Chinese hibiscus

The symptom affects acid-loving plants — hydrangea, rose, rhododendron, blueberry and others — grown on alkaline or calcareous soils. The leaf blade turns yellow while the veins remain distinctly green, producing the characteristic interveinal chlorosis. The symptoms appear first on the youngest leaves at the shoot tips, because iron is not very mobile within the plant and is not translocated into new growth. In extreme cases the leaves are almost white and their margins dry out. Crucially, the cause is usually not a lack of iron in the soil but too high a pH, which blocks its uptake by the roots. It is worth noting that not all susceptible plants are equally acid-loving — the rose prefers a slightly acidic to neutral reaction (about pH 6–7) and also develops iron chlorosis on strongly alkaline, calcareous soils, although it is not acid-loving in the way rhododendron or blueberry is. The pattern of yellowing is very regular: not only the main vein stays green but the whole fine network of veins, so that the leaf looks like green lace on a yellow-green background — this is the most reliable diagnostic feature, distinguishing interveinal chlorosis from the uniform, whole-surface yellowing typical of nitrogen deficiency. The symptoms intensify with each successive flush: older leaves at the base of the shoot may still be green or only slightly paled, and the younger the leaf, the paler it is, up to almost white, lemon-yellow tips. Under a long-lasting deficiency the margins and tips of the youngest leaves turn brown and necrotic, shoots grow more weakly and become smaller, and in blueberry the yield and fruit quality decline. Iron chlorosis is not contagious — it does not spread from plant to plant but appears simultaneously in all specimens growing on the same, overly alkaline site, which also helps to distinguish it from infectious diseases. Its severity can vary through the season: cold, wet and compacted soil, poor root aeration, excess calcium, a high bicarbonate content in the watering water, and over-fertilization with phosphorus additionally hinder iron uptake and aggravate the symptoms even when the element is not lacking in the soil.

The disease develops in overly wet, poorly aerated substrate and attacks the roots and the base of the stem. The roots turn brown, rot and lose their bark, which slips off easily, and a dark, watery rot appears at the stem base. The above-ground part wilts, yellows and dies back despite moist soil – a characteristic symptom, because the damaged roots no longer take up water. Especially at risk are houseplants, succulents and cacti (from overwatering), and in the ground conifers, rhododendrons, ericaceous plants and strawberries on waterlogged sites. The process begins inconspicuously at the root hairs and root tips, which darken and disappear, after which the rot advances towards the framework roots and the root collar. Healthy, pale, active roots waste away — what remains is a dark, rusty-brown or blackish, watery mass, and the outer layer of the root bark slips off like a glove, exposing the discoloured central cylinder. In trees and shrubs, infection of the collar and the base of the trunk (stem-base rot) shows as a dark, often streaking patch, sometimes with oozing, and the bark there is moist and sunken; in conifers and rhododendrons, browning and dieback are seen from the bottom or from the centre of the crown while the soil is still moist. Characteristic is the nested (patchy) occurrence of symptoms in the bed — where water and the pathogen accumulate locally. It is worth distinguishing the two causal oomycete genera: Pythium mainly attacks seedlings and fine roots, causing damping-off and root rot in cool, wet substrate, whereas Phytophthora also attacks older, woody plants, framework roots and the collar, more strongly in warmer, water-saturated soil. Key for diagnosis is the contradiction of the symptoms: the plant wilts and looks 'dry', although the substrate is wet — this results from the destruction of the roots, not from a shortage of water.

On leaves, shoots and fruit a black, soot-like coating forms that can be partly wiped off with a finger. These fungi do not infect the plant tissue – they grow on the sweet, sticky honeydew excreted by aphids, whiteflies, mealybugs, psyllids and scale insects. The coating therefore appears exactly where these pests feed, and its main harm is that it masks the leaf blade, reduces photosynthesis and disfigures the plants. The symptom is often seen on citrus, lime, maple and on houseplants and greenhouse plants. The coating may be thin and cobweb-like or thick, crusty and dull black — depending on the fungus species and the amount of honeydew; on evergreen plants it can persist through the whole winter. A key identifying feature: after the blade is wiped, healthy green tissue emerges from beneath the black layer — sooty mould leaves no spots, necrosis or pitting, because it does not penetrate the leaf. The extent of the symptom is also characteristic: the soot settles where the sticky honeydew drips, so it appears not only on leaves but also on objects beneath the plant — the pavement, garden furniture, a windowsill or car bodywork — which in itself reveals pests feeding higher up. The sooty mould fungi themselves are dark-pigmented (melanised) saprotrophs, which is why the layer is deeply black. Direct damage to the plant is usually moderate — weakening through leaf shading, loss of vigour, poorer appearance and reduced market value of fruit or ornamental plants — but a dense, long-lasting coating on many leaves genuinely limits assimilation and growth. Sooty mould is best treated as a visible signal that a colony of sap-sucking insects is developing higher up on the plant (or on a tree above it).

Tiny (1-2 mm) white insects resembling minuscule moths that rise in a cloud when the plant is disturbed. They feed on the underside of leaves, sucking sap and excreting honeydew. This causes yellowing and weakening. Common on tomatoes, peppers, cucumbers and pelargoniums, especially in polytunnels and greenhouses. The wings of the adults are covered with a white, waxy bloom (hence the name) and at rest lie flat, roof-tile fashion over the body. Females lay their eggs on the underside of young leaves, often in characteristic arcs or circles, on short stalks. The eggs hatch into mobile first-instar larvae (so-called crawlers), which, once they find a feeding site, lose the ability to move and, as flat, oval, translucent greenish 'scales', attach themselves to the leaf until pupation. Feeding symptoms are pale spots, yellowing and wilting of the leaves and - most troublesome of all - copious honeydew, on which sooty mould (sooty mould fungi) develops, disfiguring leaves and fruit and limiting photosynthesis. In heated greenhouses the whitefly develops without interruption, producing many overlapping generations throughout the year; in the Polish climate it does not overwinter outdoors (it is killed by frost), so the source of infestation is protected cultivation and introduced seedlings, while in summer the insects may fly out into the open ground. The whitefly can also be a vector of plant viruses (criniviruses, e.g. tomato chlorosis virus).

Small (1–3 mm) soft-bodied insects, green, black or pink, feeding in clusters on young shoots and the underside of leaves. They excrete sticky honeydew. The diagnostic feature that distinguishes aphids from other small sucking insects is the pair of siphunculi (tubular projections) at the rear of the abdomen together with a tail-like cauda; a single colony usually holds wingless forms (parthenogenetic females) alongside winged migrants that fly onto new plants in summer. In summer aphids reproduce parthenogenetically and viviparously over many generations, so the population builds up explosively; in autumn many species produce a sexual generation that lays overwintering eggs on the host, and host-alternating species move between a winter host (often a tree or shrub) and a summer host (a herbaceous plant). Feeding symptoms include curling, crinkling and distortion of young leaves, stunted growth, yellowing, and the shedding of buds and fruitlets. Abundant honeydew coats the leaves, and the sooty mould fungus that develops on it forms a black film which limits photosynthesis; the honeydew is fed on by ants, which in return defend the colony against predators. Aphids are also important vectors of plant viruses (mosaics among them), transmitting them from plant to plant even during a brief probing puncture.

Tiny (0.3–0.5 mm) arachnids, hard to spot with the naked eye, feeding on the underside of leaves. Symptoms: fine, pale speckling (feeding punctures); over time the leaves turn grey, yellow and dry out. Under heavy infestation a fine webbing becomes visible. Warm, dry air favours their development — a common problem for houseplants in winter near radiators. Spider mites are not insects but mites of the family Tetranychidae — adults have 8 legs (larvae only 6). The most common in cultivation is the two-spotted spider mite (Tetranychus urticae): yellowish-green females carry two dark spots on the sides of the body, while overwintering females turn orange-red. In orchards the fruit tree red spider mite (Panonychus ulmi) is frequent, whose red eggs overwinter in clusters on the bark of the shoots. Feeding mechanism: the mites puncture the cells of the epidermis and mesophyll on the underside of the leaf and suck out their contents, and the emptied cells fill with air — hence the characteristic, silvery-grey, fine speckling visible from the upper side of the blade (resembling a mosaic discolouration). At high numbers the leaves turn brown, curl and drop prematurely, and the whole plant weakens and loses its growth. Dense webbing spun between the leaves and shoot tips appears only under an advanced attack and is a sign of the risk of the pest spreading to neighbouring plants. The development cycle is very fast in warmth and drought — at a temperature of 25–30°C a single generation can be completed in 1–2 weeks, hence the numerous generations per season and the rapid, exponential build-up of the population. Females lay spherical, transparent eggs on the underside of the leaves. Fertilised females overwinter (in bark crevices, leaf litter and plant debris) or — in the fruit tree red spider mite — the eggs on the bark. Early infestation is easy to miss; diagnosis is helped by shaking a leaf over a white sheet of paper (moving specks become visible) or examining the underside of the leaf with a magnifying glass.

Insects covered in a white, cottony, waxy coating, gathering in leaf axils, at the base of shoots and on the underside of leaf blades. They look like tufts of cotton wool. They suck sap, weaken the plant and excrete sticky honeydew, on which black sooty mould develops. A common pest of houseplants and succulents. Adult females are oval, soft-bodied insects about 2-5 mm long, with a pinkish or grey body hidden under wax, often bearing waxy filaments along the sides and at the tip of the abdomen. Males are tiny, winged and do not feed. In most species eggs are laid in white, cottony ovisacs (cocoons), from which mobile larvae (crawlers) hatch and spread across the plant, carrying the infestation to neighbouring specimens; some mealybugs (e.g. Pseudococcus longispinus), however, are ovoviviparous and give birth to live crawlers without a distinct ovisac. In the warm, dry conditions of homes and glasshouses mealybugs develop all year round, in several overlapping generations - which is why a single treatment is rarely enough. Feeding symptoms are stunted growth, yellowing, deformation and premature leaf drop, and under heavy infestation dieback of shoots; the black sooty mould on the honeydew further reduces photosynthesis and disfigures the plant. A separate form is the root mealybug (Rhizoecus), forming white, waxy clusters on the roots and inside the root ball - the plant weakens and wilts with no visible pest on the above-ground parts. Mealybugs are often attended by ants, which eat the honeydew and protect the colonies from natural enemies.

Immobile, brown or yellowish shields (1-4 mm) attached to stems and the underside of leaves, especially along the veins. Easily mistaken for growths on the plant. They suck sap, causing yellowing and weakening, and excrete sticky honeydew. Common on figs, orchids and plants with tough leaves. The collective name covers two groups with different biology. Armoured scales (Diaspididae) have a hard waxy shield separate from the body, which can be prised off - beneath it emerges a tiny, legless and wingless female; they do not excrete honeydew. Soft scales (Coccidae) have a convex, cup-shaped cover fused with the body, and it is they that excrete it abundantly. Sticky honeydew - the domain of soft scales and other honeydew-producing scale insects, not of armoured scales - settles on the leaves and attracts sooty mould fungi (sooty mould, sooty blotch) as well as ants, which defend the colony against predators. The only mobile feeding stage is the crawler - a flat, tiny larva which, after hatching from beneath the mother's shield, disperses over the plant, then settles, inserts its stylet into the conducting tissues and loses the ability to move; adult females remain immobile for the rest of their lives. Males pupate and, as tiny, winged, short-lived insects, do not feed. In heated interiors the pest breeds all year round, producing several overlapping generations; outdoors usually 1-3 generations, and fertilised females or eggs hidden beneath the shield overwinter. Feeding symptoms are yellowing and premature leaf drop, dieback of shoots, stunting and weakening of the plant, and secondarily a black coating of sooty mould on leaves covered with honeydew. On fruit trees infested by San Jose scale (Comstockaspis perniciosa), reddish discolouration appears around the feeding sites, and heavily colonised branches die back. Because the shields are immobile and their colour blends into the bark and veins, an infestation is easily overlooked until honeydew appears or the plant is noticeably weakened - which is why regular inspection of the leaf undersides and shoot axils is key.