Q: For 20 years we had a beautiful hedge of oleanders, but about a year ago they started to dry out and die one after another. They are regularly watered but not a lot, still the same for all those years. We are at a loss for what causes that.
A: The oleanders are dying from a disease called oleander scorch that is caused by the bacterium Xylella fastidiosa. A different strain of this same bacterium species causes Pierce’s disease of grapevines. The bacterium is spread by insects, mainly glassy-winged sharpshooters. The sharpshooters feed on the plant’s xylem, the water conducting system of the plant. Once a sharpshooter feeds on an infected plant, the sharpshooter is infectious for life. After being introduced by the sharpshooter, the bacterium multiplies within the plant, blocking the xylem vessels and preventing water transport to that part of the plant.
The initial symptoms of oleander scorch are yellowing of the leaves on individual branches. As the disease progresses, the leaves turn brown and die. More and more branches are affected until the entire plant dies. This progression from initial symptoms to total death of the plant may take 3 to 5 years.
We have no cure for oleander scorch. Pruning out affected parts of the plant will improve the shrub’s appearance, but the pathogen is already throughout the plant’s system. and the plant is doomed. Glassy-winged sharpshooters are prevalent in our area throughout the year. There are no effective insecticides currently available. Early removal of infected plants may reduce the source of inoculum. For readers who would like more information on this disease, you can visit the following webpage: http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7480.html
Q: I have several potted plants growing on my patio. The recent hot weather made me realize that these plastic pots get very hot for several hours every day when the sun is hitting them. Do you think this is damaging?
A: The soil in a dark colored pot that is in direct sunlight can easily exceed 120 degrees. When plant roots are subjected to these high temperatures damage can occur. Assuming that you don’t want to relocate your plants, there are a couple of remedies you may want to consider.
You may be able to provide shade to the pots by simply placing something between the pots and the path of the sun to reflect the heat away from the pots. A few decorative concrete blocks or a short bamboo screen might be all that is needed.
A second remedy is to repot your plants in unglazed clay pots. Because unglazed pots are porous, water evaporates from the outside surface of the pots. This evaporation process provides some cooling to the soil but may increase watering requirements.
The most effective method is to double-pot your plants. Place the plant in its present pot inside a light-colored pot that is several inches larger in diameter. Fill the space between the two pots with moist sphagnum moss, vermiculite, or even foam packing material. The light color will reflect sunlight off the outside pot, and the space between the two pots will provide insulation from heat generated by the sunlight.
Ottillia “Toots” Bier has been a master gardener since 1980. Send comments and questions to features@pressenterprise.com.
