Black spots on pecans are a frequent concern for both pecan growers and homeowners, often indicating underlying issues that affect the health of the tree and the quality of its nuts. Understanding the causes behind these spots is the first step toward effective management and ensuring a healthy pecan harvest. This guide identifies and addresses common problems leading to black spots.
What Causes Black Spots on Pecans
The most common cause of black spots on pecans is Pecan Scab, a fungal disease (Fusicladium effusum). This disease thrives in warm, humid conditions and frequent rainfall, posing a persistent challenge in many pecan-growing regions. Pecan scab can infect leaves, young twigs, and especially nut shucks.
Another fungal disease, Anthracnose, also contributes to black spots. It typically manifests as brown to black, sunken lesions on leaves and nut shucks, favored by wet conditions and warm temperatures.
Beyond fungal infections, certain insect pests cause black spots. Stink bugs and leaf-footed bugs feed on developing nuts, injecting digestive enzymes that create “kernel spot”—black, sunken, and bitter spots on the nutmeat. Black pecan aphids cause yellow, angular spots on leaves that turn brown, potentially leading to premature leaf drop.
Identifying the Problem
Distinguishing the causes of black spots requires careful observation. Pecan scab lesions typically appear as small, circular, olive-green spots that darken, become velvety, and sink over time. These lesions can merge, creating larger blackened areas on leaves, nut shucks, and young twigs. Severely infected nut shucks may crack or distort.
Damage from stink bugs and leaf-footed bugs primarily affects the nut kernel. When these insects feed, they create dark brown to black, sunken spots on the kernel surface, which can be bitter to the taste. The external nut shuck may show no obvious damage until the nut is cracked open.
Black pecan aphid damage is identifiable by distinct yellow, angular spots that form on the leaves, particularly between the veins. These spots expand and eventually turn brown as the affected leaf tissue dies. Aphid damage is primarily a foliar issue that can lead to early defoliation if severe.
Effective Treatment Methods
Treating black spots on pecans involves targeted approaches based on the cause. For fungal infections like pecan scab and anthracnose, fungicide applications are common. These are most effective preventatively, applied from bud break through nut development. Various types of fungicides are available, including phosphite, tin-based compounds, and strobilurin fungicides.
Managing insect-induced black spots, particularly from stink bugs and leaf-footed bugs, often involves integrated pest management (IPM). This includes targeted insecticide applications if pest populations reach damaging levels. Pyrethroid insecticides, like bifenthrin, are often used against these pests during nut-filling stages, though they can increase aphid populations. For black pecan aphids, specific aphicides may be necessary if numbers cause significant leaf damage.
Cultural practices also aid treatment. Pruning to improve air circulation within the tree canopy helps reduce humid conditions that favor fungal growth.
Preventing Black Spots
Proactive measures are the most effective way to prevent black spots. Selecting scab-resistant pecan varieties when planting new trees is a primary preventative step. Cultivars like ‘Elliott’, ‘Kanza’, ‘Lakota’, ‘Excel’, and ‘McMillan’ show good resistance to pecan scab. Choosing appropriate varieties for your specific climate and disease pressure is important.
Maintaining good orchard hygiene is another effective preventative measure. This includes regularly removing and destroying fallen leaves, old nut shucks, and infected twigs from around the trees. This practice eliminates sites where disease-causing fungi and some insect pests can overwinter and re-infect trees, significantly reducing the inoculum load.
Appropriate cultural care also prevents black spots. Ensuring adequate watering and balanced fertilization maintains tree vigor, helping pecans resist diseases and pests. Pruning trees to improve air circulation reduces prolonged leaf wetness, creating an environment less conducive to fungal development. For stink bugs, controlling weed hosts or planting trap crops can be beneficial.
Are Black Spotted Pecans Safe to Eat?
Pecans with black spots are generally safe to consume once the outer shell is removed. While the nutmeat’s appearance may be compromised, especially by stink bug feeding, the primary concern is typically quality, not safety. These black spots, known as “kernel spot,” are caused by insect-injected digestive fluids and can impart a bitter taste to the affected area.
Though unappealing, the black spots can often be trimmed away, leaving edible kernel portions. Severe damage can lead to shriveled or discolored kernels, impacting size, taste, and fill, making nuts less desirable for eating or use in recipes.