Pecan Tree Stress
Incomplete kernel filling was realized in some orchards. Dark kernel color was a problem in some orchards. Poor shuck opening and sprouting in other orchards. Kernel rot showed up to disappoint growers in yet other orchards. And some growers had combinations of all of these problems.
The 2016 crop that appeared to withstand the onslaught of spring and early summer rainfall, disease pressure and insects seems to have withered on the tree late in the year. For many growers, living with the consequences of a less than stellar pecan crop would be easier if the reason for the outcome could be defined. The answer we (myself and my counterparts at other universities) often provide is one that growers really don’t like—“pecan tree stress”. What does that mean? Were the trees anxious, worried or under too much pressure to perform?
One East Texas grower recently wrote to me and said, “We, the growers, need more specific information other than ‘the trees were stressed’ (Guess I could install a sound system in the orchard and play Mozart or something like that).” True indeed; stress has become an often-used, ambiguous answer that needs better definition and recommendations for correction.
Stress in humans is defined by some as “any change in the environment that requires a body to react and adjust its response. The body reacts to these changes with physical, mental and emotional responses. Some short-term stress on people is positive, because it is the source of chemical blood changes that provoke fight or flight responses to potential dangers. Chronic stress though can be detrimental to peoples’ physical and mental well-being. Healthline.com lists numerous effects of stress on the human body, including increased hypertension and risk for stroke and heart attack, lack of adequate sleep, digestive problems, ulcers, diminished sexual health and fertility, and many other common effects. People faced with stress either move away from it, cope with it internally, adopt some form of stress relief (exercise), or take medication for it.
But what about plant stress? How does it differ to human stress? Rhodes and Nadolska-Orczyk (2001) define plant stress as any external factor that negatively influences plant growth, productivity, reproductive capacity or survival. Unlike people and animals, plants cannot move away from stress, forcing them to attempt to lower the effects of stressors through physiological and morphological changes (Collin, J.). These physiological tools differ among plant species and include an array of cellular and molecular processes. Plants and people are similar in that negative extrinsic forces (stress) cause unseen internal reactions. Those unseen reactions, however small, may have very visible effects.
Plant stress comes from a variety of sources but is broadly categorized as either biotic stress or abiotic stress. Biotic stresses are those that come from biological agents: insects, fungi, bacteria, viruses, animals and humans. Yes, a pecan grower pruning limbs off pecan trees is a stress agent to those trees being pruned.
Abiotic stresses are those caused by weather and environmental factors that alter favorable plant growing conditions. A flood event that restricts root oxygen is just one example of abiotic stress. Just as people create some of their own stress by taking on too many jobs or projects, pecans and other plants similarly generate their own stress by their systematic, species-driven efforts at reproduction. Charlie Graham made the following statement in the October 2016 issue of this Southeastern Shakings column: “Fruit development in pecan is a process that requires high energy, which can induce a considerable amount of stress on older trees.” So between biotic, abiotic and internal stress factors, the ongoing job of a pecan grower is stress management.
A better concept of managing pecan tree stress is gained from first defining what constitutes a stress-free pecan growing environment. In his Pecan South article on the subject of evaluating pecan stress, George Ray McEachern (Sept. 2009) outlined what might be the ideal, stress-free situation for pecan trees. He stated, “Plant on deep, well-drained soil, space trees far apart, select varieties with moderate nut size and crop load, irrigate with water free of salts, keep weeds and grass out of the trees, maintain tree health with zinc and nitrogen, prevent foliage diseases and control insects.” This description typifies what it takes for pecan trees to produce at their best and to produce the best quality pecans. In looking back at 2016 and attempting to identify why pecans were poor quality, light weight, dark in color, sprouted, etc., we have to look line by line at a checklist of the factors described by McEachern and other researchers as contributing factors to pecan tree stress.
Water stress is usually the first assessment target when pecan nut quality is unsatisfactory. Pecan foliage must transpire water daily to keep cool. The more foliage is present on a tree and the drier the weather, the more gallons of water must be transpired. Surplus water above that which is needed for transpiration can be directed by trees to nut sizing and kernel filling.
Pecan fruit development occurs in distinct phases so that water stress periods can be identified in the final nut quality. If nuts wind up smaller than average, it verifies that water stress occurred during the sizing period of June through early August. If nut size is normal or above average, but kernel development is less than optimal, water stress occurred in the post shell-hardening period of late August to late September. Small and poorly filled nuts point at water stress during the whole growing season.
The amount of water recommended in Texas during the growing season is 2 acre inches per week. Amounts less than that in mature orchards have the potential to induce stress and reduce nut quality. Many Texas pecan growers felt like they received timely rains in 2016, such that soil moisture should not have been limiting. One pitfall may have been that early season rains encouraged feeder root growth that was not sustained by irrigation practices later in the year. Due to the complexity of soil water-holding capacity and the delicate nature of shallow feeder roots that are critical to water uptake, growers need to incorporate moisture-measuring technology into their irrigation management decisions. There is otherwise too much room for human error and water stress in pecan production.
Root Oxygen Stress
A saturated soil is one that has received water from rainfall or irrigation faster than it can percolate, causing displacement of oxygen in the root zone for some period of time. Hypoxia or oxygen deficit to roots is a potentially significant stress that causes the same negative outcomes in pecan nut quality that water deficit stress does.
Research studies of flood-irrigated pecan trees have detected negative effects on photosynthesis after 5 consecutive days of saturated soil. Orchards with clay lenses or hardpans from years of equipment passage are at increased risk for this type of stress. Rainfall that occurred during the latter part of the 2016 growing season in many orchards of Texas may have caused root oxygen stress on those heavier soils that stayed saturated for several days at a time. The same technology that can help growers know when their roots are too dry can also help them know when they are too wet.
To expound upon Dr. Graham’s previously cited statement, pecan seeds are approximately 80 percent oil. The photochemical manufacturing of oil in pecan kernels requires a significant expenditure of energy. Many commercial pecan varieties have been selected for traits of large nut size and high levels of production — 2 traits that offer great potential for financial rewards; however, these traits increase the potential for stress.
When the crop exceeds about 75 percent fruiting terminals, late season over-cropping stress is likely. And a good argument could be made for stress occurring above 50-60 percent fruiting terminals on large nut varieties. Among other side effects of this form of stress is a depression of hormone levels that facilitate shuck opening and induce seed dormancy. This leads to problems like shuck dieback, stick-tights, nut splitting and sprouting, dark kernel color and lightweight kernels. The stress may also be seen next year in lack of nut production. Growers sometimes flinch at the only proven technique to manage over-cropping stress — mechanical shaking for crop thinning, but it is an essential practice for mitigating this stress and improving nut quality.
Sunlight is intrinsic to successful tree growth and fruit production, and thus reductions in total light intensity or reductions in the number of leaves receiving full sun can be considered a stress factor. Shade stress can happen for 3 reasons: trees are spaced too close to one another, which lowers the number of leaves receiving full sun; cloudy weather reduces total hours of full sunlight over the entire orchard, or leaves are impaired by insects and diseases to receive full sunlight.
For the 2016 crop year, many orchards in Texas had impaired leaves going into the latter part of the growing season. Black aphids were problematic. Yellow aphids were problematic. Leafminers were problematic. Scab damaged a lot of leaves. Downy spot damaged leaves in some orchards, and terminal dieback (Neofusicoccum sp.), a previously seldom-observed disease problem in Texas damaged a lot of pecan leaves in some areas of Texas.
These problems, coupled with rainy, cloudy weather resulted in significant shade stress for many orchards, especially those that were already overcrowded. Hedging has become the preferred method for many growers in Texas to maintain sunlight in their orchard, but it is still a work in progress in some orchards.
Pecan tree growth and nut production require sunlight, water and adequate availability of essential major and minor essential nutrients. Deficiencies in any one of these nutrients can trigger negative stress responses. A large body of research into pecan nutrition has led to generally sound recommendations for fertilizer approaches, rates and timing in Texas and other states. Systematic leaf and soil analysis should help growers avoid nutritional stress.
As stated above, the 2016 crop year came with significant challenges from fungal diseases and insects. These biotic stressors can induce shade stress by reducing the total leaf area that is available to receive sunlight. They also cause direct plant stress themselves. Black aphids, for example, inject toxins into the leaf in the process of feeding, causing yellow, necrotic blotches and leaf drop, depending on the variety and severity of infestation. No doubt there are biochemical changes from this type of insect feeding that add to problems with overall stress and nut quality. Regular and systematic scouting must be a priority for growers who want to sell pecans for high prices.
Any one of the aforementioned stress factors is enough to negatively impact pecan yield and nut quality in a given crop year. The weather in 2016 brought about multiple stresses for pecan orchards in Texas. It was too wet; then it was too dry, and then it was too wet again. The wet periods came with reduced sunlight. The crop was too heavy in some cases. In others, biotic stresses played a significant role along the way.
Although we can label stress as the overarching problem, we also need to work toward a more specific evaluation of these stresses, so that appropriate corrections and improvements can be made.