Common Plant Diseases

Asian soybean rust (Phakopsora pachyrhizi) causes tan to dark brown pustules on leaf undersides, leading to rapid defoliation and 10-80% yield loss. No durable resistance exists. Manage with preventive fungicide applications (triazole + strobilurin) at R1-R2 stage, early planting, monitoring sentinel plots, and observing the vazio sanitário (soybean-free period) in Brazil.

Banana bunchy top virus (BBTV) is the most serious banana virus globally. It causes narrow, upright, bunched leaves with dark green streaks along veins, and stunted plants that rarely fruit. There is no cure. Control requires removing infected plants, using virus-free planting material, and managing banana aphid vectors with imidacloprid.

Bean angular leaf spot (Pseudocercospora griseola) causes angular brown lesions bounded by leaf veins, reducing yield 50-80% in common bean across Africa and Latin America. The fungus is seed-borne. Manage with resistant varieties, certified disease-free seed, crop rotation (2+ years), and fungicide sprays (mancozeb + carbendazim) starting at first flower.

Bean common mosaic virus (BCMV) causes green mosaic mottling, leaf curling, and stunting in common bean. Seed transmission (5-35%) perpetuates the virus. Manage with resistant varieties carrying bc genes, certified virus-free seed, aphid management, and roguing infected plants. Avoid varieties with only the I gene in areas with Bean common mosaic necrosis virus (BCMNV).

Black Sigatoka (Mycosphaerella fijiensis) is the most damaging banana leaf disease. It causes dark brown streaks expanding into leaf-killing lesions, reducing yield by 50%+. Manage with resistant varieties (FHIA hybrids), deleafing, proper drainage, spacing for air flow, and fungicide rotations of mancozeb and systemic triazoles.

Cabbage black rot (Xanthomonas campestris pv. campestris) causes V-shaped yellow lesions from leaf margins with blackened veins. Seed-borne and spread by rain splash. Manage with certified seed, hot water seed treatment (50°C for 25 min), resistant varieties, crop rotation (2+ years away from brassicas), and copper sprays during wet weather.

Cassava brown streak disease (CBSD) is a viral disease causing dry, brown necrotic rot inside cassava tubers, making them inedible. Above-ground symptoms include yellow blotches on older leaves and brown streaks on stems. Use resistant varieties (e.g., NASE 19, Kiroba), plant clean cuttings, rogue infected plants, and harvest within 12 months before tuber necrosis worsens.

Cassava mosaic disease (CMD) is caused by begomoviruses spread by whiteflies. Symptoms include yellow-green mosaic patterns on leaves, leaf distortion, and reduced tuber yields (20-95% loss). Use CMD-resistant varieties (e.g., NASE 14, TME 419), plant clean stem cuttings, rogue infected plants, and control whiteflies to manage the disease.

Citrus canker (Xanthomonas citri) causes raised, rough, crater-like lesions with yellow halos on citrus leaves, fruit, and twigs. Spread by wind-driven rain and contaminated tools. Manage with copper sprays during flush growth, windbreaks, tool sanitation, and resistant rootstocks. Canker-affected fruit is safe to eat but unmarketable.

Citrus greening (HLB) is caused by Candidatus Liberibacter asiaticus, spread by the Asian citrus psyllid. Symptoms include blotchy yellow mottling (asymmetric), lopsided small bitter fruit, and tree decline over 3-5 years. There is no cure. Manage with psyllid control (imidacloprid, kaolin clay), removing infected trees, and using certified disease-free nursery stock.

Cocoa black pod (Phytophthora megakarya/palmivora) causes dark brown-black rot of cocoa pods, destroying 20-30% of global production annually. It thrives in heavy rain and humidity. Manage with frequent harvesting (every 2 weeks), pod sanitation, copper fungicide sprays, proper shade management, and planting tolerant varieties.

Cocoa swollen shoot virus (CSSV) causes stem and root swelling, red vein banding on young leaves, and progressive tree decline over 3-5 years. Transmitted by mealybugs in West Africa. Management requires cutting and burning infected trees, replanting with tolerant varieties, and controlling mealybug vectors. There is no cure.

Coconut lethal yellowing is caused by a phytoplasma spread by planthoppers. It causes premature nut drop, flower blackening, progressive yellowing from lower to upper fronds, and death within 3-6 months. There is no cure. Plant resistant varieties (Malayan Dwarf, Maypan hybrid), remove infected palms, and apply oxytetracycline trunk injections to slow (not cure) disease in valuable individual palms.

Coffee leaf rust (Hemileia vastatrix) causes orange-yellow powdery spots on leaf undersides, leading to defoliation and yield loss. Manage with resistant varieties (Catimor, Sarchimor), copper-based sprays during early rainy season, adequate shade management, balanced nutrition, and proper spacing for air circulation.

Cotton bacterial blight (Xanthomonas citri pv. malvacearum) causes angular water-soaked leaf spots, black arm on stems, and boll rot. Yield losses reach 10-35%. The bacterium is seed-borne. Manage with acid-delinted seed, resistant varieties, crop rotation, and copper sprays. Avoid working in wet fields. Remove and burn severely infected plants.

Cotton leaf curl disease (CLCuV complex) causes upward leaf curling, vein thickening, and enations (leaf-like outgrowths) on cotton. Transmitted by whitefly Bemisia tabaci. Yield losses reach 30-80%. Manage with resistant varieties, whitefly control (neonicotinoid seed treatment + foliar sprays), early planting, and avoiding ratoon cotton that bridges virus between seasons.

Cowpea mosaic virus (CPMV) causes severe green/yellow mosaic, leaf distortion, and blistering in cowpea, reducing yield 50-80%. Transmitted by leaf beetles (Chrysomelidae). Manage with resistant varieties, seed treatment with insecticide to protect seedlings, controlling beetle vectors, early planting, and using virus-free seed from certified sources.

Downy mildew produces angular yellow patches on upper leaf surfaces with grayish-purple fuzzy growth underneath. Caused by Peronospora or Plasmopara oomycetes, it thrives in cool wet weather. Manage with resistant varieties, preventive mancozeb or copper sprays, good air circulation, and avoiding overhead irrigation. It requires free water to infect — keep leaves dry.

Finger millet blast (Magnaporthe oryzae) causes neck rot and panicle blast, preventing grain fill and causing 50-90% yield loss. The most devastating disease of ragi/eleusine. Manage with resistant varieties, seed treatment with tricyclazole, proper spacing, balanced nitrogen, and fungicide application at heading stage.

Groundnut rosette disease causes severe stunting, leaf curling, and chlorosis in peanut crops across Africa. Caused by a unique 3-component virus complex transmitted by Aphis craccivora aphids. Manage with resistant varieties (ICG 12991), early planting at high density, aphid control, and roguing infected plants. Losses reach 100% in severe outbreaks.

Maize gray leaf spot (Cercospora zeae-maydis) causes rectangular gray-tan lesions bounded by leaf veins on maize. It thrives in humid conditions with heavy dew. Manage with resistant hybrids, crop rotation, tillage to bury residue, and fungicide applications of azoxystrobin or propiconazole at tasseling if lesions exceed 5% of leaf area.

Maize lethal necrosis (MLN) is caused by co-infection of Maize chlorotic mottle virus and Sugarcane mosaic virus. Symptoms include chlorotic mottling, leaf necrosis from the edges inward, premature plant death, and empty ears. There is no cure. Use certified MLN-free seed, control thrips and aphid vectors, practice crop rotation, and plant tolerant varieties.

Maize streak virus (MSV) causes narrow, broken yellow streaks along leaf veins in maize. Transmitted by Cicadulina leafhoppers across Africa. Early infection causes severe stunting and barren ears. Use MSV-resistant varieties, control leafhoppers, avoid late planting, and rogue infected plants early to reduce virus source.

Mango anthracnose (Colletotrichum gloeosporioides) causes flower blight, leaf spots, and dark sunken fruit rot. The fungus infects flowers, then lies dormant until fruit ripens. Manage with copper sprays during flowering, post-harvest hot water treatment (52°C for 5 min), resistant varieties, and proper orchard sanitation. Dry weather during flowering reduces losses significantly.

Mango malformation (Fusarium mangiferae) causes thickened, distorted flower panicles and bunchy vegetative shoots, preventing fruit production. The disease is spread by eriophyid mites. Manage by pruning malformed panicles 15-20 cm below the affected area, applying naphthalene acetic acid spray, controlling mites with sulfur, and maintaining tree vigor with balanced nutrition.

Onion purple blotch (Alternaria porri) causes purplish-brown zonate lesions on leaves that girdle and kill foliage, reducing bulb yield 40-60%. Thrips damage worsens the disease. Manage with mancozeb + chlorothalonil sprays every 7-10 days, thrips control, proper spacing for air flow, and avoiding overhead irrigation during bulb development.

Panama disease TR4 (Fusarium oxysporum f. sp. cubense) is an incurable soil-borne fungal wilt of banana. It blocks water transport, causing yellowing, wilting, and pseudostem splitting. The fungus persists in soil for 30+ years. There is no effective chemical treatment. Use resistant varieties, strict biosecurity, clean planting material, and never move soil from infested areas.

Papaya ringspot virus (PRSV) causes mosaic, shoe-string leaves, and concentric ring patterns on fruit. Spread by aphids non-persistently, there is no cure. Manage with tolerant varieties, roguing infected plants, barrier crops, reflective mulch, and cross-protection with mild strains. The transgenic Rainbow papaya saved Hawaii's industry.

Pepper bacterial spot (Xanthomonas euvesicatoria) causes small dark water-soaked leaf spots that turn brown with yellow halos, and raised scab-like fruit lesions. Spread by rain splash and seed. Manage with clean seed, copper + mancozeb sprays, resistant varieties, drip irrigation, and crop rotation. Copper resistance is increasing — rotate management tools.

Potato late blight (Phytophthora infestans) causes dark water-soaked lesions on leaves and stems, with white sporulation underneath. Tubers develop firm brown rot. Apply preventive mancozeb or chlorothalonil sprays from row closure, destroy infected foliage before harvest, use resistant varieties, and store only disease-free tubers.

Powdery mildew appears as white powdery spots on leaves and stems, caused by Erysiphe or Podosphaera fungi. It thrives in warm, dry weather with high humidity. Manage with sulfur sprays, potassium bicarbonate, neem oil, proper spacing for airflow, and resistant varieties. Unlike most fungal diseases, it does NOT need wet leaves to infect.

Rice bacterial leaf blight (Xanthomonas oryzae pv. oryzae) causes water-soaked yellow-green lesions advancing from leaf tips that dry to grayish-white. Yield losses reach 20-50%. Manage with resistant varieties (Xa4, Xa21 genes), balanced nitrogen, avoid flooding damage to leaves, and copper sprays. The kresek phase can kill young seedlings entirely.

Rice blast is caused by Magnaporthe oryzae, the most destructive rice disease globally. It produces diamond-shaped gray lesions on leaves and can rot the panicle neck, causing total grain loss. Manage with resistant varieties, balanced nitrogen fertilization, silicon amendments, and fungicide applications of tricyclazole or azoxystrobin at panicle initiation.

Rice sheath blight (Rhizoctonia solani) causes oval gray-green water-soaked lesions on leaf sheaths that enlarge with gray centers and brown borders. Sclerotia float in paddy water and infect at the waterline. Manage with moderate nitrogen, avoiding excess plant density, validamycin or azoxystrobin fungicide at panicle initiation, and incorporating crop residues after harvest.

Sorghum anthracnose (Colletotrichum sublineolum) causes elliptical leaf spots with acervuli, panicle rot, and grain mold. Yield losses reach 30-67% in humid tropics. Manage with resistant varieties, crop rotation with non-host crops, seed treatment with thiram, and foliar application of azoxystrobin at flag leaf if disease pressure is high.

Soybean mosaic virus (SMV) causes light and dark green mosaic mottling, leaf crinkling, stunting, and seed coat mottling. It is seed-transmitted (up to 5%) and spread by aphids. Manage by planting certified virus-free seed, using resistant varieties (Rsv genes), controlling aphids, and roguing infected plants early in the season.

Sugarcane red rot (Colletotrichum falcatum) causes internal stalk rotting with characteristic red pith and white spots. Enters through borer wounds and sett ends. Yield losses reach 50-75%. Manage with resistant varieties, hot water sett treatment (52°C for 30 min), borer control to prevent entry wounds, and strict nursery hygiene. Split stalks to inspect before planting.

Sugarcane smut (Sporisorium scitamineum) produces a diagnostic black whip-like structure from infected shoot tips. The fungus converts the growing point into a mass of black spores. Yield losses reach 30-75%. Manage with resistant varieties, hot water treatment of setts (52°C for 30 min), roguing infected stools, and avoiding planting material from smutted fields.

Sweet potato virus disease (SPVD) is caused by co-infection of SPFMV (aphid-borne) and SPCSV (whitefly-borne), causing severe stunting, leaf distortion, and 80-98% yield loss. Use virus-free planting material from certified nurseries, resistant varieties (NASPOT series), rogue infected plants, and control whitefly populations. Vine tip cuttings from healthy-looking plants reduce virus load.

Taro leaf blight (Phytophthora colocasiae) causes water-soaked brown lesions that can destroy an entire taro leaf in 2-3 days. The 1993 Samoa epidemic destroyed 95% of taro. Manage with resistant varieties, metalaxyl or mancozeb sprays, good drainage, spacing for air flow, and removing infected leaves. The disease requires warm, wet conditions.

Tea blister blight (Exobasidium vexans) causes translucent blister-like swellings on young tea leaves that turn brown and papery. It attacks only the young flush that is harvested for tea. Manage with regular plucking (7-day rounds), copper fungicide sprays, good drainage, pruning for air flow, and hexaconazole application during wet weather.

Tomato bacterial wilt (Ralstonia solanacearum) causes sudden, rapid wilting of entire plants without yellowing. Cut stems placed in water ooze milky white bacterial streaming. There is no chemical cure. Use resistant varieties, crop rotation with non-solanaceous crops, grafting onto resistant rootstock, and strict sanitation. The bacterium persists in soil and irrigation water.

Tomato early blight is caused by the fungus Alternaria solani. It produces dark brown spots with concentric rings on lower leaves, spreading upward. Manage with crop rotation, resistant varieties, mulching to prevent soil splash, and fungicide applications of chlorothalonil or copper-based products when symptoms first appear.

Tomato Fusarium wilt (Fusarium oxysporum f. sp. lycopersici) causes yellowing starting on one side of the plant, wilting, and brown vascular discoloration in stems. The fungus persists in soil for decades. Manage with resistant varieties carrying I, I-2, I-3 genes, grafting onto resistant rootstock, crop rotation, and raising soil pH to 6.5-7.0.

Tomato late blight is caused by Phytophthora infestans. It creates water-soaked gray-green lesions that rapidly turn dark brown, with white mold on leaf undersides in humid conditions. It can destroy crops in days. Apply preventive copper or mancozeb sprays, destroy infected plants immediately, and use resistant varieties like Mountain Magic or Defiant.

Tomato yellow leaf curl virus (TYLCV) is a begomovirus spread by whiteflies (Bemisia tabaci). Infected plants show upward leaf curling, yellowing leaf margins, stunted growth, and flower drop. There is no cure — management relies on resistant varieties, whitefly control with neem or imidacloprid, reflective mulches, and insect-proof netting.

Wheat blast (Magnaporthe oryzae pathotype Triticum) causes sudden head bleaching, destroying 40-100% of grain. Emerged in Brazil (1985), reached Bangladesh (2016), and threatens South Asia. Few resistant varieties exist. Manage with 2NS translocation varieties, mancozeb + tricyclazole at heading, crop rotation, and avoiding late sowing during warm wet weather.

Wheat stem rust (Puccinia graminis f. sp. tritici) causes reddish-brown pustules on stems, leaf sheaths, and leaves. The Ug99 race is a major global threat. Manage with resistant varieties carrying Sr31+Sr38 or Sr25 genes, early planting, and preventive fungicide sprays of tebuconazole or propiconazole at first sign of pustules.

Wheat stripe rust (Puccinia striiformis) produces bright yellow-orange pustule stripes along leaf veins. It prefers cool weather (10-18°C) but new warm-adapted races are expanding its range. Apply propiconazole or tebuconazole at flag leaf emergence, plant resistant varieties, and avoid late sowing in cool, wet regions.

Yam anthracnose (Colletotrichum gloeosporioides) causes dark necrotic lesions on Dioscorea alata leaves and vines, causing defoliation and 80-100% yield loss. D. rotundata is resistant. Manage with resistant D. alata varieties, seed tuber treatment, staking for air circulation, mancozeb sprays during rainy season, and crop rotation.