Common Signs of Pests or Ill Health & Likely Causes
Below is a symptom-based reference table that highlights common signs of pests or ill health in bonsai. Each row describes an observable issue (e.g., leaf discoloration, webbing, sticky residue), and then breaks down typical causes and responses for each major bonsai category. Of course, many symptoms overlap (e.g., yellowing leaves can mean overwatering, underwatering, or pests), so use this table as a starting guide—and always investigate further (check roots, look for insects, consider recent weather/fertilizer changes, etc.).
How to Use This Table§
- Observe the exact symptom: Is there sticky residue? Webbing? Localized brown spots or overall leaf yellowing?
- Check the relevant column for your tree type (pines, junipers, deciduous, tropical).
- Identify possible causes (e.g. spider mites, scale, root rot, normal seasonal drop).
- Take appropriate action: e.g., adjust watering, prune infected branches, apply insecticidal soap, fungicide, or systemic insecticide.
- Monitor progress: If symptoms persist, reevaluate root health, soil drainage, light conditions, and consider a professional diagnosis if needed.
Tip: When diagnosing, also consider recent changes (overfertilizing, temperature swings, new potting mix, location change). Many health issues result from environmental stress, which then allows pests/fungus to gain a foothold. Treat both the pest/fungus and the underlying stress for best results.
| Symptom / Sign | Pines (2- & 5-needle) | Junipers (Hardy Evergreens) | Deciduous (Maples, Elms, Oaks, etc.) | Tropicals (Ficus, Jade, Schefflera, etc.) |
|---|---|---|---|---|
| 1. Fine webbing on foliage / speckled or stippled leaves or needles | - Often spider mites, especially in hot, dry conditions.- Needles might yellow from the inside or show tiny pale spots; severe infestations turn the needles grayish or dull.- Pinch or shake foliage over white paper to see tiny moving specks.Action: Increase humidity (mist, overhead water), use a miticide or insecticidal soap. Check dryness—pines in heat can get stressed. | - Very common sign of spider mites. Junipers with a slight gray/bronze cast can also be in early mite trouble.- Mites hide in scale foliage; look closely at branch tips. Action: Rinse foliage thoroughly, apply neem or dedicated miticide. Provide strong light & airflow—dense juniper pads encourage mites if poorly ventilated. | - Leaves with tiny dots/spots, eventually yellowish or bronze coloration, plus possible webs on undersides = spider mites.- Mites favor high temps & low humidity.Action: Spray undersides of leaves with water regularly, use insecticidal soap or miticide. Improve air circulation around canopy. | - Classic symptom of spider mites indoors or in warm, dry spots. - Ficus, schefflera, jade commonly get stippled leaves and fine webs on petioles or undersides. Action: Shower foliage, raise humidity, treat with a mite‐targeted insecticidal soap or neem. |
| 2. Sticky residue on leaves / pots (honeydew), sooty black mold | - Could be aphids on new pine candles (less common than on deciduous, but still possible). - If sticky sap is on needles without visible insects, check for scale on branches. Action: Look closely for small green/black aphids or scale bumps, treat with systemic insecticide or insecticidal soap. Fungus (sooty mold) grows on that honeydew. | - Sticky tips or black sooty mold often indicates scale insects or aphids (aphids are rarer on juniper, scale more likely). Action: Inspect bark and foliage carefully. Use horticultural oil or systemic insecticide. Remove any heavily infested twigs. | - Very common with aphids, whiteflies, or scale. Sooty mold can blanket leaves if honeydew is abundant. Action: Spray insecticidal soap or neem for aphids/whiteflies. For scale, consider a systemic. Wash off mold gently, improve airflow. | - Mealybugs or soft scale often produce sticky honeydew on ficus, jade, etc. This leads to black sooty mold. Action: Identify the pest—mealybugs (white cottony lumps) or scale (small bumps). Wipe them off with cotton swab + alcohol, or use a systemic. Rinse off mold once infestation is controlled. |
| 3. Yellowing/browning needles or leaves from the inside out | - Pines naturally shed older needles each year; look at timing—if it's autumn or older needles only, it may be normal. - If new needles are browning at the tips, check for underwatering, salt buildup, or fungal needle cast. Action: Evaluate watering habit, flush soil, check for fungus or pests. A few older needles yellowing is normal. | - Inner foliage on juniper dying off can be normal "thinning," but if excessive browning or random patches, suspect poor light, overwatering, or phomopsis/tip blight. Action: Provide enough sun & airflow, prune out dead interior shoots, reduce water if soil stays soggy, apply fungicide if blight is confirmed. | - If older, lower leaves on a deciduous tree turn yellow in mid/late season, may be normal leaf aging. - Sudden widespread yellowing can mean root stress (over/underwater), or nutrient deficiency (esp. N) if it's older leaves first. Action: Check root health, consistent watering. If no pests found, try balanced feeding. | - Slight leaf drop is normal for some tropicals under stress or seasonal change. Rapid yellowing suggests overwatering, or low light. - If older leaves only, might be normal. If new leaves yellow, suspect nutrient deficiency or pests. Action: Inspect underside for scale/mites, check soil moisture/lighting, adjust fertilizer. |
| 4. Sudden wilting or drooping foliage | - Pines do not "wilt" dramatically, but new candles can droop if roots are severely dried or rotted. - Could also be a sign of root rot from waterlogged soil, especially in smaller pots. Action: Check soil moisture extremes; correct watering. Possibly slip-pot into drier mix if waterlogged. | - Juniper foliage can turn limp or dull green if severely underwatered or if the root system is damaged. - In advanced root rot, branches may appear brittle, foliage can brown suddenly. Action: Check roots for rot, prune dead roots, repot in well-draining mix. Adjust watering. | - Deciduous leaves clearly wilt if lacking water. Could also be fungal wilt (e.g., verticillium) or severe root damage. Action: Water thoroughly if the soil is dry. If soil is saturated, suspect root rot—improve drainage, possibly repot. If a single branch wilts, check for borer or canker. | - Tropical leaves wilt or curl under dryness, temperature shock, or root issues. - Many tropicals (ficus, etc.) may drop leaves quickly if severely underwatered. Action: Check root/soil moisture, correct watering or humidity. If not dryness, look for root rot (slimy, smelly roots) or pest infestation. |
| 5. White fuzzy growth / lumps on bark or stems | - Could be woolly aphids (rare on pines) or possibly fungal conks if the wood is rotting. - Check if it's soft/cottony (likely mealy/woolly pest) or firm spongy lumps (fungus). Action: For pests, use insecticidal soap; for fungal issues, remove rotted tissue carefully, apply fungicide if needed. | - Junipers can get mealybugs or scale that look cottony. Also, fungal "galls" (cedar-apple rust galls) appear as brown lumps that ooze orange spore horns in spring. Action: Remove or prune out galls, treat with fungicide if rust is widespread. If mealy, wipe off + insecticide. | - White fuzzy lumps are often mealybugs on trunk forks or underside of branches. Some fungus (powdery mildew) also appears white but more powdery across leaves. Action: Identify if insect or fungus. For mealybugs, dab with alcohol, use systemic insecticide. For mildew, use a fungicide. | - Mealybugs are common: white cottony masses especially on leaf axils or stems. - If it's a fungus, it might be fluffy mold on bark in humid conditions, indicating rotting wood. Action: For mealybugs, treat with insecticidal soap or systemic. For rotting bark, prune dead areas, improve air circulation, keep trunk base drier. |
| 6. Black/brown spots or lesions on foliage | - For pines, black or brown spots on needles can indicate needle cast fungus or scorch from fertilizer burn. - Often you see lower needles affected first. Action: Check humidity/watering. If fungal, remove affected needles, apply fungicide. If fertilizer burn, flush soil, adjust dosage. | - Junipers can have small black lesions if phomopsis or kabatina tip blight sets in, leading to tip browning. - Also watch for orange-rust galls. Action: Trim out infected tips, apply a conifer-safe fungicide. Ensure good airflow, do not overhead-water in evenings if possible. | - Leaf spot on deciduous species (maple anthracnose, black spot on roses, etc.) is common in damp conditions. Spots can coalesce, causing leaf yellowing/drop. Action: Remove infected leaves, improve ventilation, use fungicide if it persists. Water in mornings so leaves dry by evening. | - Tropical broadleaves can get fungal leaf spot or bacterial spots in high humidity. Spots may be circular, black with yellow halo. Action: Remove heavily spotted leaves, ensure good airflow, possibly fungicide. Check if watering overhead at night—try watering soil only. |
| 7. Branch dieback or sudden browning of one section | - Could be pine borer insects (tiny holes in trunk/branches) or severe root issue on that side. - Also check for pitch oozing or resin lumps. Action: Inspect bark carefully, remove severely affected branches, consider systemic insecticide if borers. Check root health. | - Juniper branch dieback can happen from mechanical damage, root rot, or canker fungi. - If bark near that branch is cracked or sunken, suspect a canker. Action: Prune off affected limb, disinfect tools. If wide-spread, a fungicidal trunk application may help. Check watering and roots. | - One branch or section wilts and dies while the rest is healthy → possibly canker, borer, or verticillium wilt if it moves up the trunk. Action: Prune well below the dead area, look for discoloration in wood. Sterilize tools. If trunk has holes (borers), treat systemically. If widespread wilt, consider soil test for verticillium. | - On ficus or other tropicals, a single branch browning might be mechanical damage or local infection. - If the trunk bark is peeling or turning black, suspect rot or canker. Action: Prune out dead area, treat any fungal/bacterial problem, ensure environment is warm & not overly wet. If insects (borers) are found, use insecticide. |
| 8. Leaf/needle tips turning crisp brown | - Tip burn can result from too much fertilizer (salt buildup) or underwatering in high heat. - Also check root stress or dryness. Action: Flush soil with pure water, adjust fertilizer concentration, ensure you're watering thoroughly. | - Junipers don't typically get "crispy tips" unless severely dehydrated or salt-burned from over-fertilizing. - Also watch for tip blight fungus, but that usually starts at shoot base. Action: Ensure adequate but not excessive fertilizer, consistent moisture, trim out blighted tips. | - Browning at leaf tips often means leaf scorch from heat, wind, or salt. Edges might crisp if pot dried out midday. Action: Provide partial shade if scorching, check watering frequency. Flush soil if salt or fertilizer is high. Could also be mild potassium deficiency—try balanced feed. | - Tropical leaves sometimes brown at the tips if humidity is too low or from salt accumulation in soil (indoor watering with hard water). Action: Increase humidity, flush the pot monthly to remove salts, moderate fertilizer. If the environment is very dry, consider daily misting or a humidity tray. |