Plant Diseases and Pests – Visual Guide to Identification and Treatment

Ať už pěstujete rajčata na záhonech, bylinky na balkoně nebo okrasné rostliny v interiéru, dříve či později narazíte na nežádoucího návštěvníka – houbu, bakterii, virus nebo hmyzího škůdce. Včasná identifikace je přitom rozhodující: čím dříve problém odhalíte, tím méně invazivní a nákladná bude náprava. Tento průvodce vám pomůže rozpoznat nejčastější choroby a škůdce podle vizuálních příznaků a nabídne ověřené postupy biologické i konvenční léčby – od záhonu po skleník.

Fungal Diseases

Fungi are the most common cause of plant diseases. They spread through spores via air, water, and contaminated substrate, and the vast majority thrive in warm, humid environments. This is why fungal infections are particularly common in greenhouses, polytunnels, and with excessive watering.

Powdery Mildew (Erysiphales)

Symptoms: White to grayish-white powdery coating on the upper leaf surface, gradually covering stems, flowers, and fruits. Affected leaves curl, turn yellow, and drop prematurely. Fruits remain small and deformed.

Life cycle: Powdery mildew spores overwinter on plant debris and spread through air. Optimal conditions for development are temperatures of 18–25 °C with alternating dry and humid periods. Unlike mildews, powdery mildew does not require free water on leaves to germinate – it thrives on air humidity alone, and therefore develops even in relatively dry weather.

Most commonly affects: Cucumbers, courgettes, squashes, roses, grapevines, apple trees, gooseberries, sage, phlox.

Biological treatment and prevention:
- Baking soda spray – 1 tablespoon baking soda + 1 tablespoon vegetable oil + a few drops of liquid soap per 1 litre of water, apply once weekly; soap acts as a spreader and improves spray adherence
- Wettable sulphur – traditional and reliable fungicide available in granulated or powder form; do not apply above 28 °C as phytotoxicity may occur
- Horsetail extract – strengthens the silica component of cell walls and increases mechanical leaf resistance
- Ensure adequate air circulation – overcrowded plants create a microclimate favourable for spore development
- Remove affected plant parts immediately and dispose of outside compost where spores would survive
- In the plant protection products category you will find both biological and conventional fungicides suitable for powdery mildew

Downy Mildew (Peronosporales) – Mildew

Symptoms: Yellowish-green to brown irregular patches on the upper leaf surface, bordered by veins. A grayish-white to violet mildew coating grows on the leaf undersurface. Leaves gradually brown and die off.

Life cycle: The pathogen spreads in humid and cooler weather (10–20 °C). Zoospores require a water film on leaves to germinate – excessive overhead watering and morning dew are therefore major risk factors for infection spread.

Most commonly affects: Grapevines, cucumbers, onions, lettuce, spinach, peas.

Prevention and treatment:
- Water exclusively at the roots, never over the leaves – drip irrigation is the ideal solution
- Copper-based preparations (Bordeaux mixture) – preventive spraying before periods of increased risk
- Ensure plant air spacing and adequate distances between plants
- Choose resistant varieties, especially for grapevines and cucumbers
- Biological preparations based on Bacillus subtilis as supporting prevention

Gray Mould – Botrytis (Botrytis cinerea)

Symptoms: Soft, brownish-gray, watery patches on leaves, stems, flowers, and fruits. A dense gray mycelial coating – the typical "fuzzy" coating that is practically unmistakable – quickly grows on affected areas. Fruits soften, rot, and disintegrate rapidly.

Life cycle: Botrytis survives as a saprophyte on dead plant material. Spores are released when tissue is mechanically damaged and spread through air. Air humidity above 90% combined with temperatures of 15–25 °C creates ideal conditions for mass spread.

Most commonly affects: Strawberries, raspberries, tomatoes, grapevines, lettuces, and ornamental flowers (begonias, pelargoniums, cyclamens).

Biological treatment and prevention:
- Preparations based on Bacillus subtilis – probiotic biological fungicide colonizing leaf surfaces and inhibiting spore germination
- Potassium bicarbonate solution – increases pH on the leaf surface and slows mycelium development
- Remove affected fruits and leaves immediately – never compost as spores easily survive in compost
- Ensure air circulation and reduce night humidity in interiors and greenhouses
- Do not damage fruits during harvest – botrytis preferentially enters through wounds and cut surfaces
- In the organic protection category you will find environmentally safe fungicides suitable for certified organic cultivation

Rust (Pucciniales)

Symptoms: Small light (yellowish-green) patches appear on the upper leaf surface. On the undersurface we find typical rust-orange to brown pustules arranged in ring-shaped clusters. In advanced stages, pustules darken – they transition to brown to black winter spores (teliospores).

Life cycle: Many rust species alternate hosts – for example, pear rust (Gymnosporangium sabinae) necessarily requires juniper as an intermediate host to complete its life cycle. Spores spread on wind over tens of kilometres, so distance from the intermediate host does not guarantee protection.

Most commonly affects: Roses, pelargoniums, tears, pears, currants, beans, garlic, chives, grasses.

Prevention and treatment:
- Remove affected leaves and dispose of outside garden compost
- Spray with fungicide thoroughly on both sides of leaves – pustules are on the underside
- Do not wet leaves during watering – humidity accelerates spore germination
- Maintain plant spacing and regularly inspect the leaf undersurface
- Remove intermediate host plants nearby (junipers near pears)

Black Spot (Diplocarpon rosae)

Symptoms: Dark brown to black circular patches with a characteristically frayed edge on rose leaves. The leaf tissue around patches yellows. Severely affected leaves fall massively; repeated defoliation permanently weakens the plant and reduces its winter hardiness.

Life cycle: Spores overwinter on fallen leaves in soil. In spring they spread through splashing water during rain or watering. The infection progresses in warm, humid weather, with spores requiring at least 7 hours of water film on the leaf to germinate.

Most commonly affects: Roses – particularly old varieties, varieties with dense habit, and tea hybrid cultivars.

Prevention and treatment:
- Carefully rake and dispose of fallen leaves – this is the primary storage site for infection material
- Mulch around roses with organic mulch – prevents spore splash from soil surface during rain
- Preventive sprays with sulphur or copper-based preparations from budbreak; apply at 7–14 day intervals
- Choose resistant varieties with ADR certification (Allgemeine Deutsche Rosenneuheitenprüfung)

Fusarium Wilt – Fusarium (Fusarium oxysporum)

Symptoms: Yellowing and wilting of lower leaves, notably often asymmetrical – on one side of the plant only. The plant partially recovers overnight, but wilts again during the day. Reliable diagnosis is a cross-section of the stem at the base – brown vascular tissue (xylem) is the characteristic sign.

Life cycle: Fusarium survives in soil as chlamydospores for decades without requiring a host. It enters the plant through roots and wounds, gradually clogging vascular bundles. The optimal temperature for development is 25–30 °C.

Most commonly affects: Tomatoes, peppers, cucumbers, bananas, carnations, asters.

Prevention and treatment:
- Strict crop rotation – do not grow the same crop or related species in the same location for at least 4 years
- Soil solarization – covering with transparent PE film for 4–6 weeks in summer reduces the number of viable propagules in the upper soil layer
- Biological preparations based on Trichoderma harzianum or Streptomyces griseoviridis (Mycostop) – work preventively as soil inoculants
- Grow resistant varieties (marked F or FF in catalogues)
- Maintain soil pH in the range 6.5–7.0 – more acidic environment favours Fusarium

Bacterial Diseases

Bacterial infections spread through water, sap-feeding insects, contaminated tools, and infected plant material. Their treatment is substantially more difficult than fungal diseases because there are no curative biological preparations of comparable efficacy – prevention and mechanical elimination of infection are therefore the primary approach.

Bacterial Canker (Erwinia amylovora)

Symptoms: Flowers brown and look as if scorched by fire – hence the English name "fire blight". Leaves and shoots blacken and bend into a shepherd's crook shape (characteristic symptom). Amber-coloured bacterial exudate occasionally appears on the bark.

Most commonly affects: Apple trees, pear trees, hawthorns, rowans, medlar – the entire Rosaceae family.

Prevention and treatment:
- Cut affected shoots at least 30 cm below the visible edge of infection and treat wounds with fungicide
- Disinfect tools after each individual cut – 70% alcohol or 10% sodium hypochlorite solution
- Avoid excessive nitrogen fertilization – lush, soft growth is especially susceptible to infection
- Preventive spray with copper preparations during flowering protects open stigmas as bacterial entry points
- Bacterial canker is a quarantine organism in the Czech Republic – report suspected infection to the regional office of the State Phytosanitary Administration (ÚKZÚZ)

Bacterial Canker (Pseudomonas syringae)

Symptoms: Sunken necrotic lesions (cankers) form on trunks and branches, from which resin or gummy exudate may leak. Doll-sized brown patches with narrow chlorotic borders occasionally appear on leaves.

Most commonly affects: Cherries, sour cherries, plums, peaches, and other stone fruits.

Prevention and treatment:
- Prune exclusively during dry summer weather – autumn and winter pruning causes open wounds under conditions favourable for infection
- Immediately treat cut wounds with grafting wax or biological healing preparation
- Preventively apply copper preparations in autumn, preferably after two-thirds of leaves have fallen

Viral Diseases

Viral infections are incurable – affected plants must be disposed of immediately to prevent further spread. Viruses are transmitted primarily by sap-feeding insects (aphids, whiteflies, thrips), mechanically contaminated tools, and infected seed. An exclusively preventive approach is therefore the only effective strategy.

Mosaic (Various Viruses – TMV, CMV, PVY)

Symptoms: Irregular light and dark green patches on leaves create a mosaic pattern. Leaves may be deformed, wrinkled, or mosaic-coloured. Plant growth slows, fruits remain small and deformed. Symptoms may vary depending on the specific virus and host.

Most commonly affects: Tomatoes, peppers, cucumbers, tobacco, beans, many ornamental plants.

Prevention:
- Use only certified virus-free seed and seedlings from reputable suppliers
- Suppress vector populations (aphids, thrips) – sticky traps enable effective monitoring and early detection
- Disinfect scissors and knives before moving between plants – particularly important for tomatoes and peppers
- Do not smoke near tomatoes or peppers – tobacco mosaic virus (TMV) survives in dried tobacco and is transmitted through direct contact
- Remove affected plants immediately from the greenhouse and burn them

ToBRFV – Brown Rugose Fruit Ripple Virus of Tomato

Symptoms: Yellow to brown patches form on tomato fruits; fruits are wrinkled and deformed with uneven ripening. Mosaic patterns and chlorotic patches may appear on leaves.

Why it is dangerous: ToBRFV is an extremely stable and highly infectious virus. Unlike classical TMV, it overcomes the Tm-2² resistance gene present in most modern hybrid tomato varieties – this makes it an exceptionally serious threat for commercial growers. It spreads mechanically through contact, tools, clothing, and by bumblebees during pollination.

Situation in the Czech Republic: The State Phytosanitary Administration (ÚKZÚZ) has recorded ToBRFV occurrence in the Czech Republic, particularly in large-scale greenhouse operations. The virus is a regulated harmful organism and is subject to phytosanitary measures with mandatory reporting requirements.

Prevention:
- Purchase seed and seedlings exclusively from certified suppliers with demonstrably negative ToBRFV test results
- Establish a disinfection protocol for tools, hands, and footwear on every entry to the growing space
- If warranted, contact the competent regional office of the State Phytosanitary Administration (ÚKZÚZ)

Living Pests – Sap-Feeding Insects

Sap-feeding insects are among the most widespread pests. By sucking plant sap from leaves and stems, they directly weaken the plant, deform its growth, and reduce yields. Many species also function as vectors of viral diseases, thereby causing secondary damage far exceeding direct feeding damage.

Aphids (Aphidoidea)

Symptoms: Colonies of tiny (1–6 mm) soft-bodied pests on the leaf undersurface, shoots, and buds. Colour varies by species – green, yellow, brown, to black. Leaves curl, surface becomes sticky from honeydew, and affected areas may be overgrown with sooty mould – a saprophytic fungus feeding on excreted honeydew.

Life cycle: Aphids reproduce by parthenogenesis (without fertilization), and a single female can produce millions of offspring in a month. In temperate environments and greenhouses they can multiply year-round without winter dormancy.

Biological treatment:
- Seven-spot ladybugs and their larvae – a single adult consumes up to 100 aphids daily; larvae are more voracious than adults
- Lacewings (Chrysoperla carnea) – predatory larvae are sometimes called "aphid lions"
- Parasitic wasp Aphidius colemani – lays eggs directly into the aphid body; parasitized individuals can be identified by their inflated bronze mummy
- Nettle extract – macerate 1 kg fresh nettles for 24 hours in 10 litres of water, then apply undiluted or diluted 1:10
- Spray with insecticidal potassium soap – contact action, safe for mammals and pollinators
- Sticky traps for monitoring arrival onset and tracking population pressure

Spider Mite (Tetranychus urticae)

Symptoms: Microscopically tiny (0.3–0.5 mm) yellowish-green to reddish mites on the leaf undersurface. The characteristic sign is fine silk webbing covering leaves and shoots. Leaves develop silvery to bronze speckling caused by emptied cells, turn yellow, and fall.

Life cycle: Mites are climate opportunists – above 30 °C and low air humidity, populations can double in just 3–5 days. In winter, fertilized females overwinter in soil and structural crevices.

Biological treatment:
- Predatory mite Phytoseiulus persimilis – specialized spider mite predator, ideal for cool greenhouses with temperatures 18–28 °C
- Predatory mite Amblyseius californicus – tolerates higher temperatures and lower humidity, suitable for Mediterranean conditions
- Increase air humidity above 60% – mites multiply significantly slower in humid conditions
- Regular misting of leaf undersurfaces with water mechanically washes away colonies and eggs
- Neem oil – disrupts moulting and reproduction in mites
- Products from the organic protection category based on potassium salts of fatty acids

Greenhouse Whitefly (Trialeurodes vaporariorum)

Symptoms: Tiny (1.5–2 mm) white winged flies with waxy coating that immediately scatter when the leaf is touched. Settled larvae resemble translucent scales on the leaf undersurface. Feeding causes leaf yellowing and honeydew excretion.

Life cycle: Females lay 200–400 eggs on the leaf undersurface in a characteristic circular pattern. Development proceeds through four larval stages – the first is mobile, the rest are sessile. The entire cycle from egg to adult takes approximately 3–4 weeks at 20 °C.

Biological treatment:
- Parasitic wasp Encarsia formosa – lays eggs in the third and fourth larval stages of whitefly; parasitized larvae blacken
- Predatory bug Macrolophus pygmaeus – generalist predator consuming eggs, larvae, and adults
- Yellow sticky traps – serve as reliable monitoring and direct reduction of adult numbers
- Insecticidal soap or neem oil as supplementary method
- Regular greenhouse ventilation – whiteflies are sensitive to air movement

Thrips (Thysanoptera)

Symptoms: Elongated insects (1–2 mm) with fringed wings, colouration from light yellow to dark brown. Leaves bear characteristic silvery deformed patches and tiny black faecal specks. Affected flowers are deformed, fruits may have surface scarification.

Life cycle: Females lay eggs directly into leaf tissue. Larvae hatch in 3–5 days and pass through two active (feeding) stages, then pupate in soil or substrate. At 25 °C, 5–8 generations occur per year. Thrips are additionally active vectors of TSWV virus (tomato spotted wilt).

Biological treatment:
- Predatory mite Amblyseius cucumeris – specialized on the first larval stage of thrips
- Predatory bug Orius laevigatus – effectively hunts all developmental stages
- Blue sticky traps – thrips demonstrably prefer blue over yellow
- Soil-dwelling predatory mite Hypoaspis miles – hunts pupae and overwintering forms in substrate
- Neem oil as contact insecticide at higher population pressure

Living Pests – Chewing Insects and Molluscs

Caterpillars (Lepidoptera – Moths and Butterflies)

Symptoms: Irregular holes in leaves, nibbled leaf edges, dark droppings (frass) on leaves and under plants. Many species are nocturnal – cabbage moth, black cutworm (Mamestra brassicae), silver y (Autographa gamma). During the day caterpillars hide at plant bases or in soil.

Most commonly affects: Cabbage, broccoli, cauliflower, tomatoes, lettuce, fruit trees.

Biological treatment:
- Bacillus thuringiensis (Bt) – selective bacterial insecticide effective exclusively on caterpillars; apply at dusk when larvae are most active
- Hand-picking in the evening with a torch – effective especially for large caterpillars
- Protective nets with 0.8–1.5 mm mesh prevent egg-laying by moths
- Supporting birds in the garden – a pair of tits brings hundreds of caterpillars to fledglings daily
- Trichogramma evanescens – parasitic wasp laying eggs in butterfly eggs

Slugs and Snails (Gastropoda)

Symptoms: Large irregular holes in leaves and fruits, glossy slime trails on leaves and soil. Active mainly at night and in wet weather. Most destructive in spring during seedling emergence, when they can completely destroy seedlings in a single night.

Most commonly affects: Lettuces, cabbage, strawberries, hostas, dahlias, impatiens, and seedlings of nearly all vegetable types.

Biological treatment:
- Parasitic nematode Phasmarhabditis hermaphrodita – apply as soil drench; particularly effective against Spanish slugs
- Beer traps – collect sunken containers with beer every morning, removing trapped slugs
- Barriers – copper tape (galvanic reaction), crushed eggshells, or ash around beds
- Granules based on iron phosphate (Ferramol) – effective yet safe for pets, birds, and earthworms
- Supporting natural enemies – hedgehogs, frogs, toads, and ground beetles are effective predators

Vine Weevil (Otiorhynchus sulcatus)

Symptoms: Adults (10–12 mm, dark gray to black weevils with punctured crown pattern) chew characteristic half-moon notches from leaf edges. Visually prominent but biologically less significant. Much greater damage is caused by white, legless, c-shaped larvae (up to 12 mm) that consume roots. Affected plants suddenly wilt without warning, and upon removal from soil have virtually no roots.

Most commonly affects: Rhododendrons, yews, laurels, strawberries, potatoes, and container plants generally – in containers larvae cannot disperse into surrounding soil, multiplying their harmfulness.

Biological treatment:
- Entomopathogenic nematodes Heterorhabditis bacteriophora or Steinernema kraussei – apply as soil drench to moist substrate in late summer or spring when larvae are active