(News article for November 2019)

In the last article, I addressed temperature aspects of the “right plant, right place” principle. This week, we’ll look at choosing plants with resistance to common disease problems.

Besides the maximum and minimum temperatures and the number of chilling hours that we get, another aspect of our climate is the large amount of rainfall that we typically receive. Our high rainfall conditions mean that southeastern Louisiana is favorable for many plant diseases. With few exceptions, water on leaves favors leaf spot diseases, and water around roots favors root rots.

Choosing disease-resistant plant species or varieties is a good way to avoid problems. In some cases, this prevents the need to spend time and money on applying fungicides or bactericides. In other cases, there are no good pesticide options for these diseases, even if you don’t mind applying them.

Phytophthora root rot, for example, affects a lot of different plants. Certain plants are more resistant than others. Particularly if your soil tends to stay wet after a rainfall, this is something to consider when choosing plants.

For example, sasanqua camellias are more resistant to Phytophthora root rot than our traditional Japanese camellias are. (Some Japanese camellias are grafted onto sasanqua rootstock, so if you prefer the appearance of Japanese camellias, look for this.) Likewise, the Southern Indica azaleas (e.g., ‘Formosa’) and Satsuki hybrids (e.g., ‘Gumpo Pink’) are typically less susceptible to Phytophthora than the Carla (e.g., ‘Sunglow’) or Kurume hybrids (e.g., ‘Coral Bell’). Rabbiteye blueberries, which are the kind that we have traditionally grown here, are more resistant than southern highbush blueberry varieties are.

If you want a small-leafed evergreen shrub for a foundation planting, dwarf yaupon hollies are less likely to have problems with root rot or root knot nematodes (microscopic worms) than boxwoods or Japanese hollies (‘Helleri’, ‘Compacta’, etc.).

Red tip photinias have largely fallen out of favor because of problems with Entomosporium leaf spot. Indian hawthorn gets the same disease, but some varieties, like Eleanor Taber (‘Conor’) and ‘Snow White’, have some resistance to it.

When choosing pecan varieties for home orchards, one of the most important considerations is resistance to pecan scab. While commercial growers can spray fungicides for this if they desire to do so, home growers typically do not have the equipment needed to get good fungicide coverage in the canopy of a large pecan tree. Considering this, the varieties ‘Caddo’ and ‘Oconee’ (Type I), and ‘Candy’, ‘Elliott’, ‘Melrose’, and ‘Sumner’ (Type II), are some varieties recommended for home plantings.

When it comes to choosing apples and pear varieties, one of the primary considerations – besides the chilling hour requirement – is resistance to the bacterial disease fire blight. Fire blight is one reason that pear varieties like ‘Baldwin’ and ‘Orient’ are recommended for southern Louisiana, while many of the ones that we see in the grocery store are not. Suggested apple varieties include ‘Anna’ and ‘Dorsett Golden’.

If you’ve ever wondered why we don’t grow grape varieties like ‘Chardonnay’ and ‘Merlot’ here, it’s due largely to something called Pierce’s disease. Fruit rots favored by our wet climate also pose challenges to growing bunch grapes like these, but the more important factor is this bacterial disease that will kill highly susceptible plants. Muscadine grapes, on the other hand, are more resistant to or tolerant of it.

Choosing disease resistant plants is a way to save time and money, reduce pesticide use, and increase your enjoyment of the garden.

Let me know if you have questions.

Contact Mary Helen Ferguson.

Pecan scab problems can be reduced by planting scab-resistant varieties. (Photo: Rebecca A. Melanson, Mississippi State University, Bugwood.org)

Dwarf yaupon hollies look similar to Japanese holly cultivars like 'Compacta' and 'Helleri' but are more resistant to several disease and nematode problems. (Photo: John Ruter, University of Georgia, Bugwood.org)