Leaf Scorch on Texas Red Oaks or Spanish Oaks

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Leaf Scorch is not a disease, it is a widespread noninfectious disorder or rather a condition by adverse environmental factors following prolonged periods of hot, dry, windy and bright sunny weather. Scorch is often call a disease but is not caused fungus, bacteria, or virus, nor does it result from insect attack. However, these problems may add to the seriousness of scorch. Our Texas Red Oaks or Spanish Oaks are highly likely to develop scorch, Black Jack, Post Oak and sometimes Live Oaks are also affected. The problem may appear on almost any tree if the weather conditions become particularly unfavorable - high temperatures, dry winds and low soil moisture. Scorch may also suddenly appear when long periods of wet, cloudy weather are followed abruptly by dry windy and hot sunny weather.
(Sometimes referred to as SUMMER SCORCH)

Symptoms and Causes

Scorch usually develops as an irregular yellowing, faded green, browning or bronzing of the tissue between the veins or along the margins and tips of the leaves. However, some trees may differ in pattern of scorch development. These same symptoms are often misdiagnosed as symptoms of oak wilt, which are very similar in appearance. When water is lost faster than it can be replaced, the resulting condition appears as leaf scorch. The normal seasonal rate of water movement to the top of some trees can be rapid and in Red Oaks it moves at the rate of  92 ft/hr as compared to about 4 ft/hr in Live Oaks. (see article “Water Movement In Trees”)  As condition progresses, the entire leaves may dry up, turn brown and become brittle. Leaves sometimes wilt rapidly, and when this happens, they remain a pale green color, even though dried out. Damage is usually more pronounced on the upper, windward or southern side of the trees. The trees may lose many leaves prematurely during summer and exhibit some twig or limb dieback. Scorch can also be a symptom of insect and disease problems that interrupt the flow of water from the roots. Root diseases can reduce root efficiency so that less water reached the leaves, which then scorch. If dry soil is the cause, watering may stop the scorching. The loss of leaves may not always be fatal but conditions causing scorch should be corrected or prevented if possible because over time, they can cause the decline or death of the tree. If the tree just does not have enough roots due to drought conditions and root dieback.

Control 

When leaf scorch is noticed, the tissue has usually dried past the point of recovery, but several steps can be taken to prevent more severe damage and may improve conditions is subsequent years. Thorough, deep watering will usually help increase water uptake and with this in mind and when conditions persist then it is time to start watering to prevent this problem before leaf scorch starts.  However, here in the Hill Country, the conditions are usually such that the effects happens so quickly that often times, very little can be done to save the tree.

Early spring applications of fertilizer with sulfur and micro nutrients may reduce the problem. Refrain from fertilizing your Red Oaks in late May or early June, unless trees are showing symptoms of nutrient deficiencies, such as chlorosis, stunted growth or deformed foliage, as trees will develop young soft growth that will scorch easily in hot dry windy weather. 

Establish a good insect and disease control program. Insects and disease damage can reduce tree vigor and encourage leaf scorch.

Soil compaction can also be a contributing factor and injecting either water or air into the root zone will help loosen the soil.

Conserve soil moisture by mulching trees with bark or other material. And if watering is necessary, be sure to water thoroughly because mulches do absorb water from the surface. Light watering will do no more than wet the mulch.  

(See article "Drought Stress")