Brown spots on pear tree leaves signal a disruption in the tree’s health, ranging from minor stress to serious disease. Accurately identifying the cause is the first step, as treatment for a fungal infection will not correct a nutrient deficiency, and vice-versa. These spots are damaged plant tissue, and their size, color, shape, and location provide clues for proper diagnosis. This information breaks down the common culprits behind leaf spotting.
Primary Fungal Causes of Brown Spots
The most frequent causes of brown or black spots on pear leaves are fungal pathogens, which thrive in wet, humid conditions. Differentiating between these diseases often depends on the specific appearance and texture of the spots.
Pear Scab (Venturia pirina)
Pear scab, caused by the fungus Venturia pirina, first appears on leaves as small, olive-green spots that are typically velvety or sooty in texture. These lesions often appear on the underside of the leaf first and gradually turn dark brown or black as the infection progresses. The fungus overwinters in infected, fallen leaves, releasing spores in the spring that are spread by rain and wind to the new foliage. If the infection is severe, the leaves may curl, shrivel, and drop prematurely, which weakens the tree over time.
Fabraea Leaf Spot (Fabraea maculata)
Fabraea leaf spot, also known as pear leaf blight, begins as tiny, purplish dots, usually on the younger, lower leaves of the tree. These spots enlarge into circular lesions up to a quarter-inch in diameter and develop a purplish-black or brown color with a distinct black center. A tell-tale sign is the formation of a small, black pimple within the spot, which, when wet, may ooze a gelatinous mass of spores. This disease can advance rapidly in warm, wet weather, leading to heavy defoliation if not managed.
Rust Diseases (Gymnosporangium spp.)
Rust diseases, such as European pear rust (Gymnosporangium sabinae), are identified by their distinct, bright coloring. Symptoms begin as yellowish-orange spots on the upper leaf surface in late spring or early summer, intensifying to a bright reddish-orange. Later, the corresponding underside of the leaf develops brown, blister-like swellings that form small, horn-shaped outgrowths. This fungus requires two separate hosts—the pear tree and a juniper shrub—to complete its development.
Non-Pathogen Causes and Environmental Stress
Not all leaf spots are caused by living organisms; many are the result of environmental stresses or imbalances in the tree’s growing conditions. These non-pathogen issues often present with a more uniform or generalized pattern of browning than fungal diseases.
Nutrient Deficiencies
A lack of certain macro and micronutrients can cause leaf tissue death that mimics spotting. Potassium (K) deficiency often manifests as marginal leaf necrosis, where the edges of older leaves turn brown and may cup or curl upward. Magnesium (Mg) deficiency appears on older leaves first, causing interveinal chlorosis where tissue between the veins turns yellow. This chlorosis can then progress into brown, dead patches. Since these are mobile nutrients, symptoms appear first on older foliage as the tree moves the limited supply to new growth.
Water Stress and Scorch
Both too little and too much water can cause leaf browning. During drought, the tree cannot supply enough water to the entire leaf, causing tips and edges to dry out and turn brown, known as leaf scorch. Waterlogging also leads to browning because saturated soil starves the roots of oxygen. This prevents roots from absorbing water and nutrients, resulting in scorch symptoms similar to drought.
Chemical Burn (Phytotoxicity)
Improper application of pesticides, herbicides, or fertilizers can cause chemical burn, or phytotoxicity, resulting in necrotic spots or scorched margins. This damage appears abruptly after application and is often localized where the chemical spray accumulated, such as leaf tips. Unlike a disease, phytotoxicity damage will not spread over time, and new growth will appear healthy. High temperatures during application increase the risk of chemical burn, leading to bleached or brown spots where spray droplets landed.
Strategies for Management and Prevention
Effective control of brown spots relies heavily on identifying the root cause, followed by a combination of cultural and chemical strategies. Cultural practices are the first line of defense, focusing on reducing the conditions that favor disease and stress.
Cultural Control and Sanitation
Improving air circulation within the tree canopy is important, as dense, humid conditions promote fungal growth. This is achieved through selective pruning to open up the tree’s interior structure. Sanitation is a fundamental practice for fungal diseases like scab and Fabraea leaf spot, which overwinter in fallen debris. Raking and removing all fallen leaves during the dormant season reduces the source of primary inoculum for the following spring.
Chemical and Environmental Treatments
For fungal issues, preventative fungicide applications are necessary, with timing being the most important factor. For diseases like pear scab, a series of copper-based sprays should begin early in the spring, starting around the bud break stage, to prevent initial infection. Dormant oil sprays applied before bud swell also help manage overwintering fungal spores and certain pests. If a nutrient issue is suspected, a professional soil test is necessary to determine the specific imbalance. Deficiencies can be corrected using appropriate soil amendments or foliar applications, while adjusting watering practices ensures deep, infrequent irrigation to manage stress.