Diagnosing

Step 1. Overall Assessment of Plant Habitus (Appearance)

Before examining each leaf, step back and evaluate the plant as a whole. Ask yourself these questions:

• Does the plant look vigorous and strong, or is it depressed and drooping? This overall impression is your first diagnosis.
• How long has the plant been in this condition? Did the problem appear suddenly (overnight) or develop gradually (over a week or two)? Sudden wilting often indicates root or vascular system issues, while gradual decline points to nutritional deficiencies.
• Is there a clear pattern to how the problem is spreading across the field or garden bed? Are only individual plants affected, the entire bed, or just those at the edge? This helps distinguish between a wind-borne disease and a problem with soil or irrigation (Yasou & Wien, 2020).

Step 2. Detailed Leaf Inspection. How to Read the Main Indicator

Leaves are the plant's "face." They are the first to respond to stress. Our inspection starts with them.

1. Start with the old leaves at the bottom of the plant. This is where problems with mobile nutrients (nitrogen, phosphorus, potassium, magnesium) most often show up first, as the plant "transports" these substances to young leaves and fruits.

• What do we see? Uniform yellowing? Yellowing between the veins? Dry edges? Dark spots?
• Then move to the middle tier of leaves. This is the "golden mean." The condition of these leaves is often the most objective indicator of plant health.
• Examine the young leaves and the growing tip (apex). Deficiencies of immobile elements (iron, manganese, calcium, boron) or viral diseases usually hit new growth first. A distorted, chlorotic (pale), or dying growing tip is a critical signal (Hochmuth & Sideman, 2023).
• Assess texture and color. Hard, "leathery," and dark green leaves? Or soft, pale, and drooping?

Step 3. Stem and Crown Inspection

Sometimes the problem is hidden not in the leaves but in the plant's "circulatory system."

• Inspect the stems for spots, cracks, or "constrictions." These could be signs of bacterial or fungal diseases, such as Phytophthora blight (Kemble et al., 2022).
• Pay attention to the base of the stem (crown). Are there dark, "water-soaked" areas, mold, or white powdery growth? These are signs of crown rot.
• Cut open the stem of a suspicious plant. Darkened vessels inside the stem indicate vascular wilt (Fusarium, Verticillium). This is one of the most dangerous problems, as it is virtually untreatable (Welbaum, 2015).

Step 4. Flowers, Ovaries, and Fruits

Flowering and fruiting are the most energy-intensive parts of a pepper's life. At the slightest imbalance, the plant may "evacuate" these organs.

• Check if flowers and newly set fruits are dropping off. This is the most common physiological problem.
• Examine developed fruits. Are there dark, "water-soaked" spots near the stem or at the blossom end (blossom-end rot)? Are they distorted? Are there spots on them like on the leaves?

Step 5. Root Zone and Soil Inspection (The Most Important but Often Skipped Step)

Very often, leaf problems are merely a consequence of what is happening underground.

Check soil moisture. Drought or waterlogging is the root of many evils. Carefully dig into the soil to a depth of 5-10 cm (2-4 inches) and assess its condition: dry, moist, or waterlogged (Kemble et al., 2022).

Carefully extract one weak plant with a clump of soil and examine the roots. What do they look like?

• Healthy roots: white or light brown, firm, well-branched.
• Diseased roots: dark, brown or black, water-soaked, easily torn. This is a sign of root rot (e.g., from Phytophthora or Pythium) (Wien & Stützel, 2020).
• Attention: the presence of knot-like swellings on the roots is a sign of root-knot nematode infestation. This is one of the most serious enemies of peppers in warm regions.

Step 6. Searching for Pests

Sometimes the cause of the problem is literally on the surface.

• Check the underside of the leaves. This is where aphids, whiteflies, and spider mites usually hide. White powdery residue or sticky honeydew on leaves is a sure sign of sap-sucking pests (Kemble et al., 2022).
• Inspect the plant for caterpillars, their droppings, or damage. Holes in leaves and fruits are the work of chewing pests.

Step 7. Considering Environmental Conditions

Finally, the most obvious but often ignored step. Assess:

• The weather over the past two weeks. Was there intense heat, drought, heavy rain, or a sharp temperature fluctuation? This is a key factor for diagnosing physiological disorders (Wien & Stützel, 2020).
• The conditions of your site. Are the peppers planted in a low spot where cold air stagnates? Are they in a draft? Are they shaded by a fence or other plants?

This step-by-step system is your main assistant. It will prevent you from getting lost in guesses and help you quickly determine where to go next. We will rely on it throughout all the following chapters.

Step 1. Are the Lower (Old) Leaves Turning Yellow?

This is the most common situation. It indicates that the plant is "starving" and is translocating nutrients from old leaves to young ones and fruits. This is most often related to macronutrients—substances needed in large quantities.

Scenario A: The leaf becomes uniformly pale green, then yellow, starting with the oldest leaves. This is the classic sign of nitrogen (N) deficiency (Hochmuth & Sideman, 2023). Nitrogen is a building block for proteins and chlorophyll. When deficient, the plant sacrifices old leaves to support new growth. Plants look stunted, with thin stems.

• What to do? Apply nitrogen fertilizer immediately. This could be a solution of urea, ammonium nitrate, or an organic brew (manure tea, nettle tea). The effect will be quick. Apply strictly according to instructions to avoid burning the roots (Kemble et al., 2022).

Scenario B: Yellowing appears as pale or yellow spots between the veins (interveinal chlorosis), while the veins themselves remain green. If this occurs on old leaves, it is magnesium (Mg) deficiency (Wien & Stützel, 2020). Magnesium is the central element of the chlorophyll molecule. Without it, photosynthesis is impossible. Magnesium deficiency often occurs on acidic soils or with excess potassium, which competes with magnesium for uptake (Welbaum, 2015).

• What to do? Foliar feeding (spraying on the leaves) with a solution of magnesium sulfate (Epsom salts) — 10-20 g per 10 liters of water. This is the fastest method. Dolomitic lime can be added to the soil, but it works slowly.

Scenario C: Yellowing starts at the edges and marginal areas of the leaf, gradually taking over the entire blade, and the edges become dry ("marginal burn"). This is potassium (K) deficiency (Hochmuth & Sideman, 2023). Potassium is responsible for water balance and tissue strength. When deficient, leaves lose water, and the edges die. Leaves may become crinkled.

• What to do? Apply a potassium fertilizer (potassium sulfate, potassium magnesium sulfate). Potassium is more mobile than phosphorus, but its uptake can be hindered on light sandy soils.

Step 2. Are the Young (Upper) Leaves and Growing Tip Turning Yellow?

This is a more alarming signal. It indicates a problem with elements that cannot be moved around the plant (immobile). The plant cannot "pull" them from older parts, so new growth suffers.

Scenario A: Young leaves become pale yellow or almost white, with veins remaining green (pronounced interveinal chlorosis). This is iron (Fe) deficiency (Wien & Stützel, 2020). Iron is necessary for chlorophyll synthesis, and its deficiency often occurs on alkaline soils (pH above 7.0) or with waterlogging, where roots cannot absorb iron (Yasou & Wien, 2020).

• What to do? First, check the soil pH. If it's high, acidify the soil (e.g., with colloidal sulfur) and use chelated iron forms (e.g., "Ferovit") for foliar spraying. Root applications of iron sulfate on alkaline soils are ineffective because the iron quickly binds and becomes unavailable.

Scenario B: Young leaves yellow between the veins, but the veins remain green, yet the contrast between veins and tissue is not as sharp as with iron deficiency. A net-like pattern may appear on the leaves. This is manganese (Mn) deficiency (Hochmuth & Sideman, 2023). Manganese is also important for photosynthesis, and its deficiency is often found on alkaline and organic-poor soils.

• What to do? Foliar feed with manganese chelate or manganese sulfate. Works quickly. You can also acidify the soil.

Scenario C: Young leaves turn yellow uniformly, without green veins remaining, becoming almost bleached, and stem growth stops. This is sulfur (S) deficiency (Welbaum, 2015). Sulfur is a component of amino acids and proteins. Its deficiency is rarer but possible on poor soils, especially without organic matter.

• What to do? Apply ammonium sulfate or just sulfur to the soil (if pH allows). This problem usually resolves when using complex fertilizers containing sulfur (e.g., potassium sulfate).

Step 3. Leaves Turn Yellow and Also Wilt?

This combination indicates that the problem is not so much nutrition but the root system's function or water status.

• Cause 1: Waterlogged soil. Roots suffocate without oxygen, begin to rot (especially from Pythium), and stop absorbing water and nutrients. Leaves turn yellow and droop, even if the soil is wet (Kemble et al., 2022). This often happens during rainy seasons or with too frequent watering.
• Cause 2: Drought. During severe drought, the plant first loses turgor (wilts), then, to survive, sheds or "shuts down" old leaves to reduce evaporation. They turn yellow and fall off. This is a protective mechanism (Wien & Stützel, 2020).
• Cause 3: Root-knot nematodes. Swellings (galls) form on the roots, disrupting water and nutrient transport. Leaves turn yellow, and the plant is stunted (Welbaum, 2015). This is a problem in regions with mild winters.

What to do in these cases? First, normalize watering. If the soil is wet, stop watering and loosen the surface crust. If it's dried out, give a thorough watering but don't flood it. For diagnosis, extract the roots and check their condition (as described in Chapter 1). If nematodes are found, this is a serious problem, and it's better to manage it preventively (crop rotation, using resistant rootstocks) than to treat an already sick plant.

Step 4. Leaves Turn Yellow in Spots, Not Uniformly

If dark or brown spots are visible on a yellowing background, or the yellowing has an irregular shape, it is most likely not a nutritional deficiency but a disease.

• Bacterial leaf spots — water-soaked, greasy spots that then turn dark.
• Fungal diseases (Alternaria, Septoria) — dry spots with concentric rings or a light center.
• Viruses — yellow-green mosaic patterns, leaf curling, deformation.

We will discuss the differences between diseases and physiological issues in detail in Chapter 7. For now, remember the main point: uniform or patterned (between veins) yellowing is most often a nutrient deficiency; spotty, chaotic yellowing is more likely a disease or pest. At the same time, nutrient deficiency weakens the plant and makes it more vulnerable to diseases, so everything is interconnected (Wien & Stützel, 2020).

Summary Table for Quick Diagnosis

Remember this algorithm, and you will never confuse a hungry pepper with a sick one.

Step 1. Leaves Curling Upward (Edges Curling Up, Resembling a Boat)

This is the most common type of curling, and in most cases, it is related to unfavorable environmental conditions.

Cause 1: High temperature and lack of moisture. This is a classic protective mechanism. In intense heat (above 32°C / 90°F during the day) and dry air, the plant tries to reduce its evaporative surface by rolling the leaf blade. The leaves often become small, stiff, and may slightly yellow at the edges (Yasou & Wien, 2020). Check: if the curling intensifies during the hottest part of the day and decreases towards evening, this is the cause.

• What to do? Ensure regular watering, especially during fruiting. Mulch the soil to retain moisture. If possible, use shade cloth, particularly in southern regions (Kemble et al., 2022).

Cause 2: Salt stress (soil salinization). If you use hard water, too much mineral fertilizer, or live in an arid region, salts can accumulate in the soil. They increase osmotic pressure, making it difficult for roots to absorb water. The plant experiences constant "thirst," even if the soil is moist. Leaves curl upward, become stiff, and white salt crystals may appear on the soil surface or pot walls (Hochmuth & Sideman, 2023).

• What to do? Review fertilizer dosages, use soft water for irrigation. If salts have already accumulated, give a heavy watering (leaching) followed by drainage to flush out excess salts. On an ongoing basis, apply fertilizers in smaller, more frequent doses (fertigation).

Step 2. Leaves Curling Downward (Edges Curling Down, Sometimes with the Tip)

This is also a fairly common symptom, often confused with lack of water. But here, the cause is almost always different.

Cause: Waterlogged soil and root problems. When roots are flooded and oxygen-deprived, they begin to rot. The supply of water and nutrients to the leaves is disrupted, and they lose turgor. Unlike with heat, the leaves become not stiff but soft, drooping, with edges curling downward. This is often accompanied by yellowing of the lower leaves (as discussed in Chapter 2) (Welbaum, 2015).

• What to do? Immediately reduce watering and allow the soil to dry out. If it's a container plant, make sure there are drainage holes. In open ground, you can create drainage channels or plant on raised beds to avoid water stagnation. Check the roots: if they have darkened, it's the beginning of root rot.

Step 3. Leaves Curling Inward (Along the Midrib) or Becoming "Curly," Deformed

These are the most dangerous scenarios, requiring immediate attention.

Cause 1: Calcium (Ca) deficiency. This is perhaps the most common physiological cause of curling in young leaves. Calcium is an immobile element, and when deficient, the tips and young tissues suffer first. Young leaves stop growing, become distorted, edges roll inward, and become "torn" or "hooked." The growing tip may die back. This symptom often accompanies blossom-end rot in fruits (discussed in the next chapter) (Wien & Stützel, 2020).

• What to do? First and foremost, ensure uniform watering—calcium is absorbed only in dissolved form. Foliar feeding with calcium nitrate (15-20 g per 10 liters of water) provides a quick but temporary effect. Dolomitic lime or gypsum can be added to the soil, but they work slowly. The key is to avoid sharp fluctuations in moisture.

Cause 2: Viral infection (especially Tomato Spotted Wilt Virus — TSWV, or Cucumber Mosaic Virus — CMV). Viruses cause severe leaf deformation. Leaves become bumpy, thread-like, edges curl inward and downward, and a characteristic yellow-green mosaic pattern or ring spots appear. Leaves often become smaller, and plant growth stops (Kemble et al., 2022). The key difference from calcium deficiency: with calcium starvation, the growing tip may simply die, but with a virus, it continues to grow, albeit with deformities.

• What to do? Viral diseases are not curable. Affected plants must be immediately removed and destroyed to prevent them from becoming a source of infection for neighbors. Control focuses on prevention — controlling vectors (aphids, thrips) and using resistant varieties.

Cause 3: Heavy infestation by aphids or other sap-sucking pests. Aphids suck sap from tissues, and the leaves respond with deformation, curling, and yellowing. Often, a sticky "honeydew" (aphid excrement) appears on the leaves, on which sooty mold then develops (black residue). When you turn the leaf over, the insects themselves or their white cast skins are clearly visible (Kemble et al., 2022). Curling can be upward or downward, depending on the aphid species.

• What to do? Apply insecticides (biological ones based on avermectins or pyrethrins are possible), or a soap solution to wash off the pests. In early stages, a strong spray of water helps. Attract natural aphid enemies—ladybugs.

Summary Table: Why Do Leaves Curl?

Remember: curling is always a signal. Assess the direction, texture, and accompanying signs, and you will quickly determine whether the pepper needs water, fertilizer, or urgent protection from pests and diseases.

Step 1. Check the Temperature. The Most Common Culprit

Peppers are heat-loving, but they do not tolerate extreme temperatures. Temperature stress is the main cause of mass flower and ovary drop.

Too hot (daytime temperature above 32°C / 90°F, nighttime above 24°C / 75°F). This is the most common problem in open fields and in greenhouses without good ventilation. At high temperatures, pollen becomes sterile, pollen tubes fail to germinate, and fertilization does not occur. The flower may look healthy but drops off after a few days. Large-fruited sweet pepper varieties are particularly sensitive to heat (Yasou & Wien, 2020).

• What to do? Provide shading (shade cloth, planting in partial shade). Improve ventilation in greenhouses. Adequate watering and mulching help lower the temperature in the root zone, facilitating water and nutrient uptake (Kemble et al., 2022).

Too cold (nighttime temperature below 15°C / 59°F). Peppers are native to the tropics, and during cool nights, their physiological processes slow down. Pollen becomes non-viable, and ovaries do not develop. In open ground, this problem often occurs early in the season or during sudden cold snaps.

• What to do? Use row covers to protect against cold nights. Choose varieties resistant to lower temperatures. In regions with cool nights, plant only when the soil and air have warmed up sufficiently.

Step 2. Check Air Humidity. An Often Underestimated Factor

Peppers prefer air humidity in the range of 60-70%. Both too dry and too humid air can disrupt pollination.

Low humidity (below 40%). In dry air, pollen dries out quickly and loses viability. This is a common problem in hot weather.

• What to do? Spraying plants with water (especially in the morning) can temporarily increase humidity. In greenhouses, misting systems are used.

High humidity (above 80%). Pollen becomes "sticky" and does not release well from the anthers, hindering pollination. Also, in high humidity, fungal diseases (e.g., gray mold) thrive and can attack flowers and ovaries (Kemble et al., 2022).

• What to do? Ensure good ventilation in greenhouses and tunnels. Do not overcrowd plants. In open ground, regularly remove weeds to improve air circulation.

Step 3. Check Watering. Balance is Critical

Flower and ovary drop is a response to water stress.

Soil drying out. With a lack of moisture, the plant halts flowering to avoid wasting water on fruit formation. The youngest ovaries drop first, as they require the highest water influx (Wien & Stützel, 2020).

• What to do? Regular, uniform watering. Maintaining stable moisture is especially important during mass flowering and fruit setting.

Waterlogged soil. When roots suffocate in water, they cannot effectively deliver moisture and nutrients to flowers and ovaries. This also leads to their drop.

• What to do? Reduce watering. Check drainage. Plant peppers on raised beds to avoid water stagnation around the roots.

Step 4. Check Nutrition. Excess Nitrogen is the Main Enemy

Over-application of nitrogen fertilizers is one of the most common mistakes. Nitrogen stimulates vigorous vegetative growth ("vegetative overgrowth"), not flowering. The plant becomes tall, robust, with dark green leaves, but it produces few flowers, and those that appear often drop. This happens because the plant spends all its resources on leaf and shoot growth rather than on reproductive organ formation (Hochmuth & Sideman, 2023).

What to do? Balance the nutrition. If you notice the pepper is "overgrowing," reduce nitrogen and increase potassium and phosphorus fertilization. Potassium is especially important for flowering and fruit setting. Also, pay attention to the availability of boron and calcium—their deficiency can disrupt pollination and lead to ovary drop (Welbaum, 2015).

Step 5. Check Lighting. Light Regime

Pepper is a short-day plant, but modern varieties are less sensitive to day length than to overall light intensity. If the plant doesn't get enough light, it cannot accumulate enough carbohydrates to sustain flowering and fruiting. In cloudy weather or with severe overcrowding, flower and ovary drop may increase (Yasou & Wien, 2020).

What to do? Ensure adequate spacing between plants. Remove lower, old leaves that shade the flowers. In greenhouses, use reflective screens and, if possible, supplemental lighting.

Step 6. Check for Pollinators. Critical in Greenhouses!

Peppers are self-pollinating, but for better fruit set (especially in protected cultivation), they need the help of pollinators. In open fields, wind and insects perform this role. In greenhouses or tunnels without insect access, pollen may not be transferred to the stigma, leading to flower drop.

What to do? In greenhouses, use bumblebees (best suited for peppers) or perform manual pollination. Simply shaking the plants in the morning (when pollen is most viable) allows pollen to fall onto the stigma (Yasou & Wien, 2020).

Step 7. Check for Diseases and Pests

Some diseases (especially viral) and pests (e.g., thrips, aphids, mites) can directly attack flowers and ovaries, causing them to drop. Assess the plant's overall health and look for other symptoms (spots, deformations, sticky residue).

What to do? If pests or diseases are found, apply appropriate protection measures as described in Chapters 6 and 7.

Summary Table: Why Do Flowers and Ovaries Drop?

Remember: flower and ovary drop is not a death sentence but a signal to act. Identify the cause and adjust your growing conditions, and your pepper will reward you with a bountiful harvest.

Step 1. Hollows Inside the Fruit, Thin Walls, "Emptiness"

This is one of the most common problems, especially in large-fruited sweet pepper varieties. The fruit looks normal on the outside, but inside it is half-empty, with thin walls and underdeveloped seeds. This is a consequence of incomplete pollination and fertilization (Wien & Stützel, 2020).

Why does this happen? For normal fruit development, a sufficient number of seeds must form in the ovary. Seeds produce hormones (auxins) that stimulate the growth and thickening of the fruit walls (pericarp). If pollination did not occur or was incomplete, few seeds are set, and the fruit walls remain thin. Such fruits may be seedless or have only a few seeds (Yasou & Wien, 2020).

Main causes of insufficient pollination:

• Extreme temperatures (above 32°C/90°F during the day or below 15°C/59°F at night during flowering) — pollen becomes sterile.
• Low or excessively high air humidity — pollen does not reach the stigma or loses viability.
• Lack of pollinators or mechanical shaking (especially in greenhouses and tunnels).
• Boron (B) deficiency — boron is critical for pollen tube germination. When deficient, fertilization is impaired even with viable pollen (Hochmuth & Sideman, 2023).

What to do?

• Ensure optimal conditions for pollination (see Chapter 4).
• In greenhouses, use bumblebees or manually shake the plants in the morning.
• If boron deficiency is suspected, apply a foliar spray of boric acid solution (1-2 g per 10 liters of water) during mass flowering. This will improve pollination quality and reduce the number of "hollow" fruits (Kemble et al., 2022).

Step 2. Curvature, Asymmetry of Fruits

Fruits may grow crooked, with twisted walls or uneven thickness. This is often related to uneven seed development within the ovary.

Cause: If more seeds developed in one part of the fruit than in another, that part grows faster, leading to curvature. This often occurs with partial pollination, where only some ovules were fertilized.

Other possible causes:

• Uneven watering. Sharp fluctuations in moisture can cause uneven growth of the fruit walls (Welbaum, 2015).
• Damage to the ovary by pests (e.g., thrips or bugs) at early stages — the damaged part grows slower, deforming the fruit.
• Viral diseases, especially Tomato Spotted Wilt Virus (TSWV) and Cucumber Mosaic Virus (CMV), cause severe fruit deformation, often with bumpiness and uneven color (Kemble et al., 2022).

What to do?

• Maintain uniform watering, avoiding drought and waterlogging.
• Control pests at early stages (see Chapter 6).
• If a viral infection is suspected, remove and destroy affected plants.

Step 3. Cracks on Fruits (Longitudinal and Transverse)

Cracks are mechanical damage to the fruit skin, caused by a sharp change in internal pressure. Internal pressure increases with rapid water influx, while the outer skin (exocarp) can no longer stretch.

Main cause: Sharp fluctuations in soil moisture. After a long drought, heavy watering or heavy rain causes rapid swelling of fruit tissues, and the skin cannot withstand the pressure. This is especially common in thin-skinned varieties (Hochmuth & Sideman, 2023).

Other factors:

• Potassium (K) deficiency. Potassium strengthens cell walls and increases skin strength. When deficient, cracks appear more frequently (Welbaum, 2015).
• Temperature fluctuations — can also contribute to cracking.

What to do?

• Keep the soil consistently moist using mulching and regular watering.
• Avoid long gaps between waterings.
• Apply potassium fertilizers during fruit growth to strengthen tissues.
• Choose varieties resistant to cracking.

Step 4. Fruits Too Small (Not Reaching Variety Size)

If fruits are small but correctly shaped, this often indicates high plant load (too many ovaries) or lack of resources (nutrients, light, water).

• Cause 1: High load. When many fruits ripen simultaneously on a plant, they compete for assimilates (photosynthesis products). As a result, all fruits remain small. This is a normal physiological mechanism (Yasou & Wien, 2020).
• Cause 2: Potassium (K) and phosphorus (P) deficiency. Potassium and phosphorus are key elements for fruit growth and development. Their deficiency directly limits fruit size and weight.
• Cause 3: Lack of light. Under low light (cloudy weather, crowded planting), the plant cannot produce enough carbohydrates to "fill" the fruits.
• Cause 4: Root system stress (waterlogging, drought, nematode damage) — limits water and nutrient uptake.

What to do?

• Manage the load: promptly remove small, weak ovaries, leaving 1-2 of the largest fruits per "cluster." This allows plants to produce a larger, more marketable yield.
• Ensure balanced nutrition with an emphasis on potassium (K) and phosphorus (P) during the fruiting period.
• Avoid overcrowding, prune the plant by removing excess shoots and old leaves.

Summary Table: Pepper Fruit Deformities

Remember: fruit quality is established in the first days after pollination. By ensuring optimal conditions for pollination and early ovary growth, you will get large, uniform, and tasty fruits. And timely removal of stress factors during fruit growth will help avoid cracks and deformities.

Step 1. Spot Location on Leaves

Where the spot appears on the leaf is often a key to diagnosis.

• Spots on lower, old leaves. This is the classic zone for many fungal diseases that start on the lower tier and move upward (e.g., Alternaria, Septoria). Spots related to magnesium or potassium deficiency can also appear here (see Chapter 2).
• Spots on upper, young leaves. This often points to bacterial infection (bacterial spot) or viral infection. Sunburn can also affect these leaves.
• Spots on fruits. This is often a separate issue. Here it's important to distinguish between blossom-end rot (physiology), anthracnose (fungus), and bacterial spot.

Step 2. Nature of the Spot: Water-Soaked, Dry, Greasy, or with Residue

This is the most important criterion for differentiation.

Dry, brown or tan spots with concentric rings (like a "target") — a classic sign of fungal diseases, such as Alternaria (early blight) or Septoria leaf spot. Fungi destroy leaf tissues, leaving dry necroses. Often, black dots (pycnidia or conidia) — fungal spore-bearing organs — can be seen on the spots (Welbaum, 2015).

Water-soaked, greasy spots that quickly darken, then become dry and crack — these are bacterial spots. Bacteria live inside tissues and cause their death. Spots are often irregularly shaped and surrounded by a yellow halo. In high humidity, bacterial exudate (droplets of cloudy liquid) appears on the spots. Bacterial spot of pepper (Xanthomonas euvesicatoria), which affects both leaves and fruits, is particularly dangerous (Kemble et al., 2022).

Greasy, dark brown or almost black spots on leaves and stems, often with a yellow border — this is Phytophthora blight (caused by Phytophthora capsici). The fungus affects all parts of the plant, causing rapid wilting and death. Dark "constrictions" appear on the stems (Kemble et al., 2022).

White, gray, or black powdery residue on the leaf surface. This is powdery mildew (white powdery residue) or gray mold (fluffy gray residue). Fungal mycelium develops on the leaf surface without deeply penetrating tissues. Powdery mildew often affects mature leaves, while gray mold affects weakened plants, flowers, and fruits (Hochmuth & Sideman, 2023).

Dry, discolored (white, light yellow) spots on the upper side of sun-exposed leaves. This is sunburn. It occurs when water droplets on the leaves (especially after rain) act like magnifying glasses, or when the plant is under stress. On fruits, sunburn appears as white or pale yellow depressions that later become dry and shriveled.

Small, dark (brown or black) dots or spots, often with sharp boundaries. These could be fertilizer or pesticide burns (chemical burns), especially if you applied treatments during hot hours. They could also be feeding marks from pests (e.g., thrips) or an early stage of viral infection.

Step 3. Residue on Fruits

Pepper fruits are affected less often than leaves, but the consequences are more severe—marketability is lost.

Soft, water-soaked spots on fruits that enlarge quickly and become covered with gray fluffy residue — this is gray mold (Botrytis cinerea). Often starts at a wound (scratch, insect bite) or from the flower remaining on the fruit (Kemble et al., 2022).

Dark, sunken spots with pink or orange spore masses in the center — this is anthracnose (fungal disease). Affects ripening fruits, especially in high humidity (Welbaum, 2015).

Dark, dry, sunken spots at the blossom end (the tip opposite the stem), which may enlarge over time. This is blossom-end rot, which is a physiological disorder, not an infection. It is caused by calcium (Ca) deficiency in the fruit tissues, resulting from irregular watering or nutritional imbalance (excess nitrogen, potassium, or magnesium). This is one of the most common pepper problems in hot weather (Yasou & Wien, 2020).

Spots that become dry, cracked, and scab-like over time — this is bacterial spot on fruits. Spots have raised edges, and the center becomes dry and cracks.

Step 4. Progression Dynamics

It's important to observe how the situation evolves.

• If spots appear and do not spread — this is most likely a localized physiological injury (burn, bruise, chemical burn) or a fading infection.
• If spots appear on new leaves, enlarge, and spread to stems and fruits — this is definitely a progressive infectious disease requiring urgent action.
• If spots appear in a limited area of the bed or on individual plants but do not spread further — this may indicate localized contamination or soil issues (e.g., a nematode hotspot or localized nutrient deficiency).

Step 5. What to Do When Spots, Residue, or Rot Are Found?

Once you've identified the nature of the problem, follow this scenario:

1. If the problem is physiological (blossom-end rot, sunburn, chemical burn):

• Eliminate the cause: establish regular watering, adjust fertilization, reduce fertilizer doses, use shading.
• Affected leaves and fruits will not recover, but new organs will be healthy.

2. If the problem is fungal or bacterial:

• Remove and destroy severely affected leaves and fruits.
• Treat with fungicides or bactericides according to recommendations for the specific disease. For prevention, use copper-based products (Bordeaux mixture, copper oxychloride) and biological fungicides (Trichoderma, Bacillus).
• In greenhouses, ensure good ventilation to reduce humidity.

3. If the problem is viral:

• Remove and burn the entire affected plant. Viruses are not curable. Control focuses on vectors (aphids, thrips, whiteflies) and removing the infection source.

Summary Table: Spots, Residue, and Rot

This table is your quick reference. As soon as you see a spot, assess its appearance and location, and you'll understand if you're dealing with a disease, a physiological issue, or a pest. In the next chapter, we'll learn to definitively distinguish between these three categories.

Three Key Questions for Differential Diagnosis

To distinguish one category from another, ask yourself just three questions. Their answers will give you an accurate diagnosis.

Summary Table: How to Distinguish Diseases, Pests, and Physiology

The Golden Rule of Diagnosis

Remember the main rule that unites our entire system:

If the problem spreads — it's an infection or a pest. If the problem does not spread, but just "stays put" or appears uniformly everywhere — it's physiology.

And one more important addition:

Physiological disorders weaken the plant and make it easy prey for diseases and pests. Nutrient deficiency or improper watering is an "open door" for infections. So always rule out physiological causes first, then move on to combating diseases and pests.

You now have a complete arsenal of diagnostic tools: from leaf inspection to identifying the nature of the problem. Use this algorithm, and you will be able to quickly and accurately determine what is happening with your pepper and make the right decisions to save your harvest.