Fungal disease is a familiar problem: there is a coating, a symptom, a product. Bacterial disease looks different: a water-soaked spot on a leaf, a slimy stem necrosis, a plant wilting for no visible root-related reason. These symptoms are easy to dismiss as mechanical damage, excess moisture, or a fungal issue — and treat with entirely the wrong thing. Bacteria in the greenhouse are not rare; they are under-diagnosed.
Quick glossary: Bacterial disease — the general term for plant diseases caused by phytopathogenic bacteria; unlike fungal infections, they produce no spore-bearing growth and are often accompanied by water-soaked or slimy lesions. Necrosis — tissue death; in bacterial necrosis, the affected area is typically wet or glassy at first and dries with a dark border later. Bacterial wilt — a form of bacterial disease where the pathogen colonises the plant's vascular system and blocks water transport; symptoms resemble vascular Fusarium wilt, but the pathogen is a bacterium and control methods differ.
Why Bacteria in the Greenhouse Are Commonplace
Phytopathogenic bacteria are present in any living environment: in water, on seeds, in substrate, on tools. A healthy plant with an intact cuticle and normal immunity keeps them outside. The problem begins when there is an entry point — mechanical damage during pruning, lamp burns, cracks from temperature and humidity swings, or when the plant is under chronic stress and its natural defences are lowered.
In an enclosed space, bacteria spread through water on leaves, splash during irrigation, untreated tools, and hands. One infected cutting or seed — and within a few weeks an outbreak spans the entire zone.
Three Main Forms and How to Recognise Them
Soft rots (Pectobacterium, Dickeya). Attack succulent tissues — stems, fruits, leaves with high water content. Characteristic sign: tissue becomes water-soaked and wet, then slimy, with an unpleasant smell. No spore coating, no clean dry boundary — the tissue simply breaks down. Spreads through water and contact. Favourable conditions: warmth (25–30°C) and high humidity.
Bacterial necroses (Pseudomonas, Xanthomonas). Begin as small water-soaked or greasy spots on leaves, often surrounded by a yellow chlorotic halo. Over time the spot dries and darkens; tissue dies with a brown or black margin. Unlike fungal spots — no mould coating, often with a "wet" centre in the early stage. Spread through water splash and damaged tissue.
Bacterial wilt (Ralstonia, Clavibacter). The plant wilts with no visible cause despite moist substrate and normal aeration — symptoms resemble vascular Fusarium wilt. Diagnostic sign: cut the stem and place it in clean water; within a few minutes a cloudy whitish exudate streams from the vascular tissue — bacterial slime. With Fusarium there is no exudate, only vascular discolouration.
Conditions That Turn Background Infection into an Outbreak
Bacteria are always present — an outbreak occurs when three conditions coincide: entry points (tissue damage), favourable temperature and humidity, and weakened plant defences.
The most critical moment is pruning and training. An untreated tool between plants carries bacteria directly into a fresh cut. Morning condensation droplets at sharp temperature drops sit on cut surfaces and provide the ideal environment for colonisation.
High relative humidity at reduced night temperatures is the standard condition for bacterial necroses and soft rots. Managing the microclimate — especially night-time humidity — is part of prevention, not just agronomic convenience.
Treatment and Control: What Actually Works
Copper-based products (copper sulphate, copper hydroxide) act as a protective barrier and have a bacteriostatic effect — useful for prevention alongside sanitisers. Streptomycin — an antibiotic used in crop production, effective against a broad range of bacteria, but with strict approval limitations and resistance risk.
Affected plants or organs — remove and destroy outside the growing area. Bacterial disease does not "clear up" with condition corrections the way fungi do — infected tissue is a reservoir and a source of re-infection.
For product selection and compatibility, see the article on fungicides and bactericides.
Tracking and outbreak mapping is part of the HACCP approach in the greenhouse: where it appeared, when, what was done. Without recorded data, it is impossible to trace the source and break the cycle.
Three Mistakes That Cost the Most
Treating bacterial disease with fungicides. The most common mistake — a wet spot appears, apply a fungicide, no effect, keep going. Fungicides have no action on bacteria. A week of lost time during an active outbreak means spread across an entire bench.
Not disinfecting tools between plants. Pruning without disinfecting the scissors inoculates every subsequent plant with material from the previous one. A hypochlorite or alcohol solution between plants — ten seconds that can save the cycle.
Removing symptoms without recording outbreaks. Clipped the affected leaf, discarded it, forgot about it. If a new outbreak appears in the same row a week later — there is no data to understand where it came from. An IPM strategy without records is reacting to problems, not managing them.
How to Tell Bacterial Disease from Fungal at a Glance
There is a coating (white, grey, olive) — most likely a fungus. No coating, wet or slimy tissue with an odour — bacteria. Necrosis with a clean dry margin and concentric rings — fungus. Necrosis with a "wet" or greasy margin in the early stage — bacteria. Plant wilting on healthy substrate with exudate from the vascular tissue — bacterial wilt.
For deeper understanding: IPM Strategy: How to Build a Defence Before Problems Appear — explains how bacterial diseases fit into the overall monitoring and prevention system, and what to check regularly to catch an outbreak at the start.