Brown spots on pepper plants signal that the plant is under duress, acting as a visible symptom rather than a disease itself. These blemishes are a common issue for growers, indicating environmental stress, a nutrient imbalance, or the presence of an infectious pathogen. Correctly identifying the specific appearance and location of the spots is the first step toward effective treatment. The causes range from simple sunburn to aggressive bacterial infections, and early recognition is important for preserving the overall health and yield of the pepper crop.
Abiotic Causes: Environmental Stress and Nutrient Issues
Non-infectious causes of brown spots often relate to abrupt environmental changes or a lack of proper nutrition, which are usually easier to correct than diseases. Sunscald occurs when fruit or leaves are suddenly exposed to intense sunlight, especially after transplanting or a period of cloudiness. The affected tissue first appears bleached before turning into dry, brown, or scorched patches, particularly noticeable on the fruit’s skin. To fix sunscald, provide the plant with temporary shade using a shade cloth during the hottest parts of the day.
Another common issue is edema, a physiological disorder resulting from the plant absorbing water faster than it can transpire it, leading to cellular rupture. This imbalance manifests as tiny, yellowish bumps or blisters on the undersides of the leaves that eventually collapse into small, dry, brown, corky spots. Addressing edema requires reducing the watering frequency and improving air circulation around the plants to help regulate humidity levels.
Nutrient imbalances also lead to characteristic brown spots, most notably Blossom End Rot (BER), caused by a localized calcium deficiency in the developing fruit. BER appears as a dark, sunken, water-soaked spot on the blossom end of the pepper fruit, which then enlarges and turns leathery and brown or black. This is usually due to inconsistent soil moisture or high nitrogen fertilization, which inhibits calcium uptake. To prevent BER, maintain consistently moist soil and avoid fertilizers high in ammoniacal nitrogen, which competes with calcium absorption.
Biotic Causes: Fungal and Bacterial Infections
Infectious brown spots are caused by pathogens that thrive in warm, wet, or highly humid conditions and require aggressive management strategies. Bacterial Leaf Spot, caused by Xanthomonas bacteria, begins as small, water-soaked lesions on the leaves, which then turn dark brown and develop angular, irregular shapes with yellow halos. The lesions may become raised or scab-like on the fruit, and the disease spreads easily by splashing water. Treatment involves removing and destroying infected plant material, followed by a protective application of a fixed copper-based bactericide to slow the spread.
Fungal diseases like Cercospora Leaf Spot, or “frog-eye spot,” are characterized by distinct, circular lesions with tan or gray centers surrounded by a dark reddish-brown border. As the spots mature, the centers often dry out and drop away, creating a “shot-hole” effect on the leaves. This fungus causes significant leaf drop, which can expose the fruit to sunscald. Management includes applying a targeted fungicide, such as those containing chlorothalonil or mancozeb, and ensuring the plant canopy dries quickly after irrigation or rain.
Anthracnose, caused by Colletotrichum fungi, is particularly damaging to the fruit, where it forms dark, water-soaked, and sunken circular lesions, often with concentric rings. Under humid conditions, these sunken spots may develop a visible mass of salmon-pink or orange spores. Control involves applying appropriate fungicides, such as those with Group 11 active ingredients, starting at the flowering and fruiting stage. Promptly removing all infected fruit and plant debris is also necessary.
Diagnostic Guide to Spot Appearance
Differentiating between the causes of brown spots relies on three main visual cues: the spot’s margin, its color, and its location on the plant. Abiotic problems like sunscald result in large, dry, bleached, or scorched areas on leaves and fruit facing the sun, lacking a distinct border. Edema presents as tiny, corky, dry brown bumps almost exclusively on the undersides of leaves.
Infectious diseases produce more defined lesions. Bacterial Leaf Spot creates angular or irregular brown spots that may appear greasy or water-soaked, particularly after rain. Fungal infections generally produce more circular spots with clear, distinct margins. Cercospora Leaf Spot has a classic “frog-eye” pattern (a light tan center with a dark rim), while Anthracnose on the fruit forms noticeably sunken, dark, and soft spots. Blossom End Rot is distinct because its dark, leathery spot is found only at the bottom tip of the pepper fruit.
Long-Term Prevention Through Cultural Practices
Preventing future outbreaks requires implementing cultural practices that discourage both pathogen development and environmental stress. Crop rotation is a fundamental step, as many pepper pathogens, including the fungi causing Anthracnose and Cercospora, can survive in the soil or on plant debris. Rotating peppers with non-solanaceous crops, like legumes or grains, helps break the disease cycle and reduces the concentration of pathogens in the soil.
Water management is another preventative measure for both biotic and abiotic issues. Avoiding overhead irrigation is paramount because wet foliage creates the ideal environment for spores and bacteria to germinate and spread via splashing water. Instead, use drip irrigation or soaker hoses to deliver water directly to the root zone, and always water early in the morning to allow moisture on the leaves to evaporate quickly.
Proper sanitation and spacing also reduce disease pressure. Ensure that plants are spaced adequately to promote good airflow, which minimizes the period of leaf wetness and reduces humidity within the plant canopy. At the end of the season, remove and destroy all infected plant material, including fallen leaves and fruit, by bagging them rather than composting them, to eliminate overwintering pathogen sources.