Geraniums (Pelargonium spp.) are affected by a wide array of bacterial, fungal, and viral diseases that can devastate crops. This comprehensive guide covers identification, biology, and integrated management of the most important geranium pathogens.
Xanthomonas campestris pv. pelargonii causes two symptom types: (1) small sunken spots on leaf undersides with V-shaped lesions; (2) marginal wilting of lower leaves with gray-brown vascular discoloration in stems. Highly contagious and destructive.
Management: Purchase culture-indexed cuttings or grow from seed. Immediately discard infected plants after confirmed diagnosis. Avoid overhead irrigation. Discard all geraniums at season end. Use hand-breaking for cuttings (no knives) or disinfest knives frequently. Wash hands frequently.
Rhodococcus fascians causes short, thick, fleshy aborted stems at base, pale green/yellow, with healthy-appearing upper plant.
Management: Purchase culture-indexed plants. Discard infected stock, cuttings, and media.
Ralstonia solanacearum causes lower leaf wilting/yellowing/fall with brown/black stem vascular tissue.
Management: Purchase culture-indexed plants. Do not bring ground-planted geraniums into production. Destroy infected plants.
Alternaria alternata causes water-soaked underside spots (1/4-1/2 inch) with concentric dark rings.
Management: Avoid overhead watering. Remove/destroy crop debris. Apply protective fungicides.
Pythium causes brown water-soaked basal rot on cuttings, progressing coal-black 3-4 inches up stem.
Management: Use sterile propagation media. Discard infected cuttings. Keep hose ends off ground.
Botrytis cinerea causes flower browning/premature drop, leaf spots from fallen petals, brown rot on cutting stubs and bases.
Management: Avoid plant damage. Remove fading flowers/leaves. Heat, ventilate, space for low humidity/good air flow. Apply fungicides (rotate chemistries to prevent resistance).
Fusarium oxysporum causes yellowing/wilting up one side with vascular discoloration.
Management: Purchase culture-indexed cuttings. Use sterile potting mix. Destroy infected plants.
Pythium causes translucent water-soaked root tips with outer tissue stripping off.
Management: Pot in sterile media. Keep hose ends off ground. Discard blackleg-infected cuttings even if rooted. Apply fungicides.
Rhizoctonia solani causes soil-line lesions with brown borders, dry shredded stems.
Management: Plant in pasteurized raised beds. Apply soil drench fungicides.
Puccinia pelargonii-zonalis causes chlorotic upper leaf specks opposite rust-colored pustules underneath in concentric "target" rings.
Management: Purchase rust-free cuttings. Avoid overhead irrigation. Discard unwanted plants season-end. Apply fungicides.
Verticillium albo-atrum or V. dahliae causes middle/upper leaf collapse/drying/fall with brown vascular tissue. Confusable with bacterial blight.
Management: Purchase culture-indexed cuttings. Use sterile mix. Destroy infected plants.
Cool, cloudy weather causes water-soaked pimples/blisters on lower leaf undersides becoming corky brown; severe cases cause leaf drop.
Management: Space for air circulation. Use well-drained mix. Avoid overwatering in cool, cloudy weather.
Multiple viruses cause mottling, ringspotting, leaf cupping, yellowing veins, malformation, spotting, and zonation loss. Singly or in combinations.
Management: Purchase virus-indexed cuttings. Maintain strict insect/mite control.
Pseudomonas cichorii causes elliptical water-soaked spots (1/4-1/2 inch) becoming dark brown/black irregular with possible yellow halos.
Management: Purchase disease-free cuttings. No overhead irrigation. Keep leaves dry.
The cornerstone of geranium disease management is clean stock: culture-indexed cuttings for bacterial/viral pathogens, virus-indexed for viruses. Combine with: sterile media; strict sanitation (tool/hand hygiene, debris removal); environmental management (low humidity, good air flow, no overhead irrigation); regular scouting; targeted preventive fungicide/bactericide applications with chemistry rotation; and immediate removal of symptomatic plants. This systems approach is essential for profitable geranium production.
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