By Bev Johnson

Master Gardener

A reasonable plant adapts itself to the world; the unreasonable one persists in trying to adapt the world to itself. That means that all progress depends on the unreasonable plant. Progress in horticulture depends also on the unreasonable breeder who works on developing varieties of these “unreasonable plants” that are resistant to certain diseases.

One would think a plant that is disease resistant would be able to completely fight off all diseases. Nope. Disease resistance is limited to a specific disease although a given plant may have multiple resistances. For instance, the letters VFN on your package of tomato seeds or on the information stick in a started plant, denotes resistance to the diseases Verticillium wilt, Fusarium wilt, and root knot nematode. This tomato can still get early blight, late blight and a host of other diseases. This is why we cover the soil as soon as we plant our tomato as most of these other diseases are in the soil. We want to prevent soil from washing up on the stem and infecting our baby plant.

I f the soil has a high nematode level and has damaged roots due to nematode feeding, the roots can become a way the Verticillium and Fusarium fungus can get into your plant. If the plant has the VFN resistance, the plant will not be seriously infected.  This is because the resistant plant is less likely to become seriously infected than a susceptible one is. A resistant plant will more likely perform better, maintain an attractive appearance and require fewer chemical inputs than a susceptible one. For this reason, disease resistance is considered to be one of the great agricultural discoveries, and one that has been underutilized by commercial growers and in home horticulture.

One reason for this underutilization is the amount of time it takes to develop a resistant cultivar. Take wheat for instance, it takes about 18 years to develop a new potato variety. This difficulty of breeding for resistance is so lengthy that some of the plants that are disease resistant that are now available are just naturally resistant.

Regardless of the level of resistance, (remember we are discussing resistance not immunity) a cultivar is supposed to possess, hosts, pathogens and environment all interact in creating a disease triangle. Because of the complexities of this triangle, you may see evaluations of daylily rust to vary from a site in Florida to a site in Arkansas. This can be explained by remembering that Florida is tropical, and Arkansas is subtropical to temperate. Both the climate and pathogens are different. Perennials grown in the Northeast will grow differently than the same plant grown here. Ours will probably be shorter.

The most famous example of natural resistance is the 2002 perennial plant of the year, ‘David’ a white phlox, that was found by volunteers of the Brandywine Conservancy near Philadelphia. They were grown from open pollinated seed collected in the Brandywine Valley’s natural areas. Richard Simon of the Bluemont Nurseries of Monkton, Maryland and F.M. Mooberry, the horticultural coordinator for the Brandywine Conservancy, suggested that this plant, a white fragrant phlox should be cultivated and introduced because it was resistant to powdery mildew, a disease that seriously affects other phlox.

Unfortunately, there are several reports of severe powdery mildew on white phlox sold as the David cultivar. It may be years before the growers find out what exactly happened and why it lost its resistance. Instead try two the University recommends, ‘Laura’ or ‘Starfire’.

Read the front of the seed packet. It will tell you if your seeds are resistant and to what. 

Information from a paper by Jana Beckerman, Extension Plant Pathologist.