• Sidebar: 5 Common Herb Diseases
• Learn the Visual Symptoms of Basil Downy Mildew
• Report Your Observation: Make Report
Q. My rosemary limbs have lost leaves from the bottom sections. Knobby growths, which look like root sprouts, are growing out from the stems. What is happening with my rosemary?
A. These rosemary disease symptoms are called “stem knot” and are caused by a systemic bacterial infection. According to Arthur Tucker and Thomas DeBaggio, in The Encyclopedia of Herbs, there is no cure; in fact, the cause is unknown, although it resembles bacterial infections caused by Pseudomonas, Agrobacterium and Xanthomonas species. These systemic types of infection are difficult to control because they grow from the inside out. However, infected plants can live for many years if good practices are followed. Prune the diseased stems back to healthy growth during the early spring. Be sure to disinfect pruning tools by dipping them in a 10 percent chlorine bleach solution or rubbing alcohol between each cut. Do not use organic material around the base of the plants. Choose coarse sand, crushed oyster shell, lava rock or white marble chunks for mulch instead. Remove all fallen tree leaves and do not allow weeds and other plants to block air circulation around the rosemary. Keep enjoying your rosemary by whipping up the rosemary mayonnaise recipe on Page 80.
Q. I am not able to keep garden sage in the ground for longer than one season in my humid Arkansas garden. The leaves and stems turn black overnight and the plants die. Is there anything I can do to grow sage and similar culinary perennials?
A. It is possible for herb gardeners in the Eastern United States to develop a successful garden spot for sage and other gray/green Mediterranean
natives. First of all, get to know the enemy. As a group, the fungal diseases that cause this fatal symptom are aptly called “sudden wilt.” They live in the soil, on seed and on almost any surface. The diseases are spread by spores and are passed on to plants by fingers, pruning tools, air currents and in water droplets.
Never replant sage or any other perennial Mediterranean herb in the same place where sudden wilt has taken a victim. Begin a new herb bed in a well-drained, full-sun location. Incorporate aggregates such as sharp sand and gravel to improve drainage. Use mineral-based mulch (rather than materials that come from plants, such as tree bark or straw) around sage. Organic mulch holds moisture longer than rocks.
Space plants so that the leaves of neighboring herbs do not block air circulation. Moisture from irrigation, dew and rain must drain away and evaporate quickly. Use drip irrigation and make it a rule to water in the early morning so that herb leaves dry out before dark. Try organic antifungal products containing the friendly bacteria Streptomyces lydicus or S. griseoviridis. According to David Hopwood in the reference book Streptomyces in Nature and Medicine: The Antibiotic Makers, these bacterial strains colonize plant tissues and the soil around roots. There they produce antimicrobial agents or enzymes that attack various fungal diseases.
Q. My basil leaves are turning yellow with a gray-fuzzy growth on the underside. What should I do?
A. This sounds like the dreaded basil downy mildew. According to plant pathology experts Meg McGrath, Andy Wyenandt and Jim Simon, there are three important tools to use in the fight against downy mildew on basil. First, learn the symptoms so that you can be a part of the monitoring system and get infected basil out of your soil as quickly as possible. Some of the symptoms include yellowing on upper leaf surfaces, and distinctive pathogen growth (that looks like perforation) and spores on lower leaf surfaces. If downy mildew is the culprit, take a bag out to the garden, put it over the basil and cut the stems into the bag. It is okay to take the bag indoors to harvest and process the good leaves. This report tracks the spread of the disease, helping major growers prepare treatments or harvest plans.
Second, look for plants and seeds that are free of or resistant to the disease. In the near future, seeds will be tested to be sure that they are powdery mildew-free. Disease-resistant basil varieties are being developed now.
Third, use planting techniques and control methods to discourage downy mildew spores. Thin basil seedlings and space plants in the greenhouse and in the garden so that the leaves do not touch. This encourages good air circulation, which dries surfaces so that the fungal spores have a difficult time growing. Use fans indoors, water early in the day and cultivate plants in a well-drained growing medium.
Become familiar with available fungicides labeled for use against this disease. According to Cornell University research, OxiDate, a fungicide containing 27 percent hydrogen dioxide, provides limited control on powdery mildew, is OMRI-approved (okay for use in organic gardens), and is labeled for use in greenhouses and outside. Actinovate, which contains the friendly bacteria Streptomyces lydicus, is labeled for use to suppress many soil-borne and foliar diseases on a variety of plants. Both fungicides are labeled for use on herbs.
To learn more about growing healthy plants, try these three book recommendations. You can purchase the Illustrated Encyclopedia of Herbs at www.herbcompanion.com/shopping.
• The Encyclopedia of Herbs: A Comprehensive Reference to Herbs of Flavor and Fragrance (Timber Press, 2009)
• Streptomyces in Nature and Medicine: The Antibiotic Makers (Oxford University Press, 2007)
• Rodale’s Illustrated Encyclopedia of Herbs (Rodale Press, 1998)
• "Downy Mildew Wars" by Margaret McGrath, Andy Wyenandt and James E. Simon (American Vegetable Grower)
• "Efficacy of Various Biological and Microbial Fungicides — Does That Really Work?" by Margaret Mcgrath (New England Vegetable & Fruit Conference)
Tina Marie Wilcox has been gardening at the Ozark Folk Center State Park in Mountain View, Arkansas, for more than 25 years. She writes a column, Yarb Tales, for www.ozarkfolkcenter.com. She and Susan Belsinger co-authored The Creative Herbal Home, available at www.herbcompanion.com/shopping.