There are many reasons to love gardening along the Texas Gulf Coast, one being the ability to successfully grow a variety of citrus trees in our landscape. Urban gardeners to small-scale orchard operators in Galveston County have natural resources of temperate weather and access to full sun to entice sun-colored fragrant fruit from their trees.
Citrus is a general term for fruit that includes the familiar lemon, key and persian lime, the unusual thick-skinned citron, the delectable sweet orange, easy to peel tangerine, huge pomelo, grape-sized kumquat and our beloved grapefruit. Citrus species have their origins in southeast Asia, including south Vietnam, south China and India. Global trade routes via European western expansion eventually brought this precious commodity to be cultivated along the Texas Gulf Coast starting in the 1880’s. It is estimated that approximately 27,000 acres of citrus are now grown in South Texas in the Lower Rio Grande Valley region.
Image courtesy Harris County AgriLife Extension
Since there is a wealth of information regarding citrus production, variety availability and horticultural management, I wish to focus on several inquiries I recently received and related to citrus plant health. A few concerned homeowners witnessed damage to their citrus foliage, reporting black and silver-colored trails on the top of curling and disfigured leaves. One homeowner sent me a great picture of foliar damage to help identify the culprit. I concluded that the leaf damage is due to activity of the Asian Citrus Leaf Miner (Phyllocnistis citrella), a species of moth originating from India to the Phillipines and detected in southern United States in the mid-1990’s. The moth is small and light colored, mostly found active March through October in the Texas Upper Gulf Coast Bend. Regarding life cycle, the female adult will lay eggs on the underside of newly growing leaves in the evening or late night. Larvae hatch and burrow into the leaf, mining as they feed through the leaf tissue. We see the damage as a silver-colored trail along the leaf surface. The larvae proceed through 4 instars (stages of development) and take up to 20 days to form pupae (think of a butterfly cocoon). Right before pupae development, the larvae proceeds toward the margin of the leaf, emerges and rolls the leaf around itself to develop into the adult, causing the leaf to distort and curl. The life cycle can take from 2 to 7 weeks to complete.
While the damage looks unsightly, moth activity is found not to reduce citrus plant health or productivity on mature trees. Damage to trees aged from one to four years may warrant control of this insect and depending on the severity. There are a number of predatory species that feed on the miner at the larval stage, and best management practice involves least toxic intervention on our part to encourage a more natural control. If you do decide to use a control method that should include general plant maintenance, best management practice involves application of horticultural oil in cooler months; keep in mind that oils used in horticultural application can damage leaves when we begin to sustain temperatures in the mid-80’s. If you have a heavy infestation of Asian Citrus Leaf Miner and have determined that they are affecting the health of your tree, chemical control recommendation is for the use of products containing Spinosad, a chemical compound derived from a family of soil bacterium and attacking the nervous system of the insect. While this chemical is a least toxic alternative, it is a broad-spectrum control and I encourage you to limit application of chemical control to continue to encourage natural predators of the moth. More information regarding citrus and related fruit production and management can be found online at the Texas Aggie Horticulture website: http://aggie-horticulture.tamu.edu/fruit-nut/.
You can find out more about our gardening programs from our website: https://galveston.agrilife.org/horticulture. I invite you to share your ideas and successes in your garden by browsing online to my Facebook webpage: www.facebook.com/horticulturiststephenb/. See you in the garden!
Frost, specifically radiation frost, is used to describe heat loss from radiant energy, accompanied by freezing temps forming ice crystals from dew collecting on surfaces. These events occur with clear skies, calm winds below 4 mph and temperature inversions. Freezes, also known as advective freezes, occur when freezing air mass displaces warmer air with wind speeds more than 4 mph, an event that can cause ice crystals to form and pierce cell walls within vegetative tissue. Thawing allows fluid to leak out of damaged cells and causing a burned look to leaf and stems. Our job prior to anticipated radiative events is to trap as much heat as possible from the surrounding environment, slowing down heat loss just enough to prevent damage to plant tissue and with appropriate materials such as porous fabric. Commercial frost cloth can be used, and additional materials can include bed sheets or surrounding the target plant with cardboard. Since the focus is slowing down heat loss, you can also use surrounding structures that reflect radiative energy such as roof overhang or even placing under the canopy of larger landscape plants to reduce cooling. The best method for covering plants is securing material at ground level and within the canopy drip line instead of tying around the trunk of the plant. Remember we are not trying to insulate the plants, rather slowing heat loss. The difference may be measured by a few degree’s, but it will be enough to lessen the effect.
Advective freezes require more than lightweight covers for protection and can include building box-shaped frames, applying row covers or hoop houses to place over your plants. Polyethylene, 4 mil sheeting is a good material to cover these structures. Make sure that there is enough headroom between the plastic and plants because plastic is conductive and can damage vegetation on contact. Always remove coverings as temps warm above freezing to avoid heat stress. Buildings and surrounding vegetation can also be useful as windbreaks, disrupting increased wind exposure during freeze events. Protecting plant crown and roots can be managed by 3 to 4-inch layer of mulch. Make sure the plants are well watered prior to these climatic events. Water and wet soils collect heat during the day and release slowly as ambient temps cool, providing additional protection to your plants. You can also collect water in used 1-gallon containers or 5-gallon buckets, allow to warm during the day and place beneath covered plants to provide further protection.
