Plant World Nursery Q&A

          Leaf scorch is the burning or drying of the edges or tips of leaves. This can happen for several reasons. Most plants have the capability, to a limited degree, of producing leaves and other plant parts that are adapted to the local and current weather and climate.

          If the leaves being formed by the plant are growing during cool weather, the plant is said to have "acclimated" to cool weather. This means that the plant has produced leaves and new plant tissue that can function best in this kind of weather. This might include leaves that are larger, thinner and without the protection of tough outside layers of thick cuticles and waxes. These leaves are described as "tender".

          These outer layers help protect the plant from harsh environments. During mild weather this added protection is not needed and so the plant doesn't produce it. When the weather changes gradually then the plant responds by producing new leaves and stems that have more protection.

          If the weather suddenly turns hot, dry and windy the plant may be caught "off guard" by these sudden changes. The leaves and stems, formerly acclimated under the cool weather of spring, are now abused in the harsh new environment of our Las Vegas summer. The leaves that formerly were doing quite well now can't tolerate the harsh and sudden changes. Leaf scorch usually occurs here when the plant can't supply enough water to the leaves. As the weather gets hotter and windier the plant's need for water increases. Leaves produced in the cool weather of a long spring begin to stress.

          The outside margins of the leaves are the last to receive water from the plant's roots so they are the first to show signs of scorching. So in technical terms "marginal leaf scorch" occurs. If the weather is extreme or the leaves very tender, then the entire leaf may dry up, die and fall from the plant. In extreme cases it can cause defoliation which is where all the leaves of the plant die and fall off.

          Excessive soil salts that occur naturally in our desert soils can also cause marginal leaf scorch. If soil salt levels are high, leaf scorch can result from the competition for water between the plant and the actual salty soil itself. Some types of salts, such as those high in sodium or boron, can cause even more damage to plants due to their natural toxic to many plants.

          Remember now is the time that many of our larger insect pests, like borers are active. Of course the best protection is having a healthy plant but there are things you can do such as not exposing larger limbs and young trunks to direct sunlight. So try not to do any major pruning now until fall. Pruning now also exposes young limbs which have not formed any bark to direct sunlight which in these light intensities often times causes sunscald. This is the time of year that we should be protecting those trees and large shrubs from potential borer problems.

          Plants that commonly get borers as they get larger are loquat, pyracantha, Arizona cypress, twisted juniper, flowering plum and other stone fruit trees. Other plants that do get them, but less frequently, are palo verde, mesquite, roses, agaves in the roots, poplars and Leylandi cypress. Reasons for their attack are unusually stress related.

          Some simple precautions to take for prevention include preventing stress in the plants by not pruning these plants severely and not letting them get into a droughty condition. Some plants are so hard hit by borers, like the loquat and cypresses, it makes you wonder if they should even be planted at all in this climate. Others are more of a management problem and can be addressed by making sure the trunk and lower limbs are well shaded by the upper canopy. There are sprays that can be applied for borer control but should be used only if the threat from borers is actual and not just a preventive treatment.

          Many plants can survive for several years after initial borer attacks. You just have to be aware of their potential damage and watch for it. For most gardeners the signs they recognize as a borer attacks are branch dieback and sap oozing from limbs or trunks. Branch dieback is oftentimes a sign when borers have been attacking the plants for several years already. Sap oozing can be from other problems such as mechanical damage to limbs or the trunk, high temperature stress damage to some trees and a few disease problems like slime flux. Even seeing holes in the trunk may not be a sure sign of borers. Some birds cause similar types of damage but usually this damage is seen along a line, in a row, nearly horizontal on the trunk.

          The cool wet weather of spring caused some other problems for us as well. Bees don't like to fly during cool wet weather. If bees aren't flying then there will be poor pollination. Look for the results of poor pollination on fruit and nut trees as well as your vegetable garden.

          The results of poor pollination will be early, sudden fruit and nut drop before they can mature. Since the bees did not pollinate the fruit and nuts then the seeds cannot mature inside the fruit or nuts. Most fruits and nuts need seeds to develop because the seeds send chemical "signals" to the fruit and nuts that allow them to develop and mature. Of course there are exceptions such as the seedless grapes and watermelons. But for most fruits and nuts, even though small fruit or nuts may form, they will not continue to maturity since the seeds inside were never formed.

          The same is true of vegetables that result from a pollinated flower. Vegetables such as tomatoes, peppers, squash, cucumbers, melons and the like all result from pollinated flowers. Even if the vegetable flower is not pollinated, a small vegetable "fruit" may result. But, like the fruits and nuts mentioned above, it will not contain seeds that can take this "fruit" to maturity.

          Soon after this vegetable "fruit" begins to develop and the gardener sees and gets real excited, this fruit withers and dies. The natural response is to think it was a disease or blame the nursery for selling bad plants or your neighbor. When things like this happen, stop and think. What was the weather like a few weeks before this happened? Without bees, all of our gardens would fail.

          Q. My lawn has a disease in it and it is dying in patches. The patches are getting bigger.

          A. When we examined the grass showing signs of the problem, we noticed several things.

          One: The lawn was a mixture of both bermuda and perennial ryegrass. The bermuda appeared more healthy than the ryegrass.

          Two: Some of the ryegrass leaf blades were dead from the tip, down about one half inch. We also saw light green "bands" across some of the leaf blades that, otherwise, appeared healthy.

          Three: Tracing the leaf blades down to the ground showed us that, other than the dead tip or the light green band, these blades appeared healthy. Finally, the soil was very dry.

          There was no obvious evidence of disease present. Healthy plant tissue is white or cream-colored. What we look for when trying to spot a disease is dark-colored spots or discolored plant tissue. First the leaf blades are inspected. We look for obvious spots on the leaves, particularly during the cool times of the year. If the leaves look healthy, then the stem of the grass plant is inspected. The stem is inspected for the same types of spots and discolored areas. Finally the soil is pulled away from the plant roots and the roots are inspected, again looking for discolored tissue.

          The grass appeared to be in good health. Most of what we saw was damage from a lack of water or drought. When our weather turned from cool to hot very quickly, it caught a lot of people, who were trying to do a good job managing their water, off guard and short an irrigation or two.
Warm weather and dry winds caused the lawns to use more water than they had just a week ago. Grasses will brown at the leaf tip and down the leaf blade when water isn't available and it is hot and dry.

          The light green bands half way down an otherwise green leaf blade is caused by heat stress. Granted, it wasn't that hot but when a rapid rise in temperature is combined with a lack of water, heat stress will occur. You could compare it to you being outside during the heat, with and without water. Without water, heat is much more uncomfortable or stressful, even at lower temperatures. When we are thirsty, we can't sweat as easily and cool ourselves.

          Perennial ryegrass is not as tolerant of drought as tall fescue and bermudagrass. The ryegrass with leaf tip damage will come back; ryegrass with more extensive damage probably will not. The bermudagrass will definitely come back. Even though tall fescue uses more water than most other grasses, it does recover from drought extremely well. It would have recovered completely from this type of drought stress at this time of year.

Information in Mr. Morris's column is provided by the University of Nevada Cooperative Extension and is public domain information. For more information concerning subjects in the column or other horticultural information contact the author by clicking here.