The right choice

It’s that time of year when growers may be planning to attend one of the field trials or open houses in the Northeast, such as the Cornell Floriculture Field Day, Penn State Field Day, Massachusetts Horticultural Society Trial Garden, or the Long Island Trial Gardens.

Trial gardens provide a great opportunity to see how plants perform in the landscape in a specific geographic area. Visit local trial gardens more than once throughout the season to see how plants perform under all kinds of weather conditions. Information on cultivar performance is useful for choosing which plants to grow for customers next year.

Growers are reminded to also think about plants’ resistance to diseases. As the trend to use fewer pesticides continues, one of the best ways to prevent serious plant disease outbreaks is to choose cultivars with demonstrated disease resistance for you as a grower and for garden and landscape customers.

Disease resistance is a genetic trait that varies within cultivars, species or among species within a genus. It does not mean that a plant is completely immune to disease. Instead, it refers to the plant’s ability to minimize or tolerate infection by the pathogen.

While disease-resistant cultivars can help prevent disease, it is equally important to manage diseases by using proper cultural practices. Avoiding too much or too little water, excess fertilization, sub-optimum growing temperatures, overcrowding and high humidity will also help to prevent disease outbreaks.
 

Botrytis elliptica on lily.

Genetic resistance
Most disease resistance is bred into plants by using plant species or cultivars that are genetically resistant to diseases. Selection of disease-resistant plants involves subjecting plants to high levels of disease pressure and using the plants that survive. These plants can then be used directly by propagation or in further breeding programs. This is done by crossing a plant having desirable horticultural qualities (susceptible to a disease) with a plant that is disease resistant (may or may not have the other desirable characteristics).

Occasionally, the first signs of disease resistance may be found by accident. For example, powdery mildew-resistant Phlox paniculata ‘David’ was first observed to be free from powdery mildew while surrounded by infected plants in a landscape. From this observation, the cultivar was tested further to confirm its resistance.

Plant cultivars vary in their levels of disease susceptibility. While some plant cultivars may be resistant to a disease, others may be less resistant. Still others may be uniquely very susceptible. For example, Rudbeckia fulgida Goldsturm is particularly susceptible to Septoria leaf spot. When choosing plants, highly susceptible cultivars should be avoided.

Cultivars within the same series can also vary in disease susceptibility. Research on verbena cultivar susceptibility to powdery mildew showed that verbena Aztec Peach was highly susceptible, while others in the series (Aztec Grape Magic, Aztec Lilac Picotee, Aztec Magic Purple and Aztec Silver Magic) were tolerant or resistant.

Powdery mildew resistance
Powdery mildews are common diseases of ornamental plants and are recognizable by white to gray powdery growth on leaves, stems and flowers. Resistant or less susceptible cultivars are available for many types of annuals and perennials and should be used whenever possible.

Look for powdery mildew resistant annuals including Verbena spp., Zinnia spp., and Phlox drummondii, as well as perennials such as Monarda spp., Delphinium, Geranium sanguineum, Geranium wallichianum, Annual Phlox, Phlox paniculata, Phlox maculata, Pulmonaria spp., Rudbeckia spp., Salvia spp., Sedum spp., and Veronica spp. Powdery mildew-resistant zinnias include Oklahoma, Profusion, Crystal White, Ruffles, Panorama Red and State Fair.

Battling botrytis
Another disease of concern is Botrytis elliptica on garden lilies. It causes leaf spots and destroys flower buds and flowers. In field plots at the Long Island Horticultural Research and Extension Center, less susceptible cultivars of lilies included Time Out, LeReve, Conca d’Or, Stargazer, Casa Blanca, Sorbonne, Pink Perfection and African Queen.

Unfortunately, disease-resistant plants are not available for all diseases. For example, there are currently no cultivars of Impatiens walleriana that are resistant to impatiens downy mildew.

Smith is a frequent contributor to Greenhouse Management and an extension floriculture specialist for the UMass Extension Greenhouse Crops and Floriculture program.

Have a question? You can write Tina at tsmith@umext.umass.edu.

Thank you to Margery Daughtrey, Long Island Horticultural Research & Extension Center, for reviewing this article.

Photos Courtesy of Tina Smith