disease on both.

Cucumber: Two serious diseases are “downy mildew” and “powdery mildew.” Downy mildew (DM) is caused by a water-mold, so fungicides are ineffective against the pathogen. Powdery mildew (PM) is caused by one of several different fungi. Both diseases can severely reduce yields and/or render the fruit inedible.

CMV= Cucumber Mosaic Virus (blotchy spots on leaves and fruit; plant may die)

TLS= cucumber target leaf spot (“Marketmore,” Sweet Success,” and other cultivars are resistant to this serious disease of the foliage)

BW= bacterial wilt caused by a species of Erwinia (symptoms begin with daytime wilting and progress to desiccation and possible death of the plant)

Many relatives of the cucumber (yellow and zucchini squash, pumpkin, watermelon, etc.) may be affected by the same or similar diseases.

Peppers:

PMV=Pepper Mosaic Virus stunted growth, mottled leaves and fruit blotches, death; as with other viruses, there is no chemical treatment available once the plant is infected)

X=Xanthomonas bacterial leaf spot (there are different “races” of the bacterium that may affect different pepper cultivars; leaf loss, distortion, fruit loss, death of plant may follow).

Many of the above-mentioned genera of pathogens affect a wide variety of crops. Some disease-causing organisms have wide host ranges; others are closely adapted to their intended targets.

As mentioned earlier, “resistance” is not the same as “immunity.” Even resistant varieties of a given crop may become diseased when the pathogen is present and the environmental conditions are right. By knowing what the plant’s needs are, stress can be reduced, and the plant will be in better condition to deal with what comes its way. Therefore, it is not enough to rely on the crop’s innate ability to ward off disease. Equally (if not more) important, are factors that ensure the proper conditions for growth and maintenance. These factors include vigilance (always scouting for tell-tale signs and symptoms of disease), adequate spacing (for light, water, nutrient uptake, rapid drying of plant surfaces), sanitation (removal of infected/diseased plant parts, cleaning up of debris), crop rotation (as a means of keeping the crop away from infective propagules), and possible use of row covers, netting, etc. (to keep out insects like thrips or white flies that can transmit pathogens from plant to plant). Along with resistance, all of these measures collectively make up what is sometimes called integrated pest management (IPM) to keep plants as healthy as possible.

Resistant or not, the best way to protect your plants from pathogens is prevention, and we can do it providing the proper conditions for our plants in the garden. Enjoy the catalogues and dreaming of next season’s bounty!

Last updated January 30, 2024