Temecula Valley Rose Society
Rose Care Corner, May 2007By Frank Brines – Consulting Rosarian
I t's that time of year when we see all the critters in the world coming to visit our rose gardens, just in time to ruin our hopes of having that rose specimen taking high honors in the rose show. I was asked many times at the show what causes brown spots seen mostly on light colored blooms. The brown spots which look like tiny bruises to the blooms are caused by a critter known as thrip. Thirps are tiny insects with piercing mouthparts that feed on plants for sap, juice, and water. They can damage most types of garden plants but they are particularly nasty to rose leaves and blooms. Leaves may turn brown and curl and may even fall off. Blooms can be spotted and scarred. In addition, their biting and sucking can transmit fungal and virus diseases.
Thirps reproduce rapidly, so it is important to take action when you first see their damage in order to keep them in check. They range in size from about 1/25 to 1/8 of an inch long. Their color can range from clear to white to yellow to brown to black—generally a color similar to the plant they are mostly feeding on. Thrips crawl, jump, walk, and fly from plant to plant. They are so light that they can move across your garden on wind currents. They flourish feed, nest, and reproduce wherever there is water or high moisture. With our warm temperatures they can remain active year round, but their populations explode as we begin watering and plants start rapid growth.
Thirps lay their eggs on plant tissue and the newly hatched young immediately begin feeding on any part of the plant which provides sap. After a week or so, these larvae pass through two more stages, eating non-stop. They stop eating when enter their third stage and may develop wings, fly off, or crawl, looking for leaf litter and mulch where they can pupate. They emerge to lay eggs. This life cycle can take as little as two weeks, so a few can become a few thousand very rapidly.
So, when you see their damage, treat as soon as you can and repeat the treatment until they are under control. A common treatment that poses little risk is insecticidal soap. It quickly kills thriving thrips and won't hurt plants but it doesn't leave much residue to keep on working. Once it dries, it's pretty much inactive, so you have to repeat treatments as often as needed (like twice a week during the growing seasons) and more often if you've got a big population. This is available at L&M Fertilizer. If the thrips are just too persistent, you may want something a stronger which will leave more residue. Most of what is available is broad spectrum, killing a wide range of insects. Although I try to stay away from heavy use of such insecticides, sometimes you just have to use them. One is pyrethrum concentrate. You can find it in well-stocked garden centers or on the Internet. It is a true insecticide which kills just about any insect in and around the garden (and that includes beneficial insects). It works well for whiteflies and aphids, too. The residue lasts 1-2 weeks. It presents little danger to mammals (like us). There are always other products coming on the market and L&M Fertilizer is a good source.
This year powdery mildew has been a problem in some areas of the Temecula Valley. If it is treated at the first sign it can be controlled, but if it goes undetected for a short time treatment will have to be more aggressive and continuous until brought under control. Here is an organic home recipe I recently got: 2 Tsp. baking soda,1 1/2 Tbs. non detergent soft soap, 1-1/2 Tbs. cooking oil, 16 shakes of Tabasco sauce per gallon of hot water. Use a sprayer to apply as the water cools. If more aggressive action is needed, use a product called Monterey Fungi-Fighter. Read the complete instructions when using this product as it is a chemical. Susan at L&M Fertilizer can help you with this product and many others they stock. They are continually getting new products, so talk to Susan. Remember, L&M gives all society members a discount, another perk for being a member of the Temecula Rose Society.
As the weather continues to get warmer be sure that your irrigation system is working properly. Now would be a good time to add 2-4 inches of composted mulch to the garden if you haven't already. Also, deadhead old blooms to stimulate another round of blooms.
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