Ask Mr. Sage: How to Prevent Root Disease in Plants for Dry Gardens
Dear Mr. Sage,
When I moved to New Mexico from the East Coast a year ago, I was so excited about growing colorful Southwestern plants. Well, now I'm worried instead of excited. The leaves of my beautiful Salvia clevelandii 'Winnifred Gilman' are yellowing and wilting. I dug up the plant and discovered a rootball that appears brown, and feels mushy. What's causing this problem? Can I rescue my beloved Winnifred?
Qué Lástima in Las Cruces, NM
Dear Qué Lástima,
I have bad and good news for you. The bad news is that your plant likely has root rot, a disease caused by a complex of pathogens (fungi and other microbial agents), including species of Phytophthora, Pythium and Rhizoctonia. Sometimes more than one of these pathogens strikes at the same time, because all are activated by excess water.
Your best option is to dispose of this plant before it infects others. If you are upset, I certainly understand. Our Cleveland Sages, including Winnifred Gilman, are among our prettiest, most drought resistant shrubs. We love their lavender flowers and fuzzy, gray-green foliage. However, when disease attacks, it is important to protect the rest of your garden. Before replacing the plant, it helps to learn how to prevent root rot. And that is the good news; root rot can be prevented.
It's improbable if not impossible to have soil or potting mix that is free of root rot pathogens. However, healthy soils and mixes also contain plenty of mycorrhizal fungi that form mutually beneficial relationships with plants .
At Flowers by the Sea, one way that we control root rot and maintain plant vigor is by using a product called Pro-Mix® LP15 Biofungicide™ + Mycorrhizae. Pro-Mix® also makes potting and seed-starting mixes for home use. Biofungicides are naturally occurring beneficial microbes, including certain fungi, that control other disease-causing microbes in conjunction with roots.
Actually, mycorrhizas (or some say "mycorrhizae" for the plural) aren't simply the fungi. When roots and a fungus work together beneficially, they become a combined structure called a mycorrhiza. Fungi can't photosynthesize, so they need plant energy. While gaining energy from the roots in which they grow or to which they cling, the fungi help form a mycorrhizal shield against attack by harmful fungi. Also, as mycorrhizas thrive, the fungi send out root-like structures called hyphae that absorb nutrients -- particularly phosphorous, which can be difficult to access -- and water, both of which get shared with the plant. Mycorrhizal structures help roots to drink water rather than drown in it. Of course, you still need to avoid overwatering to control pathogens.
At Flowers by the Sea, we rely on prevention, eagle-eyed observation and biocontrols -- such as beneficial microbes and insects -- for disease control. We don't use chemical pesticides because they are highly toxic to people and well as to small wildlife, including beneficial insects. They can even harm other, non-targeted plants.
Also, chemical fungicides aren't effective enough against root rot complex to warrant their use. For example, Oklahoma State University researchers in 1992 found that chemical fungicides did little to control the oospores of Pythium and that these "hardy" spores can last for years in soil or soilless potting mixes.
Your first step in controlling root rot is to select vigorous, disease-resistant plants. Although Salvias sometimes succumb to disease, overall the genus has a reputation for disease resistance.
Aside from selecting vigorous plants and disposing of diseased plants, you can minimize or eliminate root rot and other foliage/root diseases by taking a number of steps that help prevent infection. These actions include:
- Adding plenty of microbe-rich compost to your garden
- Maintaining good garden hygiene
- Watering at the right time of day and
- Avoiding overwatering.
Adding Compost. Well-rotted compost enriches and aerates soil as well as improving drainage. It is teeming with mycorrhizal fungi and other microfauna that help control root rot complex pathogens.
Maintaining Hygiene. Although we don't recommend trimming or heavily pruning plants that appear to be suffering from root rot, it isn't always apparent what is causing a plant to wilt or yellow. The natural inclination is to snip off dying foliage.
However, after digging up or pruning a plant that appears to be ailing, you risk spreading infection to other plants if you don't sterilize equipment. Dave's Garden offers a comprehensive tip sheet for cleaning pruners, trowels, shovels and other tools. One easy suggestion is to keep a spray can of Lysol in your gardening bucket with clean rags for wiping tools. Scrubbing tools with bleach-free disinfectant wipes is another easy choice. Don't forget to follow up by oiling the tools so they don't corrode.
Another part of good garden hygiene involves cleaning up fallen foliage from a sick plant and disposing of it along with the plant. Compost heaps don't get hot enough to kill root rot pathogens.
Finally, if you are planting in containers, make sure that you remove all soil and sterilize them as well.
Watering at the Right Time. Early morning before it gets hot is the best time to water most garden plants to avoid a number of problems, including evaporative loss. However, it is especially important for avoiding root rot, which can thrive when plant roots are moist for extended periods, such as overnight. This is especially true in soils, such as clays, that don't drain quickly.
Avoiding Overwatering. Although withering brought on by underwatering makes plants vulnerable to health problems such as spider mites, overwatering is a greater concern with desert plants. Ask Mr. Sage: How to Water Desert Plants offers useful guidelines for calculating supplemental watering.
Cleveland Sage is an excellent choice for the hot, dry, summer climate in Las Cruces and other parts of New Mexico. These pretty yet tough plants are native to the semi-arid lands of Southern California where they survive summers in the wild with little moisture.
Cleveland Sage doesn't like damp roots. I realize that this may sound crazy, but it is possible -- despite your dry climate -- that your Winnifred Gilman got too much water and that this has activated dormant, root-attacking fungi in the soil. The result is root rot, crown rot or both.
One common overwatering situation involves cultivating low-water plants in flowerbeds with the type of moisture-loving plants you were accustomed to growing on the East Coast. Be sure to group plants based on similar watering needs.
A second scenario leading to overwatering involves use of a single drip system that isn't zoned for the varying water needs of different plants.
Root rot can also occur when sages are planted in compacted clay soil with poor drainage. Planting desert sages in raised beds filled with loose, sandy loam containing plenty of compost is an excellent aid in warding off root rot.
I hope this helps. If you have more questions about root rot complex or other Salvia problems, please call or email us at Flowers by the Sea. We want your plants to be healthy.