Pecan trees are prized for their shade and high-quality nuts but are highly susceptible to diseases, primarily caused by fungi, that severely impact yield and tree health. Successful management requires proactive intervention, especially in regions with high humidity and frequent rainfall. Protecting trees involves understanding the visual signs of infection, followed by a combined strategy of cultural practices and targeted chemical treatments.
Recognizing Major Pecan Tree Diseases
Pecan Scab is the most economically significant disease, attacking the leaves, young twigs, and nut shucks. Symptoms begin as small, circular, olive-green spots that darken and become sunken or black. On leaves, these lesions may merge, causing a tattered or “shot-hole” appearance. Severe infections on the nut shuck can cause the nut to shrivel, drop prematurely, or result in “stick-tights.” Damage is greatest when the nuts are forming, as the fungus infects young, actively growing tissue.
Downy Spot primarily affects the foliage, beginning in late spring or early summer. The disease first manifests on the underside of young leaves as small yellow spots that may turn white and “downy” as spores are produced. Mature spots become dark yellow to light brown on the underside and brown on the upper surface. Although Downy Spot does not infect the nuts, severe infection reduces photosynthetic activity, leading to premature leaf drop and reduced tree vigor the following season.
Shuck Dieback and Stem End Blight are late-season disorders, typically appearing from mid-August through October. Shuck dieback starts with the shuck turning black and dying at the nut’s tip, often causing it to flare open. Stem End Blight begins as a brown or black spot near the nut’s base, which then enlarges to cover the shuck. Both conditions significantly reduce kernel quality, with earlier symptoms resulting in poorer kernel development.
Cultural Practices for Disease Control
Effective disease management begins with non-chemical controls that reduce the pathogen’s ability to survive and spread. Sanitation is foundational, involving the removal and destruction of fallen leaves and old nut shucks from the previous season. Since the fungi causing Scab and Downy Spot survive the winter in this debris, removing it before spring bud break eliminates a major source of initial infection.
Proper tree structure and spacing are important for manipulating the canopy microclimate. Pruning to remove low-hanging limbs and thin out dense growth improves air circulation and sunlight penetration. Increased airflow helps leaves and nut shucks dry more quickly after rain or dew. This limits fungal development, as most fungal spores require hours of sustained wetness to germinate and infect.
Managing water stress and soil health contributes to overall tree resilience against disease. Pecans require specific nutrients; a deficiency in zinc, for instance, can predispose the tree to fungal spots. Planting disease-resistant cultivars, such as ‘Kanza’ or ‘Sumner,’ is the most effective long-term strategy for minimizing disease pressure.
Targeted Chemical Treatments
Fungicides are often necessary, especially on susceptible cultivars in humid climates, and are used for prevention rather than cure. Timing is paramount for Pecan Scab, requiring a series of sprays starting early when new tissues are most vulnerable. The first protective sprays should coincide with bud break and pre-pollination, followed by regular applications while young tissue is expanding.
During the nut sizing period, typically starting in June, the focus shifts to protecting the rapidly growing nut shucks. Susceptible varieties should not exceed 14 days between sprays, shortening the interval to 10 days or less during frequent rain events. Fungicides containing active ingredients like propiconazole (Group 3) or tebuconazole combined with a strobilurin (Group 11) are commonly used for broad-spectrum control.
Fungicide rotation is necessary to maintain effectiveness and prevent the development of resistant fungal strains. Fungicides are categorized into groups based on their mode of action, and products from different groups must be alternated in the spray schedule. For example, materials effective against leaf scab, such as phosphite, can be rotated with combination products that offer stronger nut scab protection.
Growers should consult their local agricultural extension office for specific product recommendations and application rates, which vary by location and disease pressure. Proper equipment is necessary to achieve adequate coverage of the entire canopy. Large, mature pecan trees often require a specialized air-blast sprayer for effective treatment. Trees deficient in zinc may also require foliar zinc treatments to correct the nutritional imbalance, which can otherwise weaken the tree.